Concepts of Physics Exercises Electric Field & Potential
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Transcript of Concepts of Physics Exercises Electric Field & Potential
Concepts of Physics
Exercises
Electric Field amp Potential
1 Find the dimensional formula of 1205760
Solution
Electrostatic force between two point charges F = 1
41205871205760
11990211199022
1199032 rArr 1205760 = 11990211199022
41205871198651199032
Dimensional formula of charge q = IT
Dimensional formula of force F = MLT-2
Dimensional formula of distance r = L
Dimensional formula of permittivity of free space 1205760 = (119868119879) (119868119879)
119872119871119879minus2119871= M-1L-2T4I2
Electric Field amp Potential
2 A charge of 10 C is placed at the top of your college building and another equal charge at the top of your house
Take the separation between the two charges to be 20 km Find the force exerted by the charges on each other How
many times of your weight is this force
Solution
Charge on each particle q = 1 C
Separation between the charges r = 2 km = 2000 m
Electrostatic force between the charge F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1 119909 1
4 119909 106 = 225 x 103 N
3 At what separation should two equal charges 10 C each be placed so that the force between them equals the weight
of a 50 kg person
Solution
Charge on each particle q1 = q2 = 1 C
Force between the particles F = mg = 50 x 98 = 490 N
For separation between the particle F = 1
41205871205760
11990211199022
1199032 rArr 490 = 9 x 109 1 119909 1
1199032 rArr r = 9 119909 109
490=
3
7x 104 = 43 x 103 m
Electric Field amp Potential
4 Two equal charges are placed at a separation of 10 m What should be the magnitude of the charges so that the force
between them equals the weight of a 50 kg person
Solution
Let the charge on each particle be lsquoqrsquo
Separation between the charges r = 1 m
Magnitude of force between the charges F = mg = 50 x 98 = 490 N
For magnitude of charges F = 1
41205871205760
11990211199022
1199032 rArr 490 = 9 x 109 1199022
1rArr q =
490
9 119909 109 = 7
3 119909 104 = 23 x 10-4 C
5 Find the electric force between two protons separated by a distance of 1 fermi (1 fermi = 10minus15m) The protons in a
nucleus remain at a separation of this order
Solution
Charge on each proton q = 16 x 10-19 C
Separation between the protons r = 1 fm = 10-15 m
Electrostatic force between the protons F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
10minus30 = 9 x 1039 x 256 x 10-38 = 2304 N
Electric Field amp Potential
6 Two charges 20 times 10minus6C and 10 times 10minus6C are placed at a separation of 10 cm Where should a third charge be
placed such that it experiences no net force due to these charges
Solution
Charge on the first particle q1 = 2 x 10-6 C
Charge on the second particle q2 = 1 x 10-6 C
Separation between the particles r = 10 cm = 01 m
Let the third charge be placed at lsquoxrsquo from the larger charge
Force between the larger charge and test charge F1 = 1
41205871205760
1199021119902
1199032 = 9 x 109 2 119909 10minus6 (119902)
1199092 [repulsive]
Force between the smaller charge and test charge F2 = 1
41205871205760
1199022119902
1199032 = 9 x 109 1 119909 10minus6 (119902)
(10minus119909)2 [repulsive]
Given net force on test charge is zero F1 ndash F2 = 0 rArr F1 = F2 rArr 9 x 109 2 119909 10minus6 (119902)
1199092 = 9 x 109 1 119909 10minus6 (119902)
(10 minus 119909)2
2 119909 10minus6 (119902)
1199092 = 1 119909 10minus6
(10 minus 119909)2 rArr 2
1199092 = 1
(10 minus 119909)2 rArr 2
119909=
1
10 minus 119909rArr (1 + 2) x = 10 2 rArr x =
10 2
1 + 2= 59 cm
Electric Field amp Potential
7 Suppose the second charge in the previous problem is minus10 times 10minus6C Locate the position where a third charge will
not experience a net force
Solution
When one of the charges is negative null point is not possible between the two charges
Let the null point is located at lsquoxrsquo from smaller charge outside the two charges
Force on the test charge due to larger charge F1 = 1
41205871205760
1199021119902
1199032 = 9 x 109 2 119909 10minus6 (119902)
(10+119909)2 [repulsive]
Force on the test charge due to smaller charge F2 = 1
41205871205760
1199022119902
1199032 = 9 x 109 1 119909 10minus6 (119902)
(119909)2 [attractive]
Given net force on test charge is zero F1 ndash F2 = 0 rArr F1 = F2 rArr 9 x 109 2 119909 10minus6 (119902)
(10+119909)2 = 9 x 109 1 119909 10minus6 (119902)
(119909)2
2 119909 10minus6 (119902)
(10+119909)2 = 1 119909 10minus6 (119902)
(119909)2 rArr 2
(10+119909)2 = 1
(119909)2 rArr 2
10 + 119909=
1
119909rArr 2 minus 1 119909 = 10 rArr x =
10
2minus1=
10
0414= 241 cm
So the test charge is a distance of 10 + 241 = 341 cm from larger charge
Electric Field amp Potential
8 Two charged particles are placed at a distance 10 cm apart What is the minimum possible magnitude of the electric
force acting on each charge
Solution
Separation between the particles r = 1 cm
Minimum magnitude of electric charge charge on electron q = 16 x 10-19 C
Force between the two particle
F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
10minus4 = 9 x 1013 x 256 x 10-38 = 2304 x 10-25 N = 23 x 10-24 N
Electric Field amp Potential
9 Estimate the number of electrons in 100 g of water How much is the total negative charge on these electrons
Solution
Mass of water m = 100 g
Molecular weight of water M = 18 gmole
Number of molecules N = nNA = 119898
119872NA =
100
18x 6023 x 1023 = 335 x 1024
Number of electrons in each water molecule Ne = 2 + 16 = 18
Number of electrons in given sample of water n = 10 x 335 x 1024 = 335 x 1025
Total negative charge on the electrons q = ne = 335 x 1025 x 16 x 10-19 = 535 x 106
Electric Field amp Potential
10 Suppose all the electrons of 100 g water are lumped together to form a negatively charged particle and all the
nuclei are lumped together to form a positively charged particle If these two particles are placed 100 cm away from
each other find the force of attraction between them Compare it with your weight
Solution
Total negative charge available q1 = 535 x 106 C
Total positive charge available q2 = 535 x 106 C
Separation between the charges r = 10 cm = 01 m
Force of attraction between the charges F = 1
41205871205760
11990211199022
1199032
F = 9 x 109 x 535 119909 106 2
001= 9 x 1011 x 286 x 1012 = 256 x 1025 N
Electric Field amp Potential
11 Consider a gold nucleus to be a sphere of radius 69 fermi in which protons and neutrons are distributed Find the
force of repulsion between two protons situated at largest separation Why do these protons not fly apart under this
repulsion
Solution
Radius of the gold nucleus R = 69 fm = 69 x 10-15 m
Largest separation between protons r = 2R = 138 x 10-15 m
Charge on each proton q = 16 x 10-19 C
Force of repulsion between the protons F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
138 119909 10minus15 2 = 12 N
Electric Field amp Potential
12 Two insulting small spheres are rubbed against each other and placed 1 cm apart If they attract each other with a
force of 01 N how many electrons were transferred from one sphere to the other during rubbing
Solution
Let the charge acquired by each sphere q = ne
Separation between the spheres r = 1 cm = 10-2 m
Force of attraction between the spheres F = 01 N
Force of attraction between the spheres F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1198992 16 119909 10minus19 2
10minus4
01 = 9 x 1013 x 256 x 10-38 n2
n2 = 1024
2304=
100
2304x 1022 rArr n = 2 x 1011
Electric Field amp Potential
13 NaCI molecule is bound due to the electric force between the sodium and the chlorine ions when one electron of
sodium is transferred to chlorine Taking the separation between the ions to be 275 times 10minus8 cm find the force of
attraction between them State the assumptions (if any) that you have made
Solution
Separation between the ions r = 275 x 10-8 cm
Charge on sodium and chlorine ions q1 = q2 = q = 16 x 10-19 C
Force of attraction between the ions F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
275 119909 10minus10 2 = 305 x 10-9 N
Electric Field amp Potential
14 Find the ratio of the electric and gravitational forces between two protons
Solution
Mass of each proton m1 = m2 = m = 167 x 10-27 kg
Charge on each proton q1 = q2 = q = 16 x 10-19 C
Let the separation between the protons be lsquorrsquo
Electric force between the protons Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
1199032 = 2304 119909 10minus29
1199032
Gravitational force between the protons Fg = 11986611989811198982
1199032 = 667 119909 10minus11 119909 167 119909 10minus27 2
1199032 = 186 119909 10minus65
1199032
Ratio of electric and gravitational forces 119865119890
119865119892=
2304 119909 10minus29
186 119909 10minus65 = 123 x 1036
Electric Field amp Potential
15 Suppose an attractive nuclear force acts between two protons which may be written as F = Ceminuskrr2
(a) write down the dimensional formulae and appropriate SI units of C and k (b) suppose that k = 1 fermiminus1 and that
the repulsive electric force between the protons is just balanced by the attractive nuclear force when the separation is
5 fermi Find the value of C
Solution
(a) Nuclear force between protons F = Ceminuskrr2
Dimensions of exponential function M0L0T0
Dimensional formula for kr M0L0T0 rArr kL = M0L0T0 rArr k = M0L-1T0 (SI unit m-1)
Dimensional formula of force MLT-2
Dimensional formula of Cr2 = MLT-2 rArr C = MLT-2L2 = ML3T-2 (SI unit Nm2)
(b) Repulsive force between protons Fe = 1
41205871205760
1199022
1199032 = 9 119909 109 119909 16 119909 10minus19 2
5 119909 10minus15 2 = 2304
25x 101 = 92 N
Nuclear attractive force between protons Fn = C(eminuskr)r2 = C 119890
minus11 119909 5
5 119909 10minus15 2 = (Ce-5 x 1030 )25
Given Fe = Fn rArr 92 = 1030
251198905 C rArr C = 92 x 25e5 x 10-30 = 34 x 10-26 Nm2
Electric Field amp Potential
16 Three equal charges 20 times 10minus6C each are held fixed at the three corners of an equilateral triangle of side 5 cm
Find the Coulomb force experienced by one of the charges due to the rest two
Solution
Charge on each particle q1 = q2 = q3 = 2 x 10-6 C
Side of the triangle r = 5 cm = 5 x 10-2 m
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Net force on C Fc = 2FAC cos 602 = 2 x 144 x cos 30 = 249 N [along the angular bisector 300]
Electric Field amp Potential
17 Four equal charges 20 times 10minus6C each are fixed at the four corners of a square of side 5 cm Find the Coulomb
force experienced by one of the charges due to the rest three
Solution
Charge on each particle q1 = q2 = q3 = q4 = q = 2 x 10-6 C
Side of the square r = 5 cm = 5 x 10-2 m
Diagonal of the square R = 2r = 5 2 x 10-2 m
Force between B amp C FBC = 1
41205871205760
11990221199023
1199032 = 1
41205871205760
1199022
1199032
Force between D amp C FDC 1
41205871205760
11990241199023
1199032 = 1
41205871205760
1199022
1199032
Resultant of the above two forces FBD = 21
41205871205760
1199022
1199032 [along the diagonal angular bisector 450]
Force between A amp C FAC = 1
41205871205760
11990211199023
1198772 = 1
41205871205760
1199022
21199032
Resultant force on C FC = FAC + FBD = 1
41205871205760
1199022
21199032 + 21
41205871205760
1199022
1199032 = 1
41205871205760
1199022
1199032 2 +1
2
FC = 9 x 109 x 4 119909 10minus12
25 119909 10minus4 2 +1
2= 275 N
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
1 Find the dimensional formula of 1205760
Solution
Electrostatic force between two point charges F = 1
41205871205760
11990211199022
1199032 rArr 1205760 = 11990211199022
41205871198651199032
Dimensional formula of charge q = IT
Dimensional formula of force F = MLT-2
Dimensional formula of distance r = L
Dimensional formula of permittivity of free space 1205760 = (119868119879) (119868119879)
119872119871119879minus2119871= M-1L-2T4I2
Electric Field amp Potential
2 A charge of 10 C is placed at the top of your college building and another equal charge at the top of your house
Take the separation between the two charges to be 20 km Find the force exerted by the charges on each other How
many times of your weight is this force
Solution
Charge on each particle q = 1 C
Separation between the charges r = 2 km = 2000 m
Electrostatic force between the charge F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1 119909 1
4 119909 106 = 225 x 103 N
3 At what separation should two equal charges 10 C each be placed so that the force between them equals the weight
of a 50 kg person
Solution
Charge on each particle q1 = q2 = 1 C
Force between the particles F = mg = 50 x 98 = 490 N
For separation between the particle F = 1
41205871205760
11990211199022
1199032 rArr 490 = 9 x 109 1 119909 1
1199032 rArr r = 9 119909 109
490=
3
7x 104 = 43 x 103 m
Electric Field amp Potential
4 Two equal charges are placed at a separation of 10 m What should be the magnitude of the charges so that the force
between them equals the weight of a 50 kg person
Solution
Let the charge on each particle be lsquoqrsquo
Separation between the charges r = 1 m
Magnitude of force between the charges F = mg = 50 x 98 = 490 N
For magnitude of charges F = 1
41205871205760
11990211199022
1199032 rArr 490 = 9 x 109 1199022
1rArr q =
490
9 119909 109 = 7
3 119909 104 = 23 x 10-4 C
5 Find the electric force between two protons separated by a distance of 1 fermi (1 fermi = 10minus15m) The protons in a
nucleus remain at a separation of this order
Solution
Charge on each proton q = 16 x 10-19 C
Separation between the protons r = 1 fm = 10-15 m
Electrostatic force between the protons F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
10minus30 = 9 x 1039 x 256 x 10-38 = 2304 N
Electric Field amp Potential
6 Two charges 20 times 10minus6C and 10 times 10minus6C are placed at a separation of 10 cm Where should a third charge be
placed such that it experiences no net force due to these charges
Solution
Charge on the first particle q1 = 2 x 10-6 C
Charge on the second particle q2 = 1 x 10-6 C
Separation between the particles r = 10 cm = 01 m
Let the third charge be placed at lsquoxrsquo from the larger charge
Force between the larger charge and test charge F1 = 1
41205871205760
1199021119902
1199032 = 9 x 109 2 119909 10minus6 (119902)
1199092 [repulsive]
Force between the smaller charge and test charge F2 = 1
41205871205760
1199022119902
1199032 = 9 x 109 1 119909 10minus6 (119902)
(10minus119909)2 [repulsive]
Given net force on test charge is zero F1 ndash F2 = 0 rArr F1 = F2 rArr 9 x 109 2 119909 10minus6 (119902)
1199092 = 9 x 109 1 119909 10minus6 (119902)
(10 minus 119909)2
2 119909 10minus6 (119902)
1199092 = 1 119909 10minus6
(10 minus 119909)2 rArr 2
1199092 = 1
(10 minus 119909)2 rArr 2
119909=
1
10 minus 119909rArr (1 + 2) x = 10 2 rArr x =
10 2
1 + 2= 59 cm
Electric Field amp Potential
7 Suppose the second charge in the previous problem is minus10 times 10minus6C Locate the position where a third charge will
not experience a net force
Solution
When one of the charges is negative null point is not possible between the two charges
Let the null point is located at lsquoxrsquo from smaller charge outside the two charges
Force on the test charge due to larger charge F1 = 1
41205871205760
1199021119902
1199032 = 9 x 109 2 119909 10minus6 (119902)
(10+119909)2 [repulsive]
Force on the test charge due to smaller charge F2 = 1
41205871205760
1199022119902
1199032 = 9 x 109 1 119909 10minus6 (119902)
(119909)2 [attractive]
Given net force on test charge is zero F1 ndash F2 = 0 rArr F1 = F2 rArr 9 x 109 2 119909 10minus6 (119902)
(10+119909)2 = 9 x 109 1 119909 10minus6 (119902)
(119909)2
2 119909 10minus6 (119902)
(10+119909)2 = 1 119909 10minus6 (119902)
(119909)2 rArr 2
(10+119909)2 = 1
(119909)2 rArr 2
10 + 119909=
1
119909rArr 2 minus 1 119909 = 10 rArr x =
10
2minus1=
10
0414= 241 cm
So the test charge is a distance of 10 + 241 = 341 cm from larger charge
Electric Field amp Potential
8 Two charged particles are placed at a distance 10 cm apart What is the minimum possible magnitude of the electric
force acting on each charge
Solution
Separation between the particles r = 1 cm
Minimum magnitude of electric charge charge on electron q = 16 x 10-19 C
Force between the two particle
F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
10minus4 = 9 x 1013 x 256 x 10-38 = 2304 x 10-25 N = 23 x 10-24 N
Electric Field amp Potential
9 Estimate the number of electrons in 100 g of water How much is the total negative charge on these electrons
Solution
Mass of water m = 100 g
Molecular weight of water M = 18 gmole
Number of molecules N = nNA = 119898
119872NA =
100
18x 6023 x 1023 = 335 x 1024
Number of electrons in each water molecule Ne = 2 + 16 = 18
Number of electrons in given sample of water n = 10 x 335 x 1024 = 335 x 1025
Total negative charge on the electrons q = ne = 335 x 1025 x 16 x 10-19 = 535 x 106
Electric Field amp Potential
10 Suppose all the electrons of 100 g water are lumped together to form a negatively charged particle and all the
nuclei are lumped together to form a positively charged particle If these two particles are placed 100 cm away from
each other find the force of attraction between them Compare it with your weight
Solution
Total negative charge available q1 = 535 x 106 C
Total positive charge available q2 = 535 x 106 C
Separation between the charges r = 10 cm = 01 m
Force of attraction between the charges F = 1
41205871205760
11990211199022
1199032
F = 9 x 109 x 535 119909 106 2
001= 9 x 1011 x 286 x 1012 = 256 x 1025 N
Electric Field amp Potential
11 Consider a gold nucleus to be a sphere of radius 69 fermi in which protons and neutrons are distributed Find the
force of repulsion between two protons situated at largest separation Why do these protons not fly apart under this
repulsion
Solution
Radius of the gold nucleus R = 69 fm = 69 x 10-15 m
Largest separation between protons r = 2R = 138 x 10-15 m
Charge on each proton q = 16 x 10-19 C
Force of repulsion between the protons F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
138 119909 10minus15 2 = 12 N
Electric Field amp Potential
12 Two insulting small spheres are rubbed against each other and placed 1 cm apart If they attract each other with a
force of 01 N how many electrons were transferred from one sphere to the other during rubbing
Solution
Let the charge acquired by each sphere q = ne
Separation between the spheres r = 1 cm = 10-2 m
Force of attraction between the spheres F = 01 N
Force of attraction between the spheres F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1198992 16 119909 10minus19 2
10minus4
01 = 9 x 1013 x 256 x 10-38 n2
n2 = 1024
2304=
100
2304x 1022 rArr n = 2 x 1011
Electric Field amp Potential
13 NaCI molecule is bound due to the electric force between the sodium and the chlorine ions when one electron of
sodium is transferred to chlorine Taking the separation between the ions to be 275 times 10minus8 cm find the force of
attraction between them State the assumptions (if any) that you have made
Solution
Separation between the ions r = 275 x 10-8 cm
Charge on sodium and chlorine ions q1 = q2 = q = 16 x 10-19 C
Force of attraction between the ions F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
275 119909 10minus10 2 = 305 x 10-9 N
Electric Field amp Potential
14 Find the ratio of the electric and gravitational forces between two protons
Solution
Mass of each proton m1 = m2 = m = 167 x 10-27 kg
Charge on each proton q1 = q2 = q = 16 x 10-19 C
Let the separation between the protons be lsquorrsquo
Electric force between the protons Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
1199032 = 2304 119909 10minus29
1199032
Gravitational force between the protons Fg = 11986611989811198982
1199032 = 667 119909 10minus11 119909 167 119909 10minus27 2
1199032 = 186 119909 10minus65
1199032
Ratio of electric and gravitational forces 119865119890
119865119892=
2304 119909 10minus29
186 119909 10minus65 = 123 x 1036
Electric Field amp Potential
15 Suppose an attractive nuclear force acts between two protons which may be written as F = Ceminuskrr2
(a) write down the dimensional formulae and appropriate SI units of C and k (b) suppose that k = 1 fermiminus1 and that
the repulsive electric force between the protons is just balanced by the attractive nuclear force when the separation is
5 fermi Find the value of C
Solution
(a) Nuclear force between protons F = Ceminuskrr2
Dimensions of exponential function M0L0T0
Dimensional formula for kr M0L0T0 rArr kL = M0L0T0 rArr k = M0L-1T0 (SI unit m-1)
Dimensional formula of force MLT-2
Dimensional formula of Cr2 = MLT-2 rArr C = MLT-2L2 = ML3T-2 (SI unit Nm2)
(b) Repulsive force between protons Fe = 1
41205871205760
1199022
1199032 = 9 119909 109 119909 16 119909 10minus19 2
5 119909 10minus15 2 = 2304
25x 101 = 92 N
Nuclear attractive force between protons Fn = C(eminuskr)r2 = C 119890
minus11 119909 5
5 119909 10minus15 2 = (Ce-5 x 1030 )25
Given Fe = Fn rArr 92 = 1030
251198905 C rArr C = 92 x 25e5 x 10-30 = 34 x 10-26 Nm2
Electric Field amp Potential
16 Three equal charges 20 times 10minus6C each are held fixed at the three corners of an equilateral triangle of side 5 cm
Find the Coulomb force experienced by one of the charges due to the rest two
Solution
Charge on each particle q1 = q2 = q3 = 2 x 10-6 C
Side of the triangle r = 5 cm = 5 x 10-2 m
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Net force on C Fc = 2FAC cos 602 = 2 x 144 x cos 30 = 249 N [along the angular bisector 300]
Electric Field amp Potential
17 Four equal charges 20 times 10minus6C each are fixed at the four corners of a square of side 5 cm Find the Coulomb
force experienced by one of the charges due to the rest three
Solution
Charge on each particle q1 = q2 = q3 = q4 = q = 2 x 10-6 C
Side of the square r = 5 cm = 5 x 10-2 m
Diagonal of the square R = 2r = 5 2 x 10-2 m
Force between B amp C FBC = 1
41205871205760
11990221199023
1199032 = 1
41205871205760
1199022
1199032
Force between D amp C FDC 1
41205871205760
11990241199023
1199032 = 1
41205871205760
1199022
1199032
Resultant of the above two forces FBD = 21
41205871205760
1199022
1199032 [along the diagonal angular bisector 450]
Force between A amp C FAC = 1
41205871205760
11990211199023
1198772 = 1
41205871205760
1199022
21199032
Resultant force on C FC = FAC + FBD = 1
41205871205760
1199022
21199032 + 21
41205871205760
1199022
1199032 = 1
41205871205760
1199022
1199032 2 +1
2
FC = 9 x 109 x 4 119909 10minus12
25 119909 10minus4 2 +1
2= 275 N
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
2 A charge of 10 C is placed at the top of your college building and another equal charge at the top of your house
Take the separation between the two charges to be 20 km Find the force exerted by the charges on each other How
many times of your weight is this force
Solution
Charge on each particle q = 1 C
Separation between the charges r = 2 km = 2000 m
Electrostatic force between the charge F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1 119909 1
4 119909 106 = 225 x 103 N
3 At what separation should two equal charges 10 C each be placed so that the force between them equals the weight
of a 50 kg person
Solution
Charge on each particle q1 = q2 = 1 C
Force between the particles F = mg = 50 x 98 = 490 N
For separation between the particle F = 1
41205871205760
11990211199022
1199032 rArr 490 = 9 x 109 1 119909 1
1199032 rArr r = 9 119909 109
490=
3
7x 104 = 43 x 103 m
Electric Field amp Potential
4 Two equal charges are placed at a separation of 10 m What should be the magnitude of the charges so that the force
between them equals the weight of a 50 kg person
Solution
Let the charge on each particle be lsquoqrsquo
Separation between the charges r = 1 m
Magnitude of force between the charges F = mg = 50 x 98 = 490 N
For magnitude of charges F = 1
41205871205760
11990211199022
1199032 rArr 490 = 9 x 109 1199022
1rArr q =
490
9 119909 109 = 7
3 119909 104 = 23 x 10-4 C
5 Find the electric force between two protons separated by a distance of 1 fermi (1 fermi = 10minus15m) The protons in a
nucleus remain at a separation of this order
Solution
Charge on each proton q = 16 x 10-19 C
Separation between the protons r = 1 fm = 10-15 m
Electrostatic force between the protons F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
10minus30 = 9 x 1039 x 256 x 10-38 = 2304 N
Electric Field amp Potential
6 Two charges 20 times 10minus6C and 10 times 10minus6C are placed at a separation of 10 cm Where should a third charge be
placed such that it experiences no net force due to these charges
Solution
Charge on the first particle q1 = 2 x 10-6 C
Charge on the second particle q2 = 1 x 10-6 C
Separation between the particles r = 10 cm = 01 m
Let the third charge be placed at lsquoxrsquo from the larger charge
Force between the larger charge and test charge F1 = 1
41205871205760
1199021119902
1199032 = 9 x 109 2 119909 10minus6 (119902)
1199092 [repulsive]
Force between the smaller charge and test charge F2 = 1
41205871205760
1199022119902
1199032 = 9 x 109 1 119909 10minus6 (119902)
(10minus119909)2 [repulsive]
Given net force on test charge is zero F1 ndash F2 = 0 rArr F1 = F2 rArr 9 x 109 2 119909 10minus6 (119902)
1199092 = 9 x 109 1 119909 10minus6 (119902)
(10 minus 119909)2
2 119909 10minus6 (119902)
1199092 = 1 119909 10minus6
(10 minus 119909)2 rArr 2
1199092 = 1
(10 minus 119909)2 rArr 2
119909=
1
10 minus 119909rArr (1 + 2) x = 10 2 rArr x =
10 2
1 + 2= 59 cm
Electric Field amp Potential
7 Suppose the second charge in the previous problem is minus10 times 10minus6C Locate the position where a third charge will
not experience a net force
Solution
When one of the charges is negative null point is not possible between the two charges
Let the null point is located at lsquoxrsquo from smaller charge outside the two charges
Force on the test charge due to larger charge F1 = 1
41205871205760
1199021119902
1199032 = 9 x 109 2 119909 10minus6 (119902)
(10+119909)2 [repulsive]
Force on the test charge due to smaller charge F2 = 1
41205871205760
1199022119902
1199032 = 9 x 109 1 119909 10minus6 (119902)
(119909)2 [attractive]
Given net force on test charge is zero F1 ndash F2 = 0 rArr F1 = F2 rArr 9 x 109 2 119909 10minus6 (119902)
(10+119909)2 = 9 x 109 1 119909 10minus6 (119902)
(119909)2
2 119909 10minus6 (119902)
(10+119909)2 = 1 119909 10minus6 (119902)
(119909)2 rArr 2
(10+119909)2 = 1
(119909)2 rArr 2
10 + 119909=
1
119909rArr 2 minus 1 119909 = 10 rArr x =
10
2minus1=
10
0414= 241 cm
So the test charge is a distance of 10 + 241 = 341 cm from larger charge
Electric Field amp Potential
8 Two charged particles are placed at a distance 10 cm apart What is the minimum possible magnitude of the electric
force acting on each charge
Solution
Separation between the particles r = 1 cm
Minimum magnitude of electric charge charge on electron q = 16 x 10-19 C
Force between the two particle
F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
10minus4 = 9 x 1013 x 256 x 10-38 = 2304 x 10-25 N = 23 x 10-24 N
Electric Field amp Potential
9 Estimate the number of electrons in 100 g of water How much is the total negative charge on these electrons
Solution
Mass of water m = 100 g
Molecular weight of water M = 18 gmole
Number of molecules N = nNA = 119898
119872NA =
100
18x 6023 x 1023 = 335 x 1024
Number of electrons in each water molecule Ne = 2 + 16 = 18
Number of electrons in given sample of water n = 10 x 335 x 1024 = 335 x 1025
Total negative charge on the electrons q = ne = 335 x 1025 x 16 x 10-19 = 535 x 106
Electric Field amp Potential
10 Suppose all the electrons of 100 g water are lumped together to form a negatively charged particle and all the
nuclei are lumped together to form a positively charged particle If these two particles are placed 100 cm away from
each other find the force of attraction between them Compare it with your weight
Solution
Total negative charge available q1 = 535 x 106 C
Total positive charge available q2 = 535 x 106 C
Separation between the charges r = 10 cm = 01 m
Force of attraction between the charges F = 1
41205871205760
11990211199022
1199032
F = 9 x 109 x 535 119909 106 2
001= 9 x 1011 x 286 x 1012 = 256 x 1025 N
Electric Field amp Potential
11 Consider a gold nucleus to be a sphere of radius 69 fermi in which protons and neutrons are distributed Find the
force of repulsion between two protons situated at largest separation Why do these protons not fly apart under this
repulsion
Solution
Radius of the gold nucleus R = 69 fm = 69 x 10-15 m
Largest separation between protons r = 2R = 138 x 10-15 m
Charge on each proton q = 16 x 10-19 C
Force of repulsion between the protons F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
138 119909 10minus15 2 = 12 N
Electric Field amp Potential
12 Two insulting small spheres are rubbed against each other and placed 1 cm apart If they attract each other with a
force of 01 N how many electrons were transferred from one sphere to the other during rubbing
Solution
Let the charge acquired by each sphere q = ne
Separation between the spheres r = 1 cm = 10-2 m
Force of attraction between the spheres F = 01 N
Force of attraction between the spheres F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1198992 16 119909 10minus19 2
10minus4
01 = 9 x 1013 x 256 x 10-38 n2
n2 = 1024
2304=
100
2304x 1022 rArr n = 2 x 1011
Electric Field amp Potential
13 NaCI molecule is bound due to the electric force between the sodium and the chlorine ions when one electron of
sodium is transferred to chlorine Taking the separation between the ions to be 275 times 10minus8 cm find the force of
attraction between them State the assumptions (if any) that you have made
Solution
Separation between the ions r = 275 x 10-8 cm
Charge on sodium and chlorine ions q1 = q2 = q = 16 x 10-19 C
Force of attraction between the ions F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
275 119909 10minus10 2 = 305 x 10-9 N
Electric Field amp Potential
14 Find the ratio of the electric and gravitational forces between two protons
Solution
Mass of each proton m1 = m2 = m = 167 x 10-27 kg
Charge on each proton q1 = q2 = q = 16 x 10-19 C
Let the separation between the protons be lsquorrsquo
Electric force between the protons Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
1199032 = 2304 119909 10minus29
1199032
Gravitational force between the protons Fg = 11986611989811198982
1199032 = 667 119909 10minus11 119909 167 119909 10minus27 2
1199032 = 186 119909 10minus65
1199032
Ratio of electric and gravitational forces 119865119890
119865119892=
2304 119909 10minus29
186 119909 10minus65 = 123 x 1036
Electric Field amp Potential
15 Suppose an attractive nuclear force acts between two protons which may be written as F = Ceminuskrr2
(a) write down the dimensional formulae and appropriate SI units of C and k (b) suppose that k = 1 fermiminus1 and that
the repulsive electric force between the protons is just balanced by the attractive nuclear force when the separation is
5 fermi Find the value of C
Solution
(a) Nuclear force between protons F = Ceminuskrr2
Dimensions of exponential function M0L0T0
Dimensional formula for kr M0L0T0 rArr kL = M0L0T0 rArr k = M0L-1T0 (SI unit m-1)
Dimensional formula of force MLT-2
Dimensional formula of Cr2 = MLT-2 rArr C = MLT-2L2 = ML3T-2 (SI unit Nm2)
(b) Repulsive force between protons Fe = 1
41205871205760
1199022
1199032 = 9 119909 109 119909 16 119909 10minus19 2
5 119909 10minus15 2 = 2304
25x 101 = 92 N
Nuclear attractive force between protons Fn = C(eminuskr)r2 = C 119890
minus11 119909 5
5 119909 10minus15 2 = (Ce-5 x 1030 )25
Given Fe = Fn rArr 92 = 1030
251198905 C rArr C = 92 x 25e5 x 10-30 = 34 x 10-26 Nm2
Electric Field amp Potential
16 Three equal charges 20 times 10minus6C each are held fixed at the three corners of an equilateral triangle of side 5 cm
Find the Coulomb force experienced by one of the charges due to the rest two
Solution
Charge on each particle q1 = q2 = q3 = 2 x 10-6 C
Side of the triangle r = 5 cm = 5 x 10-2 m
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Net force on C Fc = 2FAC cos 602 = 2 x 144 x cos 30 = 249 N [along the angular bisector 300]
Electric Field amp Potential
17 Four equal charges 20 times 10minus6C each are fixed at the four corners of a square of side 5 cm Find the Coulomb
force experienced by one of the charges due to the rest three
Solution
Charge on each particle q1 = q2 = q3 = q4 = q = 2 x 10-6 C
Side of the square r = 5 cm = 5 x 10-2 m
Diagonal of the square R = 2r = 5 2 x 10-2 m
Force between B amp C FBC = 1
41205871205760
11990221199023
1199032 = 1
41205871205760
1199022
1199032
Force between D amp C FDC 1
41205871205760
11990241199023
1199032 = 1
41205871205760
1199022
1199032
Resultant of the above two forces FBD = 21
41205871205760
1199022
1199032 [along the diagonal angular bisector 450]
Force between A amp C FAC = 1
41205871205760
11990211199023
1198772 = 1
41205871205760
1199022
21199032
Resultant force on C FC = FAC + FBD = 1
41205871205760
1199022
21199032 + 21
41205871205760
1199022
1199032 = 1
41205871205760
1199022
1199032 2 +1
2
FC = 9 x 109 x 4 119909 10minus12
25 119909 10minus4 2 +1
2= 275 N
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
4 Two equal charges are placed at a separation of 10 m What should be the magnitude of the charges so that the force
between them equals the weight of a 50 kg person
Solution
Let the charge on each particle be lsquoqrsquo
Separation between the charges r = 1 m
Magnitude of force between the charges F = mg = 50 x 98 = 490 N
For magnitude of charges F = 1
41205871205760
11990211199022
1199032 rArr 490 = 9 x 109 1199022
1rArr q =
490
9 119909 109 = 7
3 119909 104 = 23 x 10-4 C
5 Find the electric force between two protons separated by a distance of 1 fermi (1 fermi = 10minus15m) The protons in a
nucleus remain at a separation of this order
Solution
Charge on each proton q = 16 x 10-19 C
Separation between the protons r = 1 fm = 10-15 m
Electrostatic force between the protons F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
10minus30 = 9 x 1039 x 256 x 10-38 = 2304 N
Electric Field amp Potential
6 Two charges 20 times 10minus6C and 10 times 10minus6C are placed at a separation of 10 cm Where should a third charge be
placed such that it experiences no net force due to these charges
Solution
Charge on the first particle q1 = 2 x 10-6 C
Charge on the second particle q2 = 1 x 10-6 C
Separation between the particles r = 10 cm = 01 m
Let the third charge be placed at lsquoxrsquo from the larger charge
Force between the larger charge and test charge F1 = 1
41205871205760
1199021119902
1199032 = 9 x 109 2 119909 10minus6 (119902)
1199092 [repulsive]
Force between the smaller charge and test charge F2 = 1
41205871205760
1199022119902
1199032 = 9 x 109 1 119909 10minus6 (119902)
(10minus119909)2 [repulsive]
Given net force on test charge is zero F1 ndash F2 = 0 rArr F1 = F2 rArr 9 x 109 2 119909 10minus6 (119902)
1199092 = 9 x 109 1 119909 10minus6 (119902)
(10 minus 119909)2
2 119909 10minus6 (119902)
1199092 = 1 119909 10minus6
(10 minus 119909)2 rArr 2
1199092 = 1
(10 minus 119909)2 rArr 2
119909=
1
10 minus 119909rArr (1 + 2) x = 10 2 rArr x =
10 2
1 + 2= 59 cm
Electric Field amp Potential
7 Suppose the second charge in the previous problem is minus10 times 10minus6C Locate the position where a third charge will
not experience a net force
Solution
When one of the charges is negative null point is not possible between the two charges
Let the null point is located at lsquoxrsquo from smaller charge outside the two charges
Force on the test charge due to larger charge F1 = 1
41205871205760
1199021119902
1199032 = 9 x 109 2 119909 10minus6 (119902)
(10+119909)2 [repulsive]
Force on the test charge due to smaller charge F2 = 1
41205871205760
1199022119902
1199032 = 9 x 109 1 119909 10minus6 (119902)
(119909)2 [attractive]
Given net force on test charge is zero F1 ndash F2 = 0 rArr F1 = F2 rArr 9 x 109 2 119909 10minus6 (119902)
(10+119909)2 = 9 x 109 1 119909 10minus6 (119902)
(119909)2
2 119909 10minus6 (119902)
(10+119909)2 = 1 119909 10minus6 (119902)
(119909)2 rArr 2
(10+119909)2 = 1
(119909)2 rArr 2
10 + 119909=
1
119909rArr 2 minus 1 119909 = 10 rArr x =
10
2minus1=
10
0414= 241 cm
So the test charge is a distance of 10 + 241 = 341 cm from larger charge
Electric Field amp Potential
8 Two charged particles are placed at a distance 10 cm apart What is the minimum possible magnitude of the electric
force acting on each charge
Solution
Separation between the particles r = 1 cm
Minimum magnitude of electric charge charge on electron q = 16 x 10-19 C
Force between the two particle
F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
10minus4 = 9 x 1013 x 256 x 10-38 = 2304 x 10-25 N = 23 x 10-24 N
Electric Field amp Potential
9 Estimate the number of electrons in 100 g of water How much is the total negative charge on these electrons
Solution
Mass of water m = 100 g
Molecular weight of water M = 18 gmole
Number of molecules N = nNA = 119898
119872NA =
100
18x 6023 x 1023 = 335 x 1024
Number of electrons in each water molecule Ne = 2 + 16 = 18
Number of electrons in given sample of water n = 10 x 335 x 1024 = 335 x 1025
Total negative charge on the electrons q = ne = 335 x 1025 x 16 x 10-19 = 535 x 106
Electric Field amp Potential
10 Suppose all the electrons of 100 g water are lumped together to form a negatively charged particle and all the
nuclei are lumped together to form a positively charged particle If these two particles are placed 100 cm away from
each other find the force of attraction between them Compare it with your weight
Solution
Total negative charge available q1 = 535 x 106 C
Total positive charge available q2 = 535 x 106 C
Separation between the charges r = 10 cm = 01 m
Force of attraction between the charges F = 1
41205871205760
11990211199022
1199032
F = 9 x 109 x 535 119909 106 2
001= 9 x 1011 x 286 x 1012 = 256 x 1025 N
Electric Field amp Potential
11 Consider a gold nucleus to be a sphere of radius 69 fermi in which protons and neutrons are distributed Find the
force of repulsion between two protons situated at largest separation Why do these protons not fly apart under this
repulsion
Solution
Radius of the gold nucleus R = 69 fm = 69 x 10-15 m
Largest separation between protons r = 2R = 138 x 10-15 m
Charge on each proton q = 16 x 10-19 C
Force of repulsion between the protons F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
138 119909 10minus15 2 = 12 N
Electric Field amp Potential
12 Two insulting small spheres are rubbed against each other and placed 1 cm apart If they attract each other with a
force of 01 N how many electrons were transferred from one sphere to the other during rubbing
Solution
Let the charge acquired by each sphere q = ne
Separation between the spheres r = 1 cm = 10-2 m
Force of attraction between the spheres F = 01 N
Force of attraction between the spheres F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1198992 16 119909 10minus19 2
10minus4
01 = 9 x 1013 x 256 x 10-38 n2
n2 = 1024
2304=
100
2304x 1022 rArr n = 2 x 1011
Electric Field amp Potential
13 NaCI molecule is bound due to the electric force between the sodium and the chlorine ions when one electron of
sodium is transferred to chlorine Taking the separation between the ions to be 275 times 10minus8 cm find the force of
attraction between them State the assumptions (if any) that you have made
Solution
Separation between the ions r = 275 x 10-8 cm
Charge on sodium and chlorine ions q1 = q2 = q = 16 x 10-19 C
Force of attraction between the ions F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
275 119909 10minus10 2 = 305 x 10-9 N
Electric Field amp Potential
14 Find the ratio of the electric and gravitational forces between two protons
Solution
Mass of each proton m1 = m2 = m = 167 x 10-27 kg
Charge on each proton q1 = q2 = q = 16 x 10-19 C
Let the separation between the protons be lsquorrsquo
Electric force between the protons Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
1199032 = 2304 119909 10minus29
1199032
Gravitational force between the protons Fg = 11986611989811198982
1199032 = 667 119909 10minus11 119909 167 119909 10minus27 2
1199032 = 186 119909 10minus65
1199032
Ratio of electric and gravitational forces 119865119890
119865119892=
2304 119909 10minus29
186 119909 10minus65 = 123 x 1036
Electric Field amp Potential
15 Suppose an attractive nuclear force acts between two protons which may be written as F = Ceminuskrr2
(a) write down the dimensional formulae and appropriate SI units of C and k (b) suppose that k = 1 fermiminus1 and that
the repulsive electric force between the protons is just balanced by the attractive nuclear force when the separation is
5 fermi Find the value of C
Solution
(a) Nuclear force between protons F = Ceminuskrr2
Dimensions of exponential function M0L0T0
Dimensional formula for kr M0L0T0 rArr kL = M0L0T0 rArr k = M0L-1T0 (SI unit m-1)
Dimensional formula of force MLT-2
Dimensional formula of Cr2 = MLT-2 rArr C = MLT-2L2 = ML3T-2 (SI unit Nm2)
(b) Repulsive force between protons Fe = 1
41205871205760
1199022
1199032 = 9 119909 109 119909 16 119909 10minus19 2
5 119909 10minus15 2 = 2304
25x 101 = 92 N
Nuclear attractive force between protons Fn = C(eminuskr)r2 = C 119890
minus11 119909 5
5 119909 10minus15 2 = (Ce-5 x 1030 )25
Given Fe = Fn rArr 92 = 1030
251198905 C rArr C = 92 x 25e5 x 10-30 = 34 x 10-26 Nm2
Electric Field amp Potential
16 Three equal charges 20 times 10minus6C each are held fixed at the three corners of an equilateral triangle of side 5 cm
Find the Coulomb force experienced by one of the charges due to the rest two
Solution
Charge on each particle q1 = q2 = q3 = 2 x 10-6 C
Side of the triangle r = 5 cm = 5 x 10-2 m
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Net force on C Fc = 2FAC cos 602 = 2 x 144 x cos 30 = 249 N [along the angular bisector 300]
Electric Field amp Potential
17 Four equal charges 20 times 10minus6C each are fixed at the four corners of a square of side 5 cm Find the Coulomb
force experienced by one of the charges due to the rest three
Solution
Charge on each particle q1 = q2 = q3 = q4 = q = 2 x 10-6 C
Side of the square r = 5 cm = 5 x 10-2 m
Diagonal of the square R = 2r = 5 2 x 10-2 m
Force between B amp C FBC = 1
41205871205760
11990221199023
1199032 = 1
41205871205760
1199022
1199032
Force between D amp C FDC 1
41205871205760
11990241199023
1199032 = 1
41205871205760
1199022
1199032
Resultant of the above two forces FBD = 21
41205871205760
1199022
1199032 [along the diagonal angular bisector 450]
Force between A amp C FAC = 1
41205871205760
11990211199023
1198772 = 1
41205871205760
1199022
21199032
Resultant force on C FC = FAC + FBD = 1
41205871205760
1199022
21199032 + 21
41205871205760
1199022
1199032 = 1
41205871205760
1199022
1199032 2 +1
2
FC = 9 x 109 x 4 119909 10minus12
25 119909 10minus4 2 +1
2= 275 N
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
6 Two charges 20 times 10minus6C and 10 times 10minus6C are placed at a separation of 10 cm Where should a third charge be
placed such that it experiences no net force due to these charges
Solution
Charge on the first particle q1 = 2 x 10-6 C
Charge on the second particle q2 = 1 x 10-6 C
Separation between the particles r = 10 cm = 01 m
Let the third charge be placed at lsquoxrsquo from the larger charge
Force between the larger charge and test charge F1 = 1
41205871205760
1199021119902
1199032 = 9 x 109 2 119909 10minus6 (119902)
1199092 [repulsive]
Force between the smaller charge and test charge F2 = 1
41205871205760
1199022119902
1199032 = 9 x 109 1 119909 10minus6 (119902)
(10minus119909)2 [repulsive]
Given net force on test charge is zero F1 ndash F2 = 0 rArr F1 = F2 rArr 9 x 109 2 119909 10minus6 (119902)
1199092 = 9 x 109 1 119909 10minus6 (119902)
(10 minus 119909)2
2 119909 10minus6 (119902)
1199092 = 1 119909 10minus6
(10 minus 119909)2 rArr 2
1199092 = 1
(10 minus 119909)2 rArr 2
119909=
1
10 minus 119909rArr (1 + 2) x = 10 2 rArr x =
10 2
1 + 2= 59 cm
Electric Field amp Potential
7 Suppose the second charge in the previous problem is minus10 times 10minus6C Locate the position where a third charge will
not experience a net force
Solution
When one of the charges is negative null point is not possible between the two charges
Let the null point is located at lsquoxrsquo from smaller charge outside the two charges
Force on the test charge due to larger charge F1 = 1
41205871205760
1199021119902
1199032 = 9 x 109 2 119909 10minus6 (119902)
(10+119909)2 [repulsive]
Force on the test charge due to smaller charge F2 = 1
41205871205760
1199022119902
1199032 = 9 x 109 1 119909 10minus6 (119902)
(119909)2 [attractive]
Given net force on test charge is zero F1 ndash F2 = 0 rArr F1 = F2 rArr 9 x 109 2 119909 10minus6 (119902)
(10+119909)2 = 9 x 109 1 119909 10minus6 (119902)
(119909)2
2 119909 10minus6 (119902)
(10+119909)2 = 1 119909 10minus6 (119902)
(119909)2 rArr 2
(10+119909)2 = 1
(119909)2 rArr 2
10 + 119909=
1
119909rArr 2 minus 1 119909 = 10 rArr x =
10
2minus1=
10
0414= 241 cm
So the test charge is a distance of 10 + 241 = 341 cm from larger charge
Electric Field amp Potential
8 Two charged particles are placed at a distance 10 cm apart What is the minimum possible magnitude of the electric
force acting on each charge
Solution
Separation between the particles r = 1 cm
Minimum magnitude of electric charge charge on electron q = 16 x 10-19 C
Force between the two particle
F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
10minus4 = 9 x 1013 x 256 x 10-38 = 2304 x 10-25 N = 23 x 10-24 N
Electric Field amp Potential
9 Estimate the number of electrons in 100 g of water How much is the total negative charge on these electrons
Solution
Mass of water m = 100 g
Molecular weight of water M = 18 gmole
Number of molecules N = nNA = 119898
119872NA =
100
18x 6023 x 1023 = 335 x 1024
Number of electrons in each water molecule Ne = 2 + 16 = 18
Number of electrons in given sample of water n = 10 x 335 x 1024 = 335 x 1025
Total negative charge on the electrons q = ne = 335 x 1025 x 16 x 10-19 = 535 x 106
Electric Field amp Potential
10 Suppose all the electrons of 100 g water are lumped together to form a negatively charged particle and all the
nuclei are lumped together to form a positively charged particle If these two particles are placed 100 cm away from
each other find the force of attraction between them Compare it with your weight
Solution
Total negative charge available q1 = 535 x 106 C
Total positive charge available q2 = 535 x 106 C
Separation between the charges r = 10 cm = 01 m
Force of attraction between the charges F = 1
41205871205760
11990211199022
1199032
F = 9 x 109 x 535 119909 106 2
001= 9 x 1011 x 286 x 1012 = 256 x 1025 N
Electric Field amp Potential
11 Consider a gold nucleus to be a sphere of radius 69 fermi in which protons and neutrons are distributed Find the
force of repulsion between two protons situated at largest separation Why do these protons not fly apart under this
repulsion
Solution
Radius of the gold nucleus R = 69 fm = 69 x 10-15 m
Largest separation between protons r = 2R = 138 x 10-15 m
Charge on each proton q = 16 x 10-19 C
Force of repulsion between the protons F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
138 119909 10minus15 2 = 12 N
Electric Field amp Potential
12 Two insulting small spheres are rubbed against each other and placed 1 cm apart If they attract each other with a
force of 01 N how many electrons were transferred from one sphere to the other during rubbing
Solution
Let the charge acquired by each sphere q = ne
Separation between the spheres r = 1 cm = 10-2 m
Force of attraction between the spheres F = 01 N
Force of attraction between the spheres F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1198992 16 119909 10minus19 2
10minus4
01 = 9 x 1013 x 256 x 10-38 n2
n2 = 1024
2304=
100
2304x 1022 rArr n = 2 x 1011
Electric Field amp Potential
13 NaCI molecule is bound due to the electric force between the sodium and the chlorine ions when one electron of
sodium is transferred to chlorine Taking the separation between the ions to be 275 times 10minus8 cm find the force of
attraction between them State the assumptions (if any) that you have made
Solution
Separation between the ions r = 275 x 10-8 cm
Charge on sodium and chlorine ions q1 = q2 = q = 16 x 10-19 C
Force of attraction between the ions F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
275 119909 10minus10 2 = 305 x 10-9 N
Electric Field amp Potential
14 Find the ratio of the electric and gravitational forces between two protons
Solution
Mass of each proton m1 = m2 = m = 167 x 10-27 kg
Charge on each proton q1 = q2 = q = 16 x 10-19 C
Let the separation between the protons be lsquorrsquo
Electric force between the protons Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
1199032 = 2304 119909 10minus29
1199032
Gravitational force between the protons Fg = 11986611989811198982
1199032 = 667 119909 10minus11 119909 167 119909 10minus27 2
1199032 = 186 119909 10minus65
1199032
Ratio of electric and gravitational forces 119865119890
119865119892=
2304 119909 10minus29
186 119909 10minus65 = 123 x 1036
Electric Field amp Potential
15 Suppose an attractive nuclear force acts between two protons which may be written as F = Ceminuskrr2
(a) write down the dimensional formulae and appropriate SI units of C and k (b) suppose that k = 1 fermiminus1 and that
the repulsive electric force between the protons is just balanced by the attractive nuclear force when the separation is
5 fermi Find the value of C
Solution
(a) Nuclear force between protons F = Ceminuskrr2
Dimensions of exponential function M0L0T0
Dimensional formula for kr M0L0T0 rArr kL = M0L0T0 rArr k = M0L-1T0 (SI unit m-1)
Dimensional formula of force MLT-2
Dimensional formula of Cr2 = MLT-2 rArr C = MLT-2L2 = ML3T-2 (SI unit Nm2)
(b) Repulsive force between protons Fe = 1
41205871205760
1199022
1199032 = 9 119909 109 119909 16 119909 10minus19 2
5 119909 10minus15 2 = 2304
25x 101 = 92 N
Nuclear attractive force between protons Fn = C(eminuskr)r2 = C 119890
minus11 119909 5
5 119909 10minus15 2 = (Ce-5 x 1030 )25
Given Fe = Fn rArr 92 = 1030
251198905 C rArr C = 92 x 25e5 x 10-30 = 34 x 10-26 Nm2
Electric Field amp Potential
16 Three equal charges 20 times 10minus6C each are held fixed at the three corners of an equilateral triangle of side 5 cm
Find the Coulomb force experienced by one of the charges due to the rest two
Solution
Charge on each particle q1 = q2 = q3 = 2 x 10-6 C
Side of the triangle r = 5 cm = 5 x 10-2 m
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Net force on C Fc = 2FAC cos 602 = 2 x 144 x cos 30 = 249 N [along the angular bisector 300]
Electric Field amp Potential
17 Four equal charges 20 times 10minus6C each are fixed at the four corners of a square of side 5 cm Find the Coulomb
force experienced by one of the charges due to the rest three
Solution
Charge on each particle q1 = q2 = q3 = q4 = q = 2 x 10-6 C
Side of the square r = 5 cm = 5 x 10-2 m
Diagonal of the square R = 2r = 5 2 x 10-2 m
Force between B amp C FBC = 1
41205871205760
11990221199023
1199032 = 1
41205871205760
1199022
1199032
Force between D amp C FDC 1
41205871205760
11990241199023
1199032 = 1
41205871205760
1199022
1199032
Resultant of the above two forces FBD = 21
41205871205760
1199022
1199032 [along the diagonal angular bisector 450]
Force between A amp C FAC = 1
41205871205760
11990211199023
1198772 = 1
41205871205760
1199022
21199032
Resultant force on C FC = FAC + FBD = 1
41205871205760
1199022
21199032 + 21
41205871205760
1199022
1199032 = 1
41205871205760
1199022
1199032 2 +1
2
FC = 9 x 109 x 4 119909 10minus12
25 119909 10minus4 2 +1
2= 275 N
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
7 Suppose the second charge in the previous problem is minus10 times 10minus6C Locate the position where a third charge will
not experience a net force
Solution
When one of the charges is negative null point is not possible between the two charges
Let the null point is located at lsquoxrsquo from smaller charge outside the two charges
Force on the test charge due to larger charge F1 = 1
41205871205760
1199021119902
1199032 = 9 x 109 2 119909 10minus6 (119902)
(10+119909)2 [repulsive]
Force on the test charge due to smaller charge F2 = 1
41205871205760
1199022119902
1199032 = 9 x 109 1 119909 10minus6 (119902)
(119909)2 [attractive]
Given net force on test charge is zero F1 ndash F2 = 0 rArr F1 = F2 rArr 9 x 109 2 119909 10minus6 (119902)
(10+119909)2 = 9 x 109 1 119909 10minus6 (119902)
(119909)2
2 119909 10minus6 (119902)
(10+119909)2 = 1 119909 10minus6 (119902)
(119909)2 rArr 2
(10+119909)2 = 1
(119909)2 rArr 2
10 + 119909=
1
119909rArr 2 minus 1 119909 = 10 rArr x =
10
2minus1=
10
0414= 241 cm
So the test charge is a distance of 10 + 241 = 341 cm from larger charge
Electric Field amp Potential
8 Two charged particles are placed at a distance 10 cm apart What is the minimum possible magnitude of the electric
force acting on each charge
Solution
Separation between the particles r = 1 cm
Minimum magnitude of electric charge charge on electron q = 16 x 10-19 C
Force between the two particle
F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
10minus4 = 9 x 1013 x 256 x 10-38 = 2304 x 10-25 N = 23 x 10-24 N
Electric Field amp Potential
9 Estimate the number of electrons in 100 g of water How much is the total negative charge on these electrons
Solution
Mass of water m = 100 g
Molecular weight of water M = 18 gmole
Number of molecules N = nNA = 119898
119872NA =
100
18x 6023 x 1023 = 335 x 1024
Number of electrons in each water molecule Ne = 2 + 16 = 18
Number of electrons in given sample of water n = 10 x 335 x 1024 = 335 x 1025
Total negative charge on the electrons q = ne = 335 x 1025 x 16 x 10-19 = 535 x 106
Electric Field amp Potential
10 Suppose all the electrons of 100 g water are lumped together to form a negatively charged particle and all the
nuclei are lumped together to form a positively charged particle If these two particles are placed 100 cm away from
each other find the force of attraction between them Compare it with your weight
Solution
Total negative charge available q1 = 535 x 106 C
Total positive charge available q2 = 535 x 106 C
Separation between the charges r = 10 cm = 01 m
Force of attraction between the charges F = 1
41205871205760
11990211199022
1199032
F = 9 x 109 x 535 119909 106 2
001= 9 x 1011 x 286 x 1012 = 256 x 1025 N
Electric Field amp Potential
11 Consider a gold nucleus to be a sphere of radius 69 fermi in which protons and neutrons are distributed Find the
force of repulsion between two protons situated at largest separation Why do these protons not fly apart under this
repulsion
Solution
Radius of the gold nucleus R = 69 fm = 69 x 10-15 m
Largest separation between protons r = 2R = 138 x 10-15 m
Charge on each proton q = 16 x 10-19 C
Force of repulsion between the protons F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
138 119909 10minus15 2 = 12 N
Electric Field amp Potential
12 Two insulting small spheres are rubbed against each other and placed 1 cm apart If they attract each other with a
force of 01 N how many electrons were transferred from one sphere to the other during rubbing
Solution
Let the charge acquired by each sphere q = ne
Separation between the spheres r = 1 cm = 10-2 m
Force of attraction between the spheres F = 01 N
Force of attraction between the spheres F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1198992 16 119909 10minus19 2
10minus4
01 = 9 x 1013 x 256 x 10-38 n2
n2 = 1024
2304=
100
2304x 1022 rArr n = 2 x 1011
Electric Field amp Potential
13 NaCI molecule is bound due to the electric force between the sodium and the chlorine ions when one electron of
sodium is transferred to chlorine Taking the separation between the ions to be 275 times 10minus8 cm find the force of
attraction between them State the assumptions (if any) that you have made
Solution
Separation between the ions r = 275 x 10-8 cm
Charge on sodium and chlorine ions q1 = q2 = q = 16 x 10-19 C
Force of attraction between the ions F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
275 119909 10minus10 2 = 305 x 10-9 N
Electric Field amp Potential
14 Find the ratio of the electric and gravitational forces between two protons
Solution
Mass of each proton m1 = m2 = m = 167 x 10-27 kg
Charge on each proton q1 = q2 = q = 16 x 10-19 C
Let the separation between the protons be lsquorrsquo
Electric force between the protons Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
1199032 = 2304 119909 10minus29
1199032
Gravitational force between the protons Fg = 11986611989811198982
1199032 = 667 119909 10minus11 119909 167 119909 10minus27 2
1199032 = 186 119909 10minus65
1199032
Ratio of electric and gravitational forces 119865119890
119865119892=
2304 119909 10minus29
186 119909 10minus65 = 123 x 1036
Electric Field amp Potential
15 Suppose an attractive nuclear force acts between two protons which may be written as F = Ceminuskrr2
(a) write down the dimensional formulae and appropriate SI units of C and k (b) suppose that k = 1 fermiminus1 and that
the repulsive electric force between the protons is just balanced by the attractive nuclear force when the separation is
5 fermi Find the value of C
Solution
(a) Nuclear force between protons F = Ceminuskrr2
Dimensions of exponential function M0L0T0
Dimensional formula for kr M0L0T0 rArr kL = M0L0T0 rArr k = M0L-1T0 (SI unit m-1)
Dimensional formula of force MLT-2
Dimensional formula of Cr2 = MLT-2 rArr C = MLT-2L2 = ML3T-2 (SI unit Nm2)
(b) Repulsive force between protons Fe = 1
41205871205760
1199022
1199032 = 9 119909 109 119909 16 119909 10minus19 2
5 119909 10minus15 2 = 2304
25x 101 = 92 N
Nuclear attractive force between protons Fn = C(eminuskr)r2 = C 119890
minus11 119909 5
5 119909 10minus15 2 = (Ce-5 x 1030 )25
Given Fe = Fn rArr 92 = 1030
251198905 C rArr C = 92 x 25e5 x 10-30 = 34 x 10-26 Nm2
Electric Field amp Potential
16 Three equal charges 20 times 10minus6C each are held fixed at the three corners of an equilateral triangle of side 5 cm
Find the Coulomb force experienced by one of the charges due to the rest two
Solution
Charge on each particle q1 = q2 = q3 = 2 x 10-6 C
Side of the triangle r = 5 cm = 5 x 10-2 m
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Net force on C Fc = 2FAC cos 602 = 2 x 144 x cos 30 = 249 N [along the angular bisector 300]
Electric Field amp Potential
17 Four equal charges 20 times 10minus6C each are fixed at the four corners of a square of side 5 cm Find the Coulomb
force experienced by one of the charges due to the rest three
Solution
Charge on each particle q1 = q2 = q3 = q4 = q = 2 x 10-6 C
Side of the square r = 5 cm = 5 x 10-2 m
Diagonal of the square R = 2r = 5 2 x 10-2 m
Force between B amp C FBC = 1
41205871205760
11990221199023
1199032 = 1
41205871205760
1199022
1199032
Force between D amp C FDC 1
41205871205760
11990241199023
1199032 = 1
41205871205760
1199022
1199032
Resultant of the above two forces FBD = 21
41205871205760
1199022
1199032 [along the diagonal angular bisector 450]
Force between A amp C FAC = 1
41205871205760
11990211199023
1198772 = 1
41205871205760
1199022
21199032
Resultant force on C FC = FAC + FBD = 1
41205871205760
1199022
21199032 + 21
41205871205760
1199022
1199032 = 1
41205871205760
1199022
1199032 2 +1
2
FC = 9 x 109 x 4 119909 10minus12
25 119909 10minus4 2 +1
2= 275 N
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
8 Two charged particles are placed at a distance 10 cm apart What is the minimum possible magnitude of the electric
force acting on each charge
Solution
Separation between the particles r = 1 cm
Minimum magnitude of electric charge charge on electron q = 16 x 10-19 C
Force between the two particle
F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
10minus4 = 9 x 1013 x 256 x 10-38 = 2304 x 10-25 N = 23 x 10-24 N
Electric Field amp Potential
9 Estimate the number of electrons in 100 g of water How much is the total negative charge on these electrons
Solution
Mass of water m = 100 g
Molecular weight of water M = 18 gmole
Number of molecules N = nNA = 119898
119872NA =
100
18x 6023 x 1023 = 335 x 1024
Number of electrons in each water molecule Ne = 2 + 16 = 18
Number of electrons in given sample of water n = 10 x 335 x 1024 = 335 x 1025
Total negative charge on the electrons q = ne = 335 x 1025 x 16 x 10-19 = 535 x 106
Electric Field amp Potential
10 Suppose all the electrons of 100 g water are lumped together to form a negatively charged particle and all the
nuclei are lumped together to form a positively charged particle If these two particles are placed 100 cm away from
each other find the force of attraction between them Compare it with your weight
Solution
Total negative charge available q1 = 535 x 106 C
Total positive charge available q2 = 535 x 106 C
Separation between the charges r = 10 cm = 01 m
Force of attraction between the charges F = 1
41205871205760
11990211199022
1199032
F = 9 x 109 x 535 119909 106 2
001= 9 x 1011 x 286 x 1012 = 256 x 1025 N
Electric Field amp Potential
11 Consider a gold nucleus to be a sphere of radius 69 fermi in which protons and neutrons are distributed Find the
force of repulsion between two protons situated at largest separation Why do these protons not fly apart under this
repulsion
Solution
Radius of the gold nucleus R = 69 fm = 69 x 10-15 m
Largest separation between protons r = 2R = 138 x 10-15 m
Charge on each proton q = 16 x 10-19 C
Force of repulsion between the protons F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
138 119909 10minus15 2 = 12 N
Electric Field amp Potential
12 Two insulting small spheres are rubbed against each other and placed 1 cm apart If they attract each other with a
force of 01 N how many electrons were transferred from one sphere to the other during rubbing
Solution
Let the charge acquired by each sphere q = ne
Separation between the spheres r = 1 cm = 10-2 m
Force of attraction between the spheres F = 01 N
Force of attraction between the spheres F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1198992 16 119909 10minus19 2
10minus4
01 = 9 x 1013 x 256 x 10-38 n2
n2 = 1024
2304=
100
2304x 1022 rArr n = 2 x 1011
Electric Field amp Potential
13 NaCI molecule is bound due to the electric force between the sodium and the chlorine ions when one electron of
sodium is transferred to chlorine Taking the separation between the ions to be 275 times 10minus8 cm find the force of
attraction between them State the assumptions (if any) that you have made
Solution
Separation between the ions r = 275 x 10-8 cm
Charge on sodium and chlorine ions q1 = q2 = q = 16 x 10-19 C
Force of attraction between the ions F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
275 119909 10minus10 2 = 305 x 10-9 N
Electric Field amp Potential
14 Find the ratio of the electric and gravitational forces between two protons
Solution
Mass of each proton m1 = m2 = m = 167 x 10-27 kg
Charge on each proton q1 = q2 = q = 16 x 10-19 C
Let the separation between the protons be lsquorrsquo
Electric force between the protons Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
1199032 = 2304 119909 10minus29
1199032
Gravitational force between the protons Fg = 11986611989811198982
1199032 = 667 119909 10minus11 119909 167 119909 10minus27 2
1199032 = 186 119909 10minus65
1199032
Ratio of electric and gravitational forces 119865119890
119865119892=
2304 119909 10minus29
186 119909 10minus65 = 123 x 1036
Electric Field amp Potential
15 Suppose an attractive nuclear force acts between two protons which may be written as F = Ceminuskrr2
(a) write down the dimensional formulae and appropriate SI units of C and k (b) suppose that k = 1 fermiminus1 and that
the repulsive electric force between the protons is just balanced by the attractive nuclear force when the separation is
5 fermi Find the value of C
Solution
(a) Nuclear force between protons F = Ceminuskrr2
Dimensions of exponential function M0L0T0
Dimensional formula for kr M0L0T0 rArr kL = M0L0T0 rArr k = M0L-1T0 (SI unit m-1)
Dimensional formula of force MLT-2
Dimensional formula of Cr2 = MLT-2 rArr C = MLT-2L2 = ML3T-2 (SI unit Nm2)
(b) Repulsive force between protons Fe = 1
41205871205760
1199022
1199032 = 9 119909 109 119909 16 119909 10minus19 2
5 119909 10minus15 2 = 2304
25x 101 = 92 N
Nuclear attractive force between protons Fn = C(eminuskr)r2 = C 119890
minus11 119909 5
5 119909 10minus15 2 = (Ce-5 x 1030 )25
Given Fe = Fn rArr 92 = 1030
251198905 C rArr C = 92 x 25e5 x 10-30 = 34 x 10-26 Nm2
Electric Field amp Potential
16 Three equal charges 20 times 10minus6C each are held fixed at the three corners of an equilateral triangle of side 5 cm
Find the Coulomb force experienced by one of the charges due to the rest two
Solution
Charge on each particle q1 = q2 = q3 = 2 x 10-6 C
Side of the triangle r = 5 cm = 5 x 10-2 m
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Net force on C Fc = 2FAC cos 602 = 2 x 144 x cos 30 = 249 N [along the angular bisector 300]
Electric Field amp Potential
17 Four equal charges 20 times 10minus6C each are fixed at the four corners of a square of side 5 cm Find the Coulomb
force experienced by one of the charges due to the rest three
Solution
Charge on each particle q1 = q2 = q3 = q4 = q = 2 x 10-6 C
Side of the square r = 5 cm = 5 x 10-2 m
Diagonal of the square R = 2r = 5 2 x 10-2 m
Force between B amp C FBC = 1
41205871205760
11990221199023
1199032 = 1
41205871205760
1199022
1199032
Force between D amp C FDC 1
41205871205760
11990241199023
1199032 = 1
41205871205760
1199022
1199032
Resultant of the above two forces FBD = 21
41205871205760
1199022
1199032 [along the diagonal angular bisector 450]
Force between A amp C FAC = 1
41205871205760
11990211199023
1198772 = 1
41205871205760
1199022
21199032
Resultant force on C FC = FAC + FBD = 1
41205871205760
1199022
21199032 + 21
41205871205760
1199022
1199032 = 1
41205871205760
1199022
1199032 2 +1
2
FC = 9 x 109 x 4 119909 10minus12
25 119909 10minus4 2 +1
2= 275 N
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
9 Estimate the number of electrons in 100 g of water How much is the total negative charge on these electrons
Solution
Mass of water m = 100 g
Molecular weight of water M = 18 gmole
Number of molecules N = nNA = 119898
119872NA =
100
18x 6023 x 1023 = 335 x 1024
Number of electrons in each water molecule Ne = 2 + 16 = 18
Number of electrons in given sample of water n = 10 x 335 x 1024 = 335 x 1025
Total negative charge on the electrons q = ne = 335 x 1025 x 16 x 10-19 = 535 x 106
Electric Field amp Potential
10 Suppose all the electrons of 100 g water are lumped together to form a negatively charged particle and all the
nuclei are lumped together to form a positively charged particle If these two particles are placed 100 cm away from
each other find the force of attraction between them Compare it with your weight
Solution
Total negative charge available q1 = 535 x 106 C
Total positive charge available q2 = 535 x 106 C
Separation between the charges r = 10 cm = 01 m
Force of attraction between the charges F = 1
41205871205760
11990211199022
1199032
F = 9 x 109 x 535 119909 106 2
001= 9 x 1011 x 286 x 1012 = 256 x 1025 N
Electric Field amp Potential
11 Consider a gold nucleus to be a sphere of radius 69 fermi in which protons and neutrons are distributed Find the
force of repulsion between two protons situated at largest separation Why do these protons not fly apart under this
repulsion
Solution
Radius of the gold nucleus R = 69 fm = 69 x 10-15 m
Largest separation between protons r = 2R = 138 x 10-15 m
Charge on each proton q = 16 x 10-19 C
Force of repulsion between the protons F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
138 119909 10minus15 2 = 12 N
Electric Field amp Potential
12 Two insulting small spheres are rubbed against each other and placed 1 cm apart If they attract each other with a
force of 01 N how many electrons were transferred from one sphere to the other during rubbing
Solution
Let the charge acquired by each sphere q = ne
Separation between the spheres r = 1 cm = 10-2 m
Force of attraction between the spheres F = 01 N
Force of attraction between the spheres F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1198992 16 119909 10minus19 2
10minus4
01 = 9 x 1013 x 256 x 10-38 n2
n2 = 1024
2304=
100
2304x 1022 rArr n = 2 x 1011
Electric Field amp Potential
13 NaCI molecule is bound due to the electric force between the sodium and the chlorine ions when one electron of
sodium is transferred to chlorine Taking the separation between the ions to be 275 times 10minus8 cm find the force of
attraction between them State the assumptions (if any) that you have made
Solution
Separation between the ions r = 275 x 10-8 cm
Charge on sodium and chlorine ions q1 = q2 = q = 16 x 10-19 C
Force of attraction between the ions F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
275 119909 10minus10 2 = 305 x 10-9 N
Electric Field amp Potential
14 Find the ratio of the electric and gravitational forces between two protons
Solution
Mass of each proton m1 = m2 = m = 167 x 10-27 kg
Charge on each proton q1 = q2 = q = 16 x 10-19 C
Let the separation between the protons be lsquorrsquo
Electric force between the protons Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
1199032 = 2304 119909 10minus29
1199032
Gravitational force between the protons Fg = 11986611989811198982
1199032 = 667 119909 10minus11 119909 167 119909 10minus27 2
1199032 = 186 119909 10minus65
1199032
Ratio of electric and gravitational forces 119865119890
119865119892=
2304 119909 10minus29
186 119909 10minus65 = 123 x 1036
Electric Field amp Potential
15 Suppose an attractive nuclear force acts between two protons which may be written as F = Ceminuskrr2
(a) write down the dimensional formulae and appropriate SI units of C and k (b) suppose that k = 1 fermiminus1 and that
the repulsive electric force between the protons is just balanced by the attractive nuclear force when the separation is
5 fermi Find the value of C
Solution
(a) Nuclear force between protons F = Ceminuskrr2
Dimensions of exponential function M0L0T0
Dimensional formula for kr M0L0T0 rArr kL = M0L0T0 rArr k = M0L-1T0 (SI unit m-1)
Dimensional formula of force MLT-2
Dimensional formula of Cr2 = MLT-2 rArr C = MLT-2L2 = ML3T-2 (SI unit Nm2)
(b) Repulsive force between protons Fe = 1
41205871205760
1199022
1199032 = 9 119909 109 119909 16 119909 10minus19 2
5 119909 10minus15 2 = 2304
25x 101 = 92 N
Nuclear attractive force between protons Fn = C(eminuskr)r2 = C 119890
minus11 119909 5
5 119909 10minus15 2 = (Ce-5 x 1030 )25
Given Fe = Fn rArr 92 = 1030
251198905 C rArr C = 92 x 25e5 x 10-30 = 34 x 10-26 Nm2
Electric Field amp Potential
16 Three equal charges 20 times 10minus6C each are held fixed at the three corners of an equilateral triangle of side 5 cm
Find the Coulomb force experienced by one of the charges due to the rest two
Solution
Charge on each particle q1 = q2 = q3 = 2 x 10-6 C
Side of the triangle r = 5 cm = 5 x 10-2 m
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Net force on C Fc = 2FAC cos 602 = 2 x 144 x cos 30 = 249 N [along the angular bisector 300]
Electric Field amp Potential
17 Four equal charges 20 times 10minus6C each are fixed at the four corners of a square of side 5 cm Find the Coulomb
force experienced by one of the charges due to the rest three
Solution
Charge on each particle q1 = q2 = q3 = q4 = q = 2 x 10-6 C
Side of the square r = 5 cm = 5 x 10-2 m
Diagonal of the square R = 2r = 5 2 x 10-2 m
Force between B amp C FBC = 1
41205871205760
11990221199023
1199032 = 1
41205871205760
1199022
1199032
Force between D amp C FDC 1
41205871205760
11990241199023
1199032 = 1
41205871205760
1199022
1199032
Resultant of the above two forces FBD = 21
41205871205760
1199022
1199032 [along the diagonal angular bisector 450]
Force between A amp C FAC = 1
41205871205760
11990211199023
1198772 = 1
41205871205760
1199022
21199032
Resultant force on C FC = FAC + FBD = 1
41205871205760
1199022
21199032 + 21
41205871205760
1199022
1199032 = 1
41205871205760
1199022
1199032 2 +1
2
FC = 9 x 109 x 4 119909 10minus12
25 119909 10minus4 2 +1
2= 275 N
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
10 Suppose all the electrons of 100 g water are lumped together to form a negatively charged particle and all the
nuclei are lumped together to form a positively charged particle If these two particles are placed 100 cm away from
each other find the force of attraction between them Compare it with your weight
Solution
Total negative charge available q1 = 535 x 106 C
Total positive charge available q2 = 535 x 106 C
Separation between the charges r = 10 cm = 01 m
Force of attraction between the charges F = 1
41205871205760
11990211199022
1199032
F = 9 x 109 x 535 119909 106 2
001= 9 x 1011 x 286 x 1012 = 256 x 1025 N
Electric Field amp Potential
11 Consider a gold nucleus to be a sphere of radius 69 fermi in which protons and neutrons are distributed Find the
force of repulsion between two protons situated at largest separation Why do these protons not fly apart under this
repulsion
Solution
Radius of the gold nucleus R = 69 fm = 69 x 10-15 m
Largest separation between protons r = 2R = 138 x 10-15 m
Charge on each proton q = 16 x 10-19 C
Force of repulsion between the protons F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
138 119909 10minus15 2 = 12 N
Electric Field amp Potential
12 Two insulting small spheres are rubbed against each other and placed 1 cm apart If they attract each other with a
force of 01 N how many electrons were transferred from one sphere to the other during rubbing
Solution
Let the charge acquired by each sphere q = ne
Separation between the spheres r = 1 cm = 10-2 m
Force of attraction between the spheres F = 01 N
Force of attraction between the spheres F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1198992 16 119909 10minus19 2
10minus4
01 = 9 x 1013 x 256 x 10-38 n2
n2 = 1024
2304=
100
2304x 1022 rArr n = 2 x 1011
Electric Field amp Potential
13 NaCI molecule is bound due to the electric force between the sodium and the chlorine ions when one electron of
sodium is transferred to chlorine Taking the separation between the ions to be 275 times 10minus8 cm find the force of
attraction between them State the assumptions (if any) that you have made
Solution
Separation between the ions r = 275 x 10-8 cm
Charge on sodium and chlorine ions q1 = q2 = q = 16 x 10-19 C
Force of attraction between the ions F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
275 119909 10minus10 2 = 305 x 10-9 N
Electric Field amp Potential
14 Find the ratio of the electric and gravitational forces between two protons
Solution
Mass of each proton m1 = m2 = m = 167 x 10-27 kg
Charge on each proton q1 = q2 = q = 16 x 10-19 C
Let the separation between the protons be lsquorrsquo
Electric force between the protons Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
1199032 = 2304 119909 10minus29
1199032
Gravitational force between the protons Fg = 11986611989811198982
1199032 = 667 119909 10minus11 119909 167 119909 10minus27 2
1199032 = 186 119909 10minus65
1199032
Ratio of electric and gravitational forces 119865119890
119865119892=
2304 119909 10minus29
186 119909 10minus65 = 123 x 1036
Electric Field amp Potential
15 Suppose an attractive nuclear force acts between two protons which may be written as F = Ceminuskrr2
(a) write down the dimensional formulae and appropriate SI units of C and k (b) suppose that k = 1 fermiminus1 and that
the repulsive electric force between the protons is just balanced by the attractive nuclear force when the separation is
5 fermi Find the value of C
Solution
(a) Nuclear force between protons F = Ceminuskrr2
Dimensions of exponential function M0L0T0
Dimensional formula for kr M0L0T0 rArr kL = M0L0T0 rArr k = M0L-1T0 (SI unit m-1)
Dimensional formula of force MLT-2
Dimensional formula of Cr2 = MLT-2 rArr C = MLT-2L2 = ML3T-2 (SI unit Nm2)
(b) Repulsive force between protons Fe = 1
41205871205760
1199022
1199032 = 9 119909 109 119909 16 119909 10minus19 2
5 119909 10minus15 2 = 2304
25x 101 = 92 N
Nuclear attractive force between protons Fn = C(eminuskr)r2 = C 119890
minus11 119909 5
5 119909 10minus15 2 = (Ce-5 x 1030 )25
Given Fe = Fn rArr 92 = 1030
251198905 C rArr C = 92 x 25e5 x 10-30 = 34 x 10-26 Nm2
Electric Field amp Potential
16 Three equal charges 20 times 10minus6C each are held fixed at the three corners of an equilateral triangle of side 5 cm
Find the Coulomb force experienced by one of the charges due to the rest two
Solution
Charge on each particle q1 = q2 = q3 = 2 x 10-6 C
Side of the triangle r = 5 cm = 5 x 10-2 m
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Net force on C Fc = 2FAC cos 602 = 2 x 144 x cos 30 = 249 N [along the angular bisector 300]
Electric Field amp Potential
17 Four equal charges 20 times 10minus6C each are fixed at the four corners of a square of side 5 cm Find the Coulomb
force experienced by one of the charges due to the rest three
Solution
Charge on each particle q1 = q2 = q3 = q4 = q = 2 x 10-6 C
Side of the square r = 5 cm = 5 x 10-2 m
Diagonal of the square R = 2r = 5 2 x 10-2 m
Force between B amp C FBC = 1
41205871205760
11990221199023
1199032 = 1
41205871205760
1199022
1199032
Force between D amp C FDC 1
41205871205760
11990241199023
1199032 = 1
41205871205760
1199022
1199032
Resultant of the above two forces FBD = 21
41205871205760
1199022
1199032 [along the diagonal angular bisector 450]
Force between A amp C FAC = 1
41205871205760
11990211199023
1198772 = 1
41205871205760
1199022
21199032
Resultant force on C FC = FAC + FBD = 1
41205871205760
1199022
21199032 + 21
41205871205760
1199022
1199032 = 1
41205871205760
1199022
1199032 2 +1
2
FC = 9 x 109 x 4 119909 10minus12
25 119909 10minus4 2 +1
2= 275 N
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
11 Consider a gold nucleus to be a sphere of radius 69 fermi in which protons and neutrons are distributed Find the
force of repulsion between two protons situated at largest separation Why do these protons not fly apart under this
repulsion
Solution
Radius of the gold nucleus R = 69 fm = 69 x 10-15 m
Largest separation between protons r = 2R = 138 x 10-15 m
Charge on each proton q = 16 x 10-19 C
Force of repulsion between the protons F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
138 119909 10minus15 2 = 12 N
Electric Field amp Potential
12 Two insulting small spheres are rubbed against each other and placed 1 cm apart If they attract each other with a
force of 01 N how many electrons were transferred from one sphere to the other during rubbing
Solution
Let the charge acquired by each sphere q = ne
Separation between the spheres r = 1 cm = 10-2 m
Force of attraction between the spheres F = 01 N
Force of attraction between the spheres F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1198992 16 119909 10minus19 2
10minus4
01 = 9 x 1013 x 256 x 10-38 n2
n2 = 1024
2304=
100
2304x 1022 rArr n = 2 x 1011
Electric Field amp Potential
13 NaCI molecule is bound due to the electric force between the sodium and the chlorine ions when one electron of
sodium is transferred to chlorine Taking the separation between the ions to be 275 times 10minus8 cm find the force of
attraction between them State the assumptions (if any) that you have made
Solution
Separation between the ions r = 275 x 10-8 cm
Charge on sodium and chlorine ions q1 = q2 = q = 16 x 10-19 C
Force of attraction between the ions F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
275 119909 10minus10 2 = 305 x 10-9 N
Electric Field amp Potential
14 Find the ratio of the electric and gravitational forces between two protons
Solution
Mass of each proton m1 = m2 = m = 167 x 10-27 kg
Charge on each proton q1 = q2 = q = 16 x 10-19 C
Let the separation between the protons be lsquorrsquo
Electric force between the protons Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
1199032 = 2304 119909 10minus29
1199032
Gravitational force between the protons Fg = 11986611989811198982
1199032 = 667 119909 10minus11 119909 167 119909 10minus27 2
1199032 = 186 119909 10minus65
1199032
Ratio of electric and gravitational forces 119865119890
119865119892=
2304 119909 10minus29
186 119909 10minus65 = 123 x 1036
Electric Field amp Potential
15 Suppose an attractive nuclear force acts between two protons which may be written as F = Ceminuskrr2
(a) write down the dimensional formulae and appropriate SI units of C and k (b) suppose that k = 1 fermiminus1 and that
the repulsive electric force between the protons is just balanced by the attractive nuclear force when the separation is
5 fermi Find the value of C
Solution
(a) Nuclear force between protons F = Ceminuskrr2
Dimensions of exponential function M0L0T0
Dimensional formula for kr M0L0T0 rArr kL = M0L0T0 rArr k = M0L-1T0 (SI unit m-1)
Dimensional formula of force MLT-2
Dimensional formula of Cr2 = MLT-2 rArr C = MLT-2L2 = ML3T-2 (SI unit Nm2)
(b) Repulsive force between protons Fe = 1
41205871205760
1199022
1199032 = 9 119909 109 119909 16 119909 10minus19 2
5 119909 10minus15 2 = 2304
25x 101 = 92 N
Nuclear attractive force between protons Fn = C(eminuskr)r2 = C 119890
minus11 119909 5
5 119909 10minus15 2 = (Ce-5 x 1030 )25
Given Fe = Fn rArr 92 = 1030
251198905 C rArr C = 92 x 25e5 x 10-30 = 34 x 10-26 Nm2
Electric Field amp Potential
16 Three equal charges 20 times 10minus6C each are held fixed at the three corners of an equilateral triangle of side 5 cm
Find the Coulomb force experienced by one of the charges due to the rest two
Solution
Charge on each particle q1 = q2 = q3 = 2 x 10-6 C
Side of the triangle r = 5 cm = 5 x 10-2 m
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Net force on C Fc = 2FAC cos 602 = 2 x 144 x cos 30 = 249 N [along the angular bisector 300]
Electric Field amp Potential
17 Four equal charges 20 times 10minus6C each are fixed at the four corners of a square of side 5 cm Find the Coulomb
force experienced by one of the charges due to the rest three
Solution
Charge on each particle q1 = q2 = q3 = q4 = q = 2 x 10-6 C
Side of the square r = 5 cm = 5 x 10-2 m
Diagonal of the square R = 2r = 5 2 x 10-2 m
Force between B amp C FBC = 1
41205871205760
11990221199023
1199032 = 1
41205871205760
1199022
1199032
Force between D amp C FDC 1
41205871205760
11990241199023
1199032 = 1
41205871205760
1199022
1199032
Resultant of the above two forces FBD = 21
41205871205760
1199022
1199032 [along the diagonal angular bisector 450]
Force between A amp C FAC = 1
41205871205760
11990211199023
1198772 = 1
41205871205760
1199022
21199032
Resultant force on C FC = FAC + FBD = 1
41205871205760
1199022
21199032 + 21
41205871205760
1199022
1199032 = 1
41205871205760
1199022
1199032 2 +1
2
FC = 9 x 109 x 4 119909 10minus12
25 119909 10minus4 2 +1
2= 275 N
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
12 Two insulting small spheres are rubbed against each other and placed 1 cm apart If they attract each other with a
force of 01 N how many electrons were transferred from one sphere to the other during rubbing
Solution
Let the charge acquired by each sphere q = ne
Separation between the spheres r = 1 cm = 10-2 m
Force of attraction between the spheres F = 01 N
Force of attraction between the spheres F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1198992 16 119909 10minus19 2
10minus4
01 = 9 x 1013 x 256 x 10-38 n2
n2 = 1024
2304=
100
2304x 1022 rArr n = 2 x 1011
Electric Field amp Potential
13 NaCI molecule is bound due to the electric force between the sodium and the chlorine ions when one electron of
sodium is transferred to chlorine Taking the separation between the ions to be 275 times 10minus8 cm find the force of
attraction between them State the assumptions (if any) that you have made
Solution
Separation between the ions r = 275 x 10-8 cm
Charge on sodium and chlorine ions q1 = q2 = q = 16 x 10-19 C
Force of attraction between the ions F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
275 119909 10minus10 2 = 305 x 10-9 N
Electric Field amp Potential
14 Find the ratio of the electric and gravitational forces between two protons
Solution
Mass of each proton m1 = m2 = m = 167 x 10-27 kg
Charge on each proton q1 = q2 = q = 16 x 10-19 C
Let the separation between the protons be lsquorrsquo
Electric force between the protons Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
1199032 = 2304 119909 10minus29
1199032
Gravitational force between the protons Fg = 11986611989811198982
1199032 = 667 119909 10minus11 119909 167 119909 10minus27 2
1199032 = 186 119909 10minus65
1199032
Ratio of electric and gravitational forces 119865119890
119865119892=
2304 119909 10minus29
186 119909 10minus65 = 123 x 1036
Electric Field amp Potential
15 Suppose an attractive nuclear force acts between two protons which may be written as F = Ceminuskrr2
(a) write down the dimensional formulae and appropriate SI units of C and k (b) suppose that k = 1 fermiminus1 and that
the repulsive electric force between the protons is just balanced by the attractive nuclear force when the separation is
5 fermi Find the value of C
Solution
(a) Nuclear force between protons F = Ceminuskrr2
Dimensions of exponential function M0L0T0
Dimensional formula for kr M0L0T0 rArr kL = M0L0T0 rArr k = M0L-1T0 (SI unit m-1)
Dimensional formula of force MLT-2
Dimensional formula of Cr2 = MLT-2 rArr C = MLT-2L2 = ML3T-2 (SI unit Nm2)
(b) Repulsive force between protons Fe = 1
41205871205760
1199022
1199032 = 9 119909 109 119909 16 119909 10minus19 2
5 119909 10minus15 2 = 2304
25x 101 = 92 N
Nuclear attractive force between protons Fn = C(eminuskr)r2 = C 119890
minus11 119909 5
5 119909 10minus15 2 = (Ce-5 x 1030 )25
Given Fe = Fn rArr 92 = 1030
251198905 C rArr C = 92 x 25e5 x 10-30 = 34 x 10-26 Nm2
Electric Field amp Potential
16 Three equal charges 20 times 10minus6C each are held fixed at the three corners of an equilateral triangle of side 5 cm
Find the Coulomb force experienced by one of the charges due to the rest two
Solution
Charge on each particle q1 = q2 = q3 = 2 x 10-6 C
Side of the triangle r = 5 cm = 5 x 10-2 m
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Net force on C Fc = 2FAC cos 602 = 2 x 144 x cos 30 = 249 N [along the angular bisector 300]
Electric Field amp Potential
17 Four equal charges 20 times 10minus6C each are fixed at the four corners of a square of side 5 cm Find the Coulomb
force experienced by one of the charges due to the rest three
Solution
Charge on each particle q1 = q2 = q3 = q4 = q = 2 x 10-6 C
Side of the square r = 5 cm = 5 x 10-2 m
Diagonal of the square R = 2r = 5 2 x 10-2 m
Force between B amp C FBC = 1
41205871205760
11990221199023
1199032 = 1
41205871205760
1199022
1199032
Force between D amp C FDC 1
41205871205760
11990241199023
1199032 = 1
41205871205760
1199022
1199032
Resultant of the above two forces FBD = 21
41205871205760
1199022
1199032 [along the diagonal angular bisector 450]
Force between A amp C FAC = 1
41205871205760
11990211199023
1198772 = 1
41205871205760
1199022
21199032
Resultant force on C FC = FAC + FBD = 1
41205871205760
1199022
21199032 + 21
41205871205760
1199022
1199032 = 1
41205871205760
1199022
1199032 2 +1
2
FC = 9 x 109 x 4 119909 10minus12
25 119909 10minus4 2 +1
2= 275 N
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
13 NaCI molecule is bound due to the electric force between the sodium and the chlorine ions when one electron of
sodium is transferred to chlorine Taking the separation between the ions to be 275 times 10minus8 cm find the force of
attraction between them State the assumptions (if any) that you have made
Solution
Separation between the ions r = 275 x 10-8 cm
Charge on sodium and chlorine ions q1 = q2 = q = 16 x 10-19 C
Force of attraction between the ions F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
275 119909 10minus10 2 = 305 x 10-9 N
Electric Field amp Potential
14 Find the ratio of the electric and gravitational forces between two protons
Solution
Mass of each proton m1 = m2 = m = 167 x 10-27 kg
Charge on each proton q1 = q2 = q = 16 x 10-19 C
Let the separation between the protons be lsquorrsquo
Electric force between the protons Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
1199032 = 2304 119909 10minus29
1199032
Gravitational force between the protons Fg = 11986611989811198982
1199032 = 667 119909 10minus11 119909 167 119909 10minus27 2
1199032 = 186 119909 10minus65
1199032
Ratio of electric and gravitational forces 119865119890
119865119892=
2304 119909 10minus29
186 119909 10minus65 = 123 x 1036
Electric Field amp Potential
15 Suppose an attractive nuclear force acts between two protons which may be written as F = Ceminuskrr2
(a) write down the dimensional formulae and appropriate SI units of C and k (b) suppose that k = 1 fermiminus1 and that
the repulsive electric force between the protons is just balanced by the attractive nuclear force when the separation is
5 fermi Find the value of C
Solution
(a) Nuclear force between protons F = Ceminuskrr2
Dimensions of exponential function M0L0T0
Dimensional formula for kr M0L0T0 rArr kL = M0L0T0 rArr k = M0L-1T0 (SI unit m-1)
Dimensional formula of force MLT-2
Dimensional formula of Cr2 = MLT-2 rArr C = MLT-2L2 = ML3T-2 (SI unit Nm2)
(b) Repulsive force between protons Fe = 1
41205871205760
1199022
1199032 = 9 119909 109 119909 16 119909 10minus19 2
5 119909 10minus15 2 = 2304
25x 101 = 92 N
Nuclear attractive force between protons Fn = C(eminuskr)r2 = C 119890
minus11 119909 5
5 119909 10minus15 2 = (Ce-5 x 1030 )25
Given Fe = Fn rArr 92 = 1030
251198905 C rArr C = 92 x 25e5 x 10-30 = 34 x 10-26 Nm2
Electric Field amp Potential
16 Three equal charges 20 times 10minus6C each are held fixed at the three corners of an equilateral triangle of side 5 cm
Find the Coulomb force experienced by one of the charges due to the rest two
Solution
Charge on each particle q1 = q2 = q3 = 2 x 10-6 C
Side of the triangle r = 5 cm = 5 x 10-2 m
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Net force on C Fc = 2FAC cos 602 = 2 x 144 x cos 30 = 249 N [along the angular bisector 300]
Electric Field amp Potential
17 Four equal charges 20 times 10minus6C each are fixed at the four corners of a square of side 5 cm Find the Coulomb
force experienced by one of the charges due to the rest three
Solution
Charge on each particle q1 = q2 = q3 = q4 = q = 2 x 10-6 C
Side of the square r = 5 cm = 5 x 10-2 m
Diagonal of the square R = 2r = 5 2 x 10-2 m
Force between B amp C FBC = 1
41205871205760
11990221199023
1199032 = 1
41205871205760
1199022
1199032
Force between D amp C FDC 1
41205871205760
11990241199023
1199032 = 1
41205871205760
1199022
1199032
Resultant of the above two forces FBD = 21
41205871205760
1199022
1199032 [along the diagonal angular bisector 450]
Force between A amp C FAC = 1
41205871205760
11990211199023
1198772 = 1
41205871205760
1199022
21199032
Resultant force on C FC = FAC + FBD = 1
41205871205760
1199022
21199032 + 21
41205871205760
1199022
1199032 = 1
41205871205760
1199022
1199032 2 +1
2
FC = 9 x 109 x 4 119909 10minus12
25 119909 10minus4 2 +1
2= 275 N
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
14 Find the ratio of the electric and gravitational forces between two protons
Solution
Mass of each proton m1 = m2 = m = 167 x 10-27 kg
Charge on each proton q1 = q2 = q = 16 x 10-19 C
Let the separation between the protons be lsquorrsquo
Electric force between the protons Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
1199032 = 2304 119909 10minus29
1199032
Gravitational force between the protons Fg = 11986611989811198982
1199032 = 667 119909 10minus11 119909 167 119909 10minus27 2
1199032 = 186 119909 10minus65
1199032
Ratio of electric and gravitational forces 119865119890
119865119892=
2304 119909 10minus29
186 119909 10minus65 = 123 x 1036
Electric Field amp Potential
15 Suppose an attractive nuclear force acts between two protons which may be written as F = Ceminuskrr2
(a) write down the dimensional formulae and appropriate SI units of C and k (b) suppose that k = 1 fermiminus1 and that
the repulsive electric force between the protons is just balanced by the attractive nuclear force when the separation is
5 fermi Find the value of C
Solution
(a) Nuclear force between protons F = Ceminuskrr2
Dimensions of exponential function M0L0T0
Dimensional formula for kr M0L0T0 rArr kL = M0L0T0 rArr k = M0L-1T0 (SI unit m-1)
Dimensional formula of force MLT-2
Dimensional formula of Cr2 = MLT-2 rArr C = MLT-2L2 = ML3T-2 (SI unit Nm2)
(b) Repulsive force between protons Fe = 1
41205871205760
1199022
1199032 = 9 119909 109 119909 16 119909 10minus19 2
5 119909 10minus15 2 = 2304
25x 101 = 92 N
Nuclear attractive force between protons Fn = C(eminuskr)r2 = C 119890
minus11 119909 5
5 119909 10minus15 2 = (Ce-5 x 1030 )25
Given Fe = Fn rArr 92 = 1030
251198905 C rArr C = 92 x 25e5 x 10-30 = 34 x 10-26 Nm2
Electric Field amp Potential
16 Three equal charges 20 times 10minus6C each are held fixed at the three corners of an equilateral triangle of side 5 cm
Find the Coulomb force experienced by one of the charges due to the rest two
Solution
Charge on each particle q1 = q2 = q3 = 2 x 10-6 C
Side of the triangle r = 5 cm = 5 x 10-2 m
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Net force on C Fc = 2FAC cos 602 = 2 x 144 x cos 30 = 249 N [along the angular bisector 300]
Electric Field amp Potential
17 Four equal charges 20 times 10minus6C each are fixed at the four corners of a square of side 5 cm Find the Coulomb
force experienced by one of the charges due to the rest three
Solution
Charge on each particle q1 = q2 = q3 = q4 = q = 2 x 10-6 C
Side of the square r = 5 cm = 5 x 10-2 m
Diagonal of the square R = 2r = 5 2 x 10-2 m
Force between B amp C FBC = 1
41205871205760
11990221199023
1199032 = 1
41205871205760
1199022
1199032
Force between D amp C FDC 1
41205871205760
11990241199023
1199032 = 1
41205871205760
1199022
1199032
Resultant of the above two forces FBD = 21
41205871205760
1199022
1199032 [along the diagonal angular bisector 450]
Force between A amp C FAC = 1
41205871205760
11990211199023
1198772 = 1
41205871205760
1199022
21199032
Resultant force on C FC = FAC + FBD = 1
41205871205760
1199022
21199032 + 21
41205871205760
1199022
1199032 = 1
41205871205760
1199022
1199032 2 +1
2
FC = 9 x 109 x 4 119909 10minus12
25 119909 10minus4 2 +1
2= 275 N
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
15 Suppose an attractive nuclear force acts between two protons which may be written as F = Ceminuskrr2
(a) write down the dimensional formulae and appropriate SI units of C and k (b) suppose that k = 1 fermiminus1 and that
the repulsive electric force between the protons is just balanced by the attractive nuclear force when the separation is
5 fermi Find the value of C
Solution
(a) Nuclear force between protons F = Ceminuskrr2
Dimensions of exponential function M0L0T0
Dimensional formula for kr M0L0T0 rArr kL = M0L0T0 rArr k = M0L-1T0 (SI unit m-1)
Dimensional formula of force MLT-2
Dimensional formula of Cr2 = MLT-2 rArr C = MLT-2L2 = ML3T-2 (SI unit Nm2)
(b) Repulsive force between protons Fe = 1
41205871205760
1199022
1199032 = 9 119909 109 119909 16 119909 10minus19 2
5 119909 10minus15 2 = 2304
25x 101 = 92 N
Nuclear attractive force between protons Fn = C(eminuskr)r2 = C 119890
minus11 119909 5
5 119909 10minus15 2 = (Ce-5 x 1030 )25
Given Fe = Fn rArr 92 = 1030
251198905 C rArr C = 92 x 25e5 x 10-30 = 34 x 10-26 Nm2
Electric Field amp Potential
16 Three equal charges 20 times 10minus6C each are held fixed at the three corners of an equilateral triangle of side 5 cm
Find the Coulomb force experienced by one of the charges due to the rest two
Solution
Charge on each particle q1 = q2 = q3 = 2 x 10-6 C
Side of the triangle r = 5 cm = 5 x 10-2 m
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Net force on C Fc = 2FAC cos 602 = 2 x 144 x cos 30 = 249 N [along the angular bisector 300]
Electric Field amp Potential
17 Four equal charges 20 times 10minus6C each are fixed at the four corners of a square of side 5 cm Find the Coulomb
force experienced by one of the charges due to the rest three
Solution
Charge on each particle q1 = q2 = q3 = q4 = q = 2 x 10-6 C
Side of the square r = 5 cm = 5 x 10-2 m
Diagonal of the square R = 2r = 5 2 x 10-2 m
Force between B amp C FBC = 1
41205871205760
11990221199023
1199032 = 1
41205871205760
1199022
1199032
Force between D amp C FDC 1
41205871205760
11990241199023
1199032 = 1
41205871205760
1199022
1199032
Resultant of the above two forces FBD = 21
41205871205760
1199022
1199032 [along the diagonal angular bisector 450]
Force between A amp C FAC = 1
41205871205760
11990211199023
1198772 = 1
41205871205760
1199022
21199032
Resultant force on C FC = FAC + FBD = 1
41205871205760
1199022
21199032 + 21
41205871205760
1199022
1199032 = 1
41205871205760
1199022
1199032 2 +1
2
FC = 9 x 109 x 4 119909 10minus12
25 119909 10minus4 2 +1
2= 275 N
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
16 Three equal charges 20 times 10minus6C each are held fixed at the three corners of an equilateral triangle of side 5 cm
Find the Coulomb force experienced by one of the charges due to the rest two
Solution
Charge on each particle q1 = q2 = q3 = 2 x 10-6 C
Side of the triangle r = 5 cm = 5 x 10-2 m
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6 2
25 119909 10minus4 = 36 119909 10minus3
25 119909 10minus4 = 144 N
Net force on C Fc = 2FAC cos 602 = 2 x 144 x cos 30 = 249 N [along the angular bisector 300]
Electric Field amp Potential
17 Four equal charges 20 times 10minus6C each are fixed at the four corners of a square of side 5 cm Find the Coulomb
force experienced by one of the charges due to the rest three
Solution
Charge on each particle q1 = q2 = q3 = q4 = q = 2 x 10-6 C
Side of the square r = 5 cm = 5 x 10-2 m
Diagonal of the square R = 2r = 5 2 x 10-2 m
Force between B amp C FBC = 1
41205871205760
11990221199023
1199032 = 1
41205871205760
1199022
1199032
Force between D amp C FDC 1
41205871205760
11990241199023
1199032 = 1
41205871205760
1199022
1199032
Resultant of the above two forces FBD = 21
41205871205760
1199022
1199032 [along the diagonal angular bisector 450]
Force between A amp C FAC = 1
41205871205760
11990211199023
1198772 = 1
41205871205760
1199022
21199032
Resultant force on C FC = FAC + FBD = 1
41205871205760
1199022
21199032 + 21
41205871205760
1199022
1199032 = 1
41205871205760
1199022
1199032 2 +1
2
FC = 9 x 109 x 4 119909 10minus12
25 119909 10minus4 2 +1
2= 275 N
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
17 Four equal charges 20 times 10minus6C each are fixed at the four corners of a square of side 5 cm Find the Coulomb
force experienced by one of the charges due to the rest three
Solution
Charge on each particle q1 = q2 = q3 = q4 = q = 2 x 10-6 C
Side of the square r = 5 cm = 5 x 10-2 m
Diagonal of the square R = 2r = 5 2 x 10-2 m
Force between B amp C FBC = 1
41205871205760
11990221199023
1199032 = 1
41205871205760
1199022
1199032
Force between D amp C FDC 1
41205871205760
11990241199023
1199032 = 1
41205871205760
1199022
1199032
Resultant of the above two forces FBD = 21
41205871205760
1199022
1199032 [along the diagonal angular bisector 450]
Force between A amp C FAC = 1
41205871205760
11990211199023
1198772 = 1
41205871205760
1199022
21199032
Resultant force on C FC = FAC + FBD = 1
41205871205760
1199022
21199032 + 21
41205871205760
1199022
1199032 = 1
41205871205760
1199022
1199032 2 +1
2
FC = 9 x 109 x 4 119909 10minus12
25 119909 10minus4 2 +1
2= 275 N
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
18 A hydrogen atom contains one proton and one electron It may be assumed that the electron revolves in a circle of
radius 053 angstrom (1 angstrom = 10minus10119898 and is abbreviated as Å) with the proton at the centre The hydrogen atom
is said to be in the ground state in this case Find the magnitude of the electric force between the proton and the
electron of a hydrogen atom in its ground state
Solution
Charge on the proton q1 = 16 x 10-19 C
Charge on the electron q2 = 16 x 10-19 C
Separation between proton and electron radius of the orbit r = 053 A0 = 053 x 10-10 m
Electric force between proton and electron F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 16 119909 10minus19 2
053 119909 10minus10 2 = 82 x 10-8 N
19 Find the speed of the electron in the ground state of a hydrogen atom The description of ground state is given in the previous problem
Solution
Centripetal force for circular motion of the electron is provided by electrostatic force between proton amp electron
1198981199072
119903=
1
41205871205760
11990211199022
1199032 rArr v2 = 119903
119898
1
41205871205760
11990211199022
1199032 = 053 119909 10minus10
911 119909 10minus31 x 82 x 10-8 = 53 119909 82
911x 1012 = 48 x 1012
v = 48 119909 1012 = 218 x 106 ms
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
20 Ten positively charged particles are kept fixed on the x-axis at points x = 10 cm 20 cm 30 cm 100 cm The
first particle has a charge 10 times 10minus8C the second 8 times 10minus8C the third 27 times 10minus8C and so on The tenth particle has a
charge 1000 times 10minus8C Find the magnitude of the electric force acting on a 1 C charge placed at the origin
Solution
Charges on the particles 1 119909 10minus2 3 2 119909 10minus2 3 3 119909 10minus2 3 4 119909 10minus2 3 5 119909 10minus2 3 hellip 10 119909 10minus2 3
Distances of the particles 10 cm 2 x 10 cm 3 x 10 cm helliphellip10 x 10 cm
Charge on the particle at origin q0 = 1 C
Electric force on the particle placed at origin F = 1
41205871205760
11990201199021
11990312 + helliphellip
1
41205871205760
119902011990210
119903102
F = 9 x 109 x 10minus8
01 2
13
12 +23
22 +33
32 + ⋯ hellip103
102 = 9000 [1 + 2 + 3 + helliphellip10] =
F = 9000 x 10 119909 (10+1)
2= 9000 x 55 = 495 x 105 N
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
21 Two charged particles having charge 20 times 10minus8119862 each are joined by an insulting string of length 1 m and the
system is kept on a smooth horizontal table Find the tension in the string
Solution
Charge on each particle q1 = q2 = q = 2 x 10-8 C
Separation between the particle length of the string r = 1 m
Tension in the string electric force between the particle F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus16
1= 36 x 10-6 N
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
22 Two identical balls each having a charge of 200 times 10minus7C and a mass of 100 g are suspended from a common
point by two insulating strings each 50 cm long The balls are held at a separation 50 cm apart and then released Find
(a) the electric force on one of the charged balls (b) the components of the resultant force on it along and perpendicular
to the string (c) the tension in the string (d) the acceleration of one of the balls Answers are to be obtained only for the
instant just after the release
Solution
Charge on each ball q1 = q2 = q = 2 x 10-7 C
Mass of each ball m1 = m2 = m = 100 g = 01 kg
Length of each string L = 50 cm = 05 m
Initial separation between the balls r = 5 cm = 005 m
Gravitational force on the ball Fg = mg = 100 x 10-3 x 98 = 098 N
(a) Electric force between the balls Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 4 119909 10minus14
25 119909 10minus4 = 36
25x 10-1 = 0144 N
From the diagram sin 120579 = 25
50=
1
20and cos θ asymp 1
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
Solution
(b) Component of force along the string F1 = 1198981199072
119897=
119898(0)
119897= 0 [the speed of the ball is zero just after release]
component of force perpendicular to string F2 = Fe cos θ ndash Fg sin θ = 0144 (1) ndash 098 (005) = 0095 N
(c) Tension in the string T = Fe sin θ + Fg cos 120579 = 0144 x 005 + 098 399
400= 0986 N
(d) Acceleration of the ball a = 1198652
119898=
0095
01= 095 ms2
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
23 Two identical pith balls are charged by rubbing against each other They are suspended from a horizontal rod
through two strings of length 20 cm each the separation between the suspension points being 5 cm In equilibrium the
separation between the balls is 3 cm Find the mass of each ball and the tension in the strings The charge on each ball
has a magnitude 20 times 10minus8C
Solution
Charge on each ball q = 2 x 10-8 C
Gravitational force (weight) on each ball Fg = mg
Electrostatic force on each ball Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
9 119909 10minus4 = 4 x 10-3 N
For equilibrium of the ball T cos θ = mg ---- (1)
For equilibrium of the ball T sin θ = Fe ---- (2)
From (1) and (2) tan θ = 119865119890
119898119892rArr
1
20=
4 119909 10minus3
98119898rArr m =
80
98= 82 gm
(for small angles tan θ = sin θ)
Tension in the string T = 1198651198902 + 119865119892
2 = (4 119909 10minus3)2+(82 119909 10minus3 119909 98)2 = 805 x 10-3 N = 81 x 10-2 N
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
24 Two small spheres each having a mass of 20 g are suspended from a common point by two insulating strings of
length 40 cm each The spheres are identically charged and the separation between the balls at equilibrium is found to
be 4 cm Find the charge on each sphere
Solution
Mass of each sphere m1 = m2 = m = 20 gm = 20 x 10-3 kg
Length of each string L1 = L2 = L = 40 cm = 04 m
Charge on each sphere q1 = q2 = q
Separation between the spheres at equilibrium r = 4 cm = 004 m
Gravitational force on each sphere Fg = mg = 002 x 98 = 0196 N
Electrostatic force on each sphere Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 1199022
16 119909 10minus4 = 9
16x 1013 q2
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892
1
20=
9
16x 1013 q2 1
0196rArr q2 = 174 x 10-16 rArr q = 417 x 10-8 C
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
25 Two identical pith balls each carrying a charge q are suspended from a common point by two strings of equal
length l Find the mass of each ball if the angle between the strings is 2θ in equilibrium
Solution
Let the mass of each ball be lsquomrsquo
Gravitational force on the ball Fg = mg
Electrostatic force on the ball Fe = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
1199022
1199032 [r is separation between the balls at equilibrium]
For equilibrium of the system T cos θ = Fg and T sin θ = Fe rArr tan θ = 119865119890
119865119892=
1
41205871205760
1199022
1199032 119898119892---- (1)
From the diagram sin θ = Τ119903
2
119897rArr r = 2119897 sin θ ---- (2)
From (1) amp (2) tan θ = 1
41205871205760
1199022
2119897 sin 120579 2 119898119892rArr m =
119954120784 119836119848119853 120637
120783120788120645120634120782119944119949120784119956119946119951120784120637
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
26 A particle having a charge of 20 times 10minus4C is placed directly below and at separation of 10 cm from the bob of a
simple pendulum at rest The mass of the bob is 100 g What charge should the bob be given so that the string becomes
loose
Solution
Charge on the particle q = 20 x 10-4 C
Distance of the particle from the bob d = 10 cm = 01 m
Mass of the bob m = 100 g = 01 kg
Let the charge on the bob be lsquoQrsquo
The string becomes loose when the weight of the bob is balanced by the electrostatic force of repulsion
Weight of the bob Fg = mg = 01 x 98 = 098 N
Electrostatic force of repulsion Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus4119876
001= 18 x 107Q
For equilibrium Fg = Fe rArr 098 = 18 x 107Q rArr Q = 54 x 10-9 C
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
27 Two particles A and B having charges q and 2q respectively are placed on a smooth table with a separation d A
third particle C is to be clamped on the table in such a way that the particles A and B remain at rest on the table under
electrical forces What should be the charge on C and where should it be clamped
Solution
Given the particle at C is clamped to the table
Particles A and B should remain at rest on the table under electrostatic forces
Resultant electrostatic force on particle at A FA = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092
For the particle at A to be at rest FA = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
1199092 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
119902119876
1199092 rArr 2119902
1198892 = minus119876
1199092 ---- (1)
Resultant electrostatic force on particle at B FB = 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2
For the particle at B to be at rest FB = 0 rArr 1
41205871205760
21199022
1198892 + 1
41205871205760
119902119876
(119889minus119909)2 = 0 rArr 1
41205871205760
21199022
1198892 = minus1
41205871205760
2119902119876
(119889minus119909)2 rArr 2119902
1198892 = minus2119876
(119889minus119909)2 --- (2)
From (1) amp (2) minus119876
1199092 = minus2119876
(119889minus119909)2 rArr x = 119889
2+1= d 120784 minus 120783
From (1) 2119902
1198892 = minus119876
1198892 2minus12 = minusq (6 ndash 4 120784 )
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
28 Two identically charged particle are fastened to the two ends of a spring of spring constant 100 N mminus1 and natural
length 10 cm The system rests on a smooth horizontal table If the charge on each particle is 20 times 10minus8C find the
extension in the length of the spring Assume that the extension is small as compared to the natural length Justify this
assumption after you solve the problem
Solution
Force constant of the spring k = 100 Nm
Natural length of the spring L = 10 cm = 01 m
Charge on each particle q = 2 x 10-8 C
Let the extension in the spring be lsquoxrsquo
Restoring force in the sprint Fs = kx = 100x
Electrostatic force of repulsion between the charges Fe = 1
41205871205760
11990211199022
1199032 = 9 x 109 4 119909 10minus16
01+119909 2 = 36 119909 10minus7
001+02119909
For equilibrium of the system 100x = 36 119909 10minus7
001+02119909rArr x + 20x2 = 36 x 10-7 rArr x = 36 x 10-6 m
(as x is negligibly small x2 term is neglected in the simplification process)
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
29 A particle A having a charge of 20 times 10minus6C is held fixed on a horizontal table A second charged particle of mass
80 g stays in equilibrium on the table at a distance of 10 cm from the first charge The coefficient of friction between
the table and this second particle is μ = 02 Find the range within which the charge of this second particle may lie
Solution
Charge of the fixed particle q1 = 2 x 10-6 C
Let the charge on the second particle be lsquoqrsquo
Mass of the second particle m = 80 gm = 80 x 10-3 kg
Separation between the particles r = 10 cm = 01 m
Coefficient of friction between the particle and the table surface μ = 02
Electric force between the particles F = 1
41205871205760
11990211199022
1199032 = 9 x 109 x 2 119909 10minus6119902
001= 18 x 105q N
Force of friction between the particle and surface f = μN = μmg = 02 x 80 x 10-3 x 98 = 01568 N
For equilibrium of the particle f = F rArr 01568 = 18 x 105q rArr q = 871 x 10-8 C
The same calculation can be repeated when the second particle is negative
So the range of charges for equilibrium plusmn 871 x 10-8 C
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
30 A particle A having a charge of 20 times 10minus6C and a mass of 100 g is placed at the bottom of a smooth inclined plane
of inclination 30o Where should another particle B having same charge and mass be placed on the incline so that it
may remain in equilibrium
Solution
Charge on the particle A qA = 2 x 10-6 C
Mass of the particle A mA = 100 g = 01 kg
Inclination of the plane θ = 300
Given mass and charge of the second particle are same as that of the first one
For equilibrium of the second particle mg sin θ = 1
41205871205760
11990211199022
1199032
01 x 98 x sin 30 = 9 x 109 x 4 119909 10minus12
1199032
049 = 36 x 10-3 1
1199032 rArr r2 = 7346 x 10-4 rArr r = 7346 x 10-2 = 27 cm
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
31 Two particles A and B each having a charge Q are placed a distance d apart Where should a particle of charge q
be placed on the perpendicular bisector of AB so that it experiences maximum force What is the magnitude of this
maximum force
Solution
Charge on both particles is same qA = qB = Q and Charge on the third particle q
Separation between the particles d
Let the third particle be placed at a distance y on the perpendicular bisector
Net force on the particle F = 2Fe cos 180minus2120579
2= 2Fe cos (90 - 120579) = 2Fe sin θ
F = 2 1
41205871205760
119876119902
1199102+119889
2
22 sin θ = 2
1
41205871205760
119876119902
1199102+119889
2
22
119910
1199102+119889
2
2=
119876119902
21205871205760
119910
1199102+119889
2
2 32
Apply maxima-minima condition 119889119865
119889119910= 0 rArr
119876119902
21205871205760
1 1199102+119889
2
2 32
minus 1199103
21199102+
119889
2
2 12
2119910
1199102+119889
2
2 3 = 0
1 1199102 +119889
2
2 32
minus 1199103
21199102 +
119889
2
2 12
2119910 = 0 rArr 1199102 +119889
2
2 32
= 1199103
21199102 +
119889
2
2 12
2119910 rArr y = 119941
120784 120784
Magnitude of maximum force Fmax = 119876119902
21205871205760
119889
2 2
119889
2 2
2+
119889
2
2 32 = 308 119928119954
120786120645120634120782119941120784
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
32 Two particles A and B each carrying a charge Q are held fixed with a separation d between them A particle C
having mass m and charge q is kept at the middle point of the line AB (a) if it is displaced through a distance x
perpendicular to AB what would be the electric force experienced by it (b) assuming xltltd show that this force is
proportional to x (c) under what conditions will the particle C execute simple harmonic motion if it is released after
such a small displacement Find the time period of the oscillations if these conditions are satisfied
Solution
Force between A amp C FAC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119876119902
1199092+1198892
4
(a) Resultant force F = 2FAC cos θ = 2 1
41205871205760
119876119902
1199092+1198892
4
119909
1199092+1198892
4
= 119824119850119857
120784120529120518120782 119857120784+119837120784
120786
120785120784
(b) For x ltlt d F = Qqx
2πε0d2
4
32
= 4119876119902
πε01198893 119909 rArr F prop x
(C) ma = 4119876119902
πε01198893 119909 rArr a = 4119876119902
π119898ε01198893 119909 [a = ω2x]
Angular frequency ω = 4119876119902
π119898ε01198893 rArr T = 2120587π119898ε01198893
4119876119902=
119950120645120785120634120782119941120785
119928119954
120783
120784
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
33 Repeat the previous problem if the particle C is displaced through a distance x along the line AB
Solution
Force between A amp C FAC = = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2+ 119909
2 and Force between B amp C FBC = 1
41205871205760
11990211199022
1199032 = 1
41205871205760
119902119876
119889
2minus 119909
2
Resultant force F = FBC ndash FAC = 1
41205871205760
119902119876
119889
2minus 119909
2 minus1
41205871205760
119902119876
119889
2+ 119909
2 = 119876119902
41205871205760
1
119889
2minus 119909
2 minus1
119889
2+ 119909
2 = 119876119902
41205871205760
2119909119889
1198892
4minus 1199092
2
When x ltlt d F = 119876119902
41205871205760
2119909119889
1198892
4
2 = 119876119902
41205871205760
32119909
1198893 = 119876119902
1205871205760
8119909
1198893 = 119876119902
1205871205760
8
1198893 (119909) rArr ma = 119876119902
1205871205760
8
1198893 (119909) rArr a = 119876119902
1198981205871205760
8
1198893 (119909) [a = ω2x]
Angular frequency of oscillation ω = 119876119902
1198981205871205760
8
1198893 rArr 2120587
119879=
119876119902
1198981205871205760
8
1198893 rArr T = 212058711989812058712057601198893
8119902119876=
119950120645120785120634120782119941120785
120784119954119928
120783
120784
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
34 The electric force experienced by a charge of 10 times 10minus6 C is 15 times 10minus3N Find the magnitude of the electric field
at the position of the charge
Solution
Charge on the particle q = 1 x 10-6 C
Force experienced by the particle F = 15 x 10-3 N
Magnitude of electric field E = 119865
119902=
15 119909 10minus3
1 119909 10minus6 = 15 x 103 = 1500 NC
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
35 Two particles A and B having charges of +200 times 10minus6C and of minus400 times 10minus6C respectively are held fixed at a
separation of 200 cm Locate the point(s) on the line AB where (a) the electric field is zero (b) the electric potential is
zero
Solution
Charge on the first particle qA = 2 x 10-6 C and Charge on the second particle qB = 4 x 10-6 C
Separation between the particles r = 20 cm = 02 m
Since the particles are of opposite sign zero field is not possible between the charges
(a) Let us consider a point P on the line AB outside the charges at a distance x from A
Field at P due to positive charge EA = 1
41205871205760
119902119860
1199092 = 9 x 109 x 2 119909 10minus6
1199092 = 18000
1199092 (away from A)
Field at P due to negative charge EB = 1
41205871205760
119902119861
(119909+02)2 = 9 x 109 x 4 119909 10minus6
(119909+02)2 = 36000
(119909+02)2 (towards B)
Net field at P = 0 rArr EA ndash EB = 0 rArr EA = EB rArr 18000
1199092 = 36000
(119909+02)2 rArr 1
1199092 = 2
(119909+02)2 rArr 1
119909=
2
119909 + 02rArr x =
02
2minus1= 483 cm
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
Solution
(b) For zero potential consider a point P at a distance x from A between the charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20minus119909= 9 x 109 x
minus4 119909 10minus6
20minus119909=
minus36000
20minus119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20minus119909= 0 rArr
18000
119909=
36000
20minus119909rArr
1
119909=
2
20minus119909rArr 2x = 20 minus 119909 rArr x =
120784120782
120785cm
Consider a point P at a distance x from A outside the two charges
Potential at P due to A VA = 1
41205871205760
119902119860
119909= 9 x 109 x
2 119909 10minus6
119909=
18000
119909
Potential at P due to B VB = 1
41205871205760
119902119861
20+119909= 9 x 109 x
minus4 119909 10minus6
20+119909=
minus36000
20+119909
Net potential at P V = VA + VB = 18000
119909minus
36000
20+119909= 0 rArr
18000
119909=
36000
20+119909rArr
1
119909=
2
20+119909rArr 2x = 20 + 119909 rArr x = 20 cm
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
36 A point charge produces an electric field of magnitude 50 N Cminus1 at a distance of 40 cm from it What is the
magnitude of the charge
Solution
Magnitude of electric field E = 5 NC
Distance of point r = 40 cm = 04 m
Magnitude of electric field E = 1
41205871205760
119902
1199032 = 9 x 109 x 119902
016= 5 rArr q = 89 x 10-11 C
37 A water particle of mass 100 mg and having a charge of 150 times 10minus6C stays suspended in a room What is the
magnitude of electric field in the room What is its direction
Solution
Mass of water particle m = 10 mg = 10 x 10-6 kg
Charge on the particle q = 15 x 10-6 C
Gravitational force on the water particle Fg = mg = 10-5 x 98 N (downward)
Electrostatic force on the particle Fe = Eq = E (15 x 10-6) (upward)
For equilibrium Fg = Fe rArr 98 x 10-5 = 15 x 10-6 E rArr E = 98
15= 653 NC
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
38 Three identical charges each having a value 10 times 10minus8C are placed at the corners of an equilateral triangle of
side 20 cm Find the electric field and potential at the centre of the triangle
Solution
Charge on each particle q1 = q2 = q3 = q = 1 x 10-8 C
Side of the equilateral triangle a = 20 cm = 02 m
Distance of center of the triangle from vertex r = 119886
3=
02
3m
Electric field at the center of the triangle due to one particle E = 1
41205871205760
119902
1199032 = 1
41205871205760
10minus8
02
3
2 = 1
41205871205760
3
4x 10-6 NC
Electric field at the center of the triangle due to all particles E1 = 0 (triangle law)
Electric potential at the center of the triangle due to one particle V = 1
41205871205760
119902
119903=
1
41205871205760
10minus8
02
3
= 10 3
2
1
41205871205760x 10-8 volt
Electric potential at the center of the triangle due to all particles V1 = 3V = 3 x 10 3
2
1
41205871205760x 10-8
V1 = 15 31
41205871205760x 10-8 = 15 3 x 9 x 109 x 10-8 = 23 x 102 volt
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
39 Positive charge Q is distributed uniformly over a circular ring of radius R A particle having a mass m and a
negative charge q is placed on its axis at a distance x from the centre Find the force on the particle Assuming xltltR
find the time period of oscillation of the particle if it is released from there
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Force on the particle placed on the axis of the ring F = Eq = 1
41205871205760
119876119902119909
1198772+1199092 32
Force on the particle for x ltlt R F = 1
41205871205760
119876119902119909
1198772 32 = 1
41205871205760
119876119902119909
1198773
ma = 119876119902
412058712057601198773 (x) rArr a = 119876119902
412058712057601198981198773 (x) [compare with a = ω2x]
ω2 = 119876119902
412058712057601198981198773 rArr ω = 119876119902
412058712057601198981198773 rArr 2120587
119879=
119876119902
412058712057601198981198773 rArr T = 120783120788120645120785120634120782119950119929120785
119928119954
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
40 A rod of length L has a total charge Q distributed uniformly along its length It is bent in the shape of a semicircle
Find the magnitude of the electric field at the centre of curvature of the semicircle
Solution
Linear charge density λ = 119876
119871
Consider an elemental length lsquod119897rsquo of the semicircle at an angle θ with the vertical
Charge of this element dq = λ d119897 = 119876
119871d119897
Field intensity due to the element dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119876
119871119889119897
1199032
Field intensity due to the element along the vertical dE cos θ = 1
41205871205760
119876
119871119889119897
1199032 cos θ = 1
41205871205760
119876
1198711199032 119903 119889θ cos θ
Total electric field E = 119889119864 cos 120579 = 1
41205871205760
119876
119871119903
minus120587
2
120587
2 cos 120579 119889120579 = = 1
41205871205760
119876
119871119903sin θ limits minus
120587
2to
120587
2
E = 1
41205871205760
119876
119871119903(1 ndash (minus1)) =
1
21205871205760
119876
119871119903=
1
21205871205760
119876
119871119871
120587
= 119928
120784120634120782119923120784
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
41 A 10-cm long rod carries a charge of +50 μC distributed uniformly along its length Find the magnitude of the
electric field at a point 10 cm from both the ends of the rod
Solution
Consider a rod of length L and charge q uniformly distributed on it
Linear charge density of the rod λ = 119902
119871
Consider a point at a distance d from the mid point of the rod at which we
need to find electric field intensity
Take an elemental length lsquodyrsquo of the rod at a distance lsquoyrsquo from the mid point
Charge of this element lsquodqrsquo (point charge) = λ dy = 119902
119871dy
Field intensity due to elemental charge dE = 1
41205871205760
119889119902
1199032 = 1
41205871205760
119902
119871 1198892+1199102 dy
Resultant field intensity due to entire rod E = 119889119864 cos 120579 = 1
41205871205760
119902
1198711198712minus
1198712 119889119910
1198892+1199102
119889
1198892+1199102=
1
41205871205760
119902119889
1198711198712minus
1198712 119889119910
1198892+1199102 32
Put y = d tan θ rArr dy = d sec2 θ dθ and d2 + y2 = d2 + d2 tan2 θ = d2 sec2 θ
E = 1
41205871205760
119902119889
1198711198712minus
1198712 d sec2 θ dθ11988931199041198901198883120579
= 1
41205871205760
119902
1198711198891198712minus
1198712cos θ 119889θ =
1
41205871205760
119902
119871119889sin θ =
1
41205871205760
119902
119871119889
119910
1198892+1199102[limits minusL2 to L2]
E = 1
41205871205760
119902
119889
1
1198892+1198712
4
= 9 x 109 x 50 119909 10minus6
0087
1
(0087)2+001
4
= 52 x 107 NC
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
42 Consider a uniformly charged ring of radius R Find the point on the axis where the electric field is maximum
Solution
Electric field intensity on the axis of a charged ring E = 1
41205871205760
119876119909
1198772+1199092 32
Apply maxima-minima condition 119889119864
119889119909= 0 rArr
119889
119889119909
1
41205871205760
119876119909
1198772+1199092 32 = 119876
41205871205760
119889
119889119909
119909
1198772+1199092 32
119889119864
119889119909=
119876
41205871205760
1 1198772+1199092 32minus119909
3
21198772+1199092
12 2119909
1198772+1199092 3 = 0
1198772 + 1199092 32 minus 1199093
21198772 + 1199092
1
2 2119909 = 0
1198772 + 1199092 32 = 3x2 1198772 + 11990921
2
1198772 + 1199092 = 3x2 rArr R2 = 2x2 rArr x = 119929
120784
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
43 A wire is bent in the form of a regular hexagon and a total charge q is distributed uniformly on it What is the
electric field at the centre You may answer this part without making any numerical calculations
Solution
Electric field intensity on the perpendicular bisector of a charge wire E = 1
41205871205760
119902
119889
1
1198892+1198712
4
[directed away from the wire along the bisector]
In the case of regular hexagon all the six electric fields at the center have equal magnitudes and separated by 600 as
shown in figure
From polygon law of vectors the resultant field is zero
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
44 A circular wire-loop of radius lsquoarsquo carries a total charge Q distributed uniformly over its length A small length dL of
the wire is cut off Find the electric field at the centre due to the remaining wire
Solution
Linear charge density of the wire λ = 119902
119897=
119876
2120587119886
Charge on small length of the wire dQ = λ dL = 119876
2120587119886dL
The electric field generated by any small part of the wire at the center is cancelled by diametrically opposite part
When dL part of the wire is cut off the field created by the diametrically opposite part remains
Field intensity at the center due to elemental charge dE = 1
41205871205760
119889119876
1198862
dE = 1
41205871205760
119876
2120587119886dL
1198862 = 1
41205871205760
119876 119889119871
21205871198863 = 119954 119941119923
120790120645120784120634120782119938120785
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
45 A positive charge q is placed in front of a conducting solid cube at a distance d from its centre Find the electric
field at the centre of the cube due to the charges appearing on its surface
Solution
Since the cube is made of conducting material the electric field inside it is zero
Electric field due to point charge E = 120783
120786120645120634120782
119954
119941120784
Electric field due to induced charge Ei
Net electric field at the center E = E + Ei rArr 0 = E + Ei rArr Ei = minusE
So the field due to induced charge is equal in magnitude to that of point charge and in opposite direction of the field
due to point charge
The field is towards the point charge
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
46 A pendulum bob of mass 80 mg and carrying a charge of 2 times 10minus8C is at rest in a uniform horizontal electric field
of 20 kVmminus1 Find the tension in the thread
Solution
Mass of the bob m = 80 mg = 80 x 10-6 kg
Charge on the bob q = 2 x 10-8 C
Strength of electric field E = 20 kVm = 2 x 104 Vm
Gravitational force on the bob Fg = mg = 80 x 10-6 x 98 = 784 x 10-4 N
Electrostatic force on the bob Fe = Eq = 2 x 104 x 2 x 10-8 = 4 x 10-4 N
Tension in the thread T = 1198651198902 + 119865119892
2 = 10-4 42 + 7842 = 88 x 10-4 N
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
47 A particle of mass m and charge q is thrown at a speed u against a uniform electric field E How much distance will
it travel before coming to momentary rest
Solution
Mass of the particle m
Charge on the particle q
Initial speed of the particle u
Final speed of the particle v = 0
Strength of electric field E
Force on the particle F = Eq
Acceleration of the particle a = 119865
119898=
119864119902
119898
Distance travelled by the particle before momentarily coming to rest d = 1199062
2119886=
119950119958120784
120784119916119954
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
48 A particle of mass 1 g and charge 25 times 10minus4C is released from rest in an electric field of 12 times 104 N Cminus1 (a) find
the electric force and the force of gravity acting on this particle Can one of these forces be neglected in comparison
with the other for approximate analysis (b) How long will it take for the particle to travel a distance of 40 cm
(c) what will be the speed of the particle after travelling this distance (d) how much is the work done by the electric
force on the particle during this period
Solution
Mass of the particle m = 1 g = 10-3 kg and Charge on the particle q = 25 x 10-4 C
Strength of the electric field E = 12 x 104 NC
(a) Electric force on the particle Fe= Eq = 12 x 104 x 25 x 10-4 = 3 N
Gravitational force on the particle Fg = mg = 10-3 x 98 = 98 x 10-3 N
Gravitational force can be neglected as we can see from the numerical values above
Acceleration of the particle a = 119865
119898=
3
0001= 3 x 103 ms2
(b) Time taken by the particle to travel 40 cm t = 2119904
119886=
2 119909 04
3 119909 103 = 163 x 10-2 sec
(c) Speed of the particle v2 ndash u2 = 2as rArr v2 ndash 0 = 2 x 3 x 103 x 04 rArr v = 2400 = 49 ms
(d) Work done by electric force on the particle w = Fd = 3 x 04 = 12 J
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
49 A ball of mass 100 g and having a charge of 49 times 10minus5C is released from rest in a region where a horizontal
electric field of 20 times 104 N Cminus1 exists (a) find the resultant force acting on the ball (b) what will be the path of the
ball (c) where will the ball be at the end of 2 s
Solution
Mass of the ball m = 100 g = 01 kg
Charge on the ball q = 49 x 10-5 C
Strength of electric field E = 2 x 104 NC [horizontal]
Electrical force on the ball Fe = Eq = 2 x 104 x 49 x 10-5 = 098 N
Gravitational force on the ball Fg = mg = 01 x 98 = 098 N
(a) Resultant force on the ball F = 1198651198902 + 119865119892
2 = (098)2+(098)2 = 098 2 = 14 N (450 to the horizontal)
(b) The particle follows straight line path along the resultant direction
(c) Displacement of the particle s = ut + 1
2at2 = 0 +
1
2
119865
119898t2 = 05 x
14
01x 4 = 28 m along the straight line
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
50 The bob of a simple pendulum has a mass of 40 g and a positive charge of 40 times 10minus6C It makes 20 oscillations in
45 s A vertical electric field pointing upward and of magnitude 25 times 104 N Cminus1 is switched on How much time will
it now take to complete 20 oscillations
Solution
Time period of oscillation of the bob T1 = 45
20= 225 sec
Charge on the bob q = 4 x 10-6 C
Mass of the bob m = 40 g = 004 kg
Strength of electric field E = 25 x 104 NC
Effective acceleration of the bob geff = g ndash a = g minus119864119902
119898= 10 minus
25 119909 104 119909 4 119909 10minus6
004= 10 ndash 25 = 75 ms2
Ratio of time periods 1198791
1198792=
119892119890119891119891
119892rArr
225
1198792=
75
10rArr T2 = 225
10
75= 26 sec
Time taken for 20 oscillations t = 20T = 20 x 26 = 52 sec
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
51 A block mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an
unstressed spring of spring constant k as shown in figure A horizontal electric field E parallel to the spring is switched
on Find the amplitude of the resulting SHM of the block
Solution
Electrostatic force on the block Fe = Eq (towards right)
Restoring force in the spring Fs = kx (towards mean position)
Let the amplitude of oscillation be lsquoArsquo
At the extreme position kA = Eq rArr A = 119916119954
119948
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
52 A block of mass m containing a net positive charge q is placed on a smooth horizontal table which terminates in a
vertical wall as shown in figure The distance of the block from the wall is d A horizontal electric field E towards right
is switched on Assuming elastic collisions (if any) find the time period of the resulting oscillatory motion Is it a
simple harmonic motion
Solution
Charge on the block q
Mass of the block m
Electric field intensity E
Force on the block F = Eq
Acceleration of the block a = 119865
119898=
119864119902
119898
Time taken to travel upto the wall s = ut + 1
2at2 rArr d = 0 +
1
2
119864119902
119898t2 rArr t =
2119889119898
119864119902
Time taken for round trip time period T = 2t = 22119889119898
119864119902=
120790119941119950
119916119954
since the acceleration is constant the oscillations are not simple harmonic
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
53 A uniform electric field of 10 N Cminus1 exists in the vertically down ward direction Find the increase in the electric
potential as one goes up through a height of 50 cm
Solution
Strength of the electric field E = 10 NC [vertically downward]
Distance travelled d = 50 cm = 05 m
Increase in potential ∆V = Ed = 10 x 05 = 5 volt
54 12 J work has to be done against an existing electric field to take a charge of 001 C from A to B How much is the
potential difference 119881119861 minus 119881119860
Solution
Charge on the particle q = 001 C
Work done against electric field w = 12 J
Since work is done against electric field B is at higher potential than A
Work done w = VBAq = VBA (001) = 12 rArr VB ndash VA = 1200 volt
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
55 Two equal charges 20 times 10minus7C each are held fixed at a separation of 20 cm A third charge of equal magnitude is
placed midway between the two charges It is now moved to a point 20 cm from both the charges How much work is
done by the electric field during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-7 C
Separation between the particles r = 20 cm = 02 m
Charge on the third particle q3 = 2 x 10-7 C
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Ui = 9 x 109 4 119909 10minus14
02+
4 119909 10minus14
01+
4 119909 10minus14
01= 36 x 10-5 1
02+
1
01+
1
01= 36 x 10-5 (5 + 10 + 10) = 9 x 10-3 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 x 109 x 3 x 4 119909 10minus14
02= 540 x 10-5 = 54 x 10-3 J
Work done by electric field w = Ui ndash Uf = (9 ndash 54) x 10-3 = 36 x 10-3 J
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
56 An electric field of 20 N Cminus1 exists along the x-axis in space Calculate the potential difference VB minus VA where the
points A and B are given by
(a) A = (0 0) B = (4 m 2 m) (b) A = (4 m 2 m) B = (6 m 5 m) (c) A = (0 0) B = (6 m 5 m)
Do you find any relation between the answers of parts (a) (b) and (c)
Solution
Electric field intensity E = 20 NC along x axis
Relation between potential difference and field intensity E = minus119889119881
119889119909rArr dV = minus E dx rArr 1198811
1198812 119889119881 = minus 1199091
1199092 119864 119889119909
(a) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 4] = minus(80 ndash 0) = minus80 volt
(b) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 4 to 6] = minus(120 ndash 80) = minus40 volt
(c) Potential difference 119860
119861119889119881 = minus 1199091
1199092 119864 119889119909 rArr VBminusVA = minus20x [limits 0 to 6] = minus(120 ndash 0) = minus120 volt
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
57 Consider the situation of the previous problem A charge of minus20 times 10minus4C is moved from the point A to the point
B Find the change in electrical potential energy UB minus UA for the cases (a) (b) and (c)
Solution
Change in potential energy ∆119880 = UB ndash UA = qVBA = q(VB ndash VA)
(a) Change in potential energy in first case UB ndash UA = minus20 times 10minus4 x minus80 = 0016 J
(b) Change in potential energy in second case UB ndash UA = minus20 times 10minus4 x minus40 = 0008 J
(c) Change in potential energy in third case UB ndash UA = minus20 times 10minus4 x minus120 = 0024 J
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
58 An electric field E = Ԧi20 + Ԧj30 N Cminus1 exists in the space If the potential at the origin is taken to be zero find the
potential at (2 m 2 m)
Solution
Electric field intensity in the region E = Ԧi20 + Ԧj30 N Cminus1
Potential at the origin V0 = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903rArr dV = minus ത119864 d ҧ119903
dV = minus 20 Ƹ119894 + 30 Ƹ119895 (119889119909 Ƹ119894 + 119889119910 Ƹ119895) = minus (20 dx + 30 dy)
119881
0119889119881 = minus 2
020 119889119909 minus 2
030 119889119910
(0 ndash V) = minus20x (limits 2 to 0) minus 30y (limits 2 to 0)
V = 20 (0 ndash 2) + 30 (0 ndash 2)
V = minus40minus60 = minus100 volt
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
59 An electric field E = Ԧ119894Ax exists in the space where A = 10 V mminus2 Take the potential at (10 m 20 m) to be zero
Find the potential at the origin
Solution
The electric field intensity in the given region E = 119860119909Ԧ119894
Potential at (10 m 20 m) V = 0 volt
Relation between potential difference and electric field intensity E = minus119889119881
119889119903
Since the field is changing only along x axis E = minus119889119881
119889119909rArr dV = minusE dx
0
1198810 119889119881 = minus 10
0119860119909 rArr V0 = minus A
1199092
2[limits 10 to 0] =
10
2(100 ndash 0) = 500 volt
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
60 The electric potential existing in space is V(x y z) = A(xy + yz + zx) (a) write the dimensional formula of A
(b) Find the expression for the electric field (c) If A is 10 SI units find the magnitude of the electric field at
(1 m 1 m 1 m)
Solution
The electric potential existing in space is V(x y z) = A(xy + yz + zx)
Here x y and z are distances (dimensional formula L)
Dimensional formula of potential V = 119908
119902=
1198721198712119879minus2
119868119879= ML2T-3I-1
(a) So the dimensional formula of A ML2Tminus3Iminus1
1198712 = MT-3I-1
(b) Electric field intensity E = minus120597119881
120597119909Ƹ119894 minus
120597119881
120597119910119895 minus
120597119881
120597119911119896 = minus119860(y + 0 + z) Ƹ119894 minus119860 (x + z + 0) 119895 minus119860 (0 + y + x) 119896
E = minus119912 [(y + z) Ƹ119946 minus119912 (x + z) 119947 minus119912 (y + x) 119948]
(c) Magnitude of electric field E = 10 [(1 + 1) Ƹ119894 + (1 + 1) 119895 + (1 + 1) 119896] = 20 ( Ƹ119894 + 119895 + 119896) = 20 3 = 346 NC
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
61 Two charged particles having equal charges of 20 times 10minus5C each are brought from infinity to within a separation
of 10 cm Find the increase in the electric potential energy during the process
Solution
Charge on each particle q1 = q2 = q = 2 x 10-5 C
Initial separation between the particles ri = infinity
Final separation between the particles rf = 10 cm = 01 m
Initial potential energy of the system Ui = 1
41205871205760
11990211199022
119903119894=
1
41205871205760
1199022
infin= 0 J
Final potential energy of the system Uf = 1
41205871205760
11990211199022
119903119891= 9 x 109 x
2 119909 10minus5 2
01= 9 x 1010 x 4 x 10-10 = 36 J
Increase in potential energy of the system ∆U = Uf ndash Ui = 36 ndash 0 = 36 J
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
62 Some equipotential surfaces are shown in figure What can you say about the magnitude and the direction of the
electric field
Solution
(a) Magnitude of electric field E = ∆119881
∆119903=
20minus10
01 sin 30=
10
01 119909 05= 200 Vm
Electric field is directed from high potential to low potential and perpendicular to the equi-potentials 1200
(b) The field is directed radially outward (in the direction of decreasing potential)
From the diagram potential at any point V = 6
119903 (119888119898)
Electric field intensity E = minus119889119881
119889119903= minus minus
6
1199032 = 120788
119955120784 Vcm
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
63 Consider a circular ring of radius r uniformly charged with linear charge density λ Find the electric potential at a
point on the axis at a distance x from the centre of the ring Using this expression for the potential find the electric
field at this point
Solution
Radius of the ring r
Linear charge density λ
Consider an elemental part (point charge) of the ring of length d119897 (charge dq = λ d119897)
Potential due to elemental ring on the axis dV = 119896 119889119902
119877=
119896λ 119889119897
1199092+1199032
Total potential due to the ring V = 119889119881 = 119896λ
1199092+1199032 119889119897 = 119896λ
1199092+1199032(2120587r) =
1
41205871205760
λ
1199092+1199032(2120587r) =
119955λ
120784120634120782 119961120784+119955120784
Electric field at the given point E = minus119889119881
119889119909= minus
119889
119889119909
119903λ
21205760 1199092+1199032= minus
119889
119889119909
λ119903
212057601199032 + 1199092 minus12
E = minusλ119903
21205760minus
1
21199032 + 1199092 minus
3
2(2119909) = 119955119961120640
120784120634120782 119955120784+119961120784 120785120784
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
64 An electric field of magnitude 1000 N Cminus1 is produced between two parallel plates having a separation of 20 cm
as shown in figure (a) what is the potential difference between the plates (b) with what minimum speed should an
electron be projected from the lower plate in the direction of the field so that it may reach the upper plate (c) suppose
the electron is projected from the lower plate with the speed calculated in part (d) the direction of projection makes an
angle of 60o with the field Find the maximum height reached by the electron
Solution
Magnitude of the electric field E = 1000 NC
Separation between the parallel plates d = 2 cm = 2 x 10-2 m
(a) Potential difference between the plates ∆V = Ed = 1000 x 2 x 10-2 = 20 volt
(b) Work done by electric field on the electron w = minusEqd = minus1000 x 16 x 10-19 x 2 x 10-2 = minus32 x 10-18 J
change in kinetic energy of the electron ∆k = kf ndash ki = 0 minus1
2mv2 = minus05 x 911 x 10-31 v2 = minus455 x 10-31 v2
work energy theorem w = ∆k rArr minus32 x 10-18 = minus455 x 10-31 v2 rArr v = 7 119909 1012 = 265 x 106 ms
(c) Acceleration provided by electric field a = 119865
119898=
119864119902
119898= 103 x 176 x 1011 = 176 x 1014 ms
Maximum height reached by the electron H = 11990721199041198941198992120579
2119886=
7 119909 1012 119909 12 2
2 119909 176 119909 1014 = 05 cm
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
65 A uniform field of 20 N Cminus1 exists in space in x-direction (a) taking the potential at the origin to be zero write an
expression for the potential at a general point (x y z) (b) at which points the potential is 25 V (c) if the potential at
the origin is taken to be 100 V what will be the expression for the potential at a general point (d) what will be the
potential at a general point (d) what will be the potential at the origin if the potential at infinity is taken to be zero Is
it practical to chose the potential at infinity to be zero
Solution
Magnitude of electric field E = 2 NC
Given potential at the origin is zero
Relation between potential and field intensity E = minus∆119881
∆119909rArr V ndash 0 = minusE ∆119909 rArr V = minus2 (Vm) x
The points at which potential is 25 V 25 = minus2x rArr x = minus125 m
Given potential at the origin is 100 volt
Potential at any point V ndash 100 = minus2x rArr V = 100 ndash 2x
Given potential at infinity is zero
Potential at the origin 2 = 0minus119881
infinrArr V = infinity
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
66 How much work has to be done in assembling three charged particles at the vertices of an equilateral triangle as
shown in figure
Solution
Initially the particles are at infinite separation potential energy is zero Ui = 0
Potential energy in the final configuration Uf = 1
41205871205760
11990211199022
11990312+
1
41205871205760
11990221199023
11990323+
1
41205871205760
11990231199021
11990331
Uf = 9 119909 109
01[2 x 10-5 x 4 x 10-5 + 4 x 10-5 x 3 x 10-5 + 2 x 10-5 x 3 x 10-5]
Uf = 9 x 1010 x 10-10 [8 + 12 + 6] = 234 J
Work done in assembling the charges w = Uf ndash Ui = 234 ndash 0 = 234 J
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
67 The kinetic energy of a charged particle decreases by 10 J as it moves from a point at potential 100 V to a point at
potential 200 V Find the charge on the particle
Solution
Potential of initial position V1 = 100 volt
Potential of final position V2 = 200 volt
Decrease in kinetic energy of the particle ∆k =10 J
Work done by the field w = Fd = Eqd = ∆119881
119889qd =
200minus100
119889qd = 100q
Work energy theorem w = ∆k rArr 10 = 100q rArr q = 01 C
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
68 Two identical particles each having a charge of 20 times 10minus4C and mass of 10 g are kept a separation of 10 cm and
then released What would be the speeds of the particles when the separation becomes large
Solution
Charge on each particle q1 = q2 = q = 2 x 10-4 C
Mass of each particle m1 = m2 = m = 10 g = 10 x 10-3 kg
Initial separation between the particles r1 = 10 cm
Final separation between the particles r2 = large (infin)
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
4 119909 10minus8
10 119909 10minus2 = 3600 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
4 119909 10minus8
infin= 0 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 10 x 10-3 v2 = 10-2 v2
Conservation of energy U1 + k1 = U2 + k2 rArr 3600 + 0 = 0 + 10-2 v2 rArr v = 600 ms
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
69 Two particles have equal masses of 50 g each and opposite charges of +40 times 10minus5C and minus40 times 10minus5C They are
released from rest with a separation of 10 m between them Find the speeds of the particles when the separation is
reduced to 50 cm
Solution
Mass of each particle m1 = m2 = m = 5 g = 5 x 10-3 kg
Charge on the first particle q1 = 4 x 10-5 C
Charge on the second particle q2 = minus40 times 10minus5 C
Initial separation between the charges r1 = 1 m = 100 cm
Final separation between the charges r2 = 50 cm
Initial potential energy of the system U1 = 1
41205871205760
11990211199022
1199031= 9 x 109 x
minus16 119909 10minus10
100 119909 10minus2 = 144 x 10-1 = minus144 J
Final potential energy of the system U2 = 1
41205871205760
11990211199022
1199032= 9 x 109 x
minus16 119909 10minus10
50 119909 10minus2 = minus288 J
Initial kinetic energy of the system k1 = 0 J
Final kinetic energy of the system k2 = 2 1
2mv2 = mv2 = 5 x 10-3 v2
Conservation of energy U1 + k1 = U2 + k2 rArr minus144 + 0 = minus288 + 5 x 10-3 v2 rArr v = 144 119909 103
5= 536 ms
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
70 A sample of HCI gas is placed in an electric field of 25 times 104 N Cminus1 The dipole moment of each HCI molecule is
34 times 10minus30Cm Find the maximum torque that can act on a molecule
Solution
Electric field intensity E = 25 x 104 NC
Dipole moment of the molecule P = 34 x 10-30 Cm
Torque on the dipole τ = PE sin θ
Maximum torque on the dipole τmax = PE = 34 x 10-30 x 25 x 104 = 85 x 10-26 Nm
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
71 Two particles A and B having opposite charges 20 times 10minus6C and minus20 times 10minus6C are placed at a separation of
10 cm (a) write down the electric dipole moment of this pair (b) calculate the electric field at a point on the axis of
the dipole 10 m away from the centre (c) calculate the electric field at a point on the perpendicular bisector of the
dipole and 10 m away from the centre
Solution
Charge on the first particle qA= 2 x 10-6 C
Charge on the second particle qB = minus20 times 10minus6 C
Separation between the charges r = 1 cm = 001 m
(a) Electric dipole moment of the combination P = q(d) = 2 x 10-6 x 001 = 2 x 10-8 Cm
(b) Electric field intensity on the axis of the dipole Eax = 1
41205871205760
2119875
1198893 = 9 x 109 x 2 119909 2 119909 10minus8
1= 36 x 101 = 360 NC
(c) Electric field intensity on the equatorial line of the dipole Eeq = 1
41205871205760
119875
1198893 = 9 x 109 x 2 119909 10minus8
1= 18 x 101 = 180 NC
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
72 Three charges are arranged on the vertices of an equilateral triangle as shown in figure Find the dipole moment of
the combination
Solution
Dipole moment of AB PAB = qd (from A to B)
Dipole moment of BC PBC = qd (from C to B)
Angle between the two moments 600
Resultant dipole moment P = 2PAB cos 300 = 2qd 3
2= 120785 qd (along the angular bisector)
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
73 Find the amplitude of the electric field at the point P in the configuration shown in figure for dgtgta Take 2qa = p
Solution
(a) Electric field intensity due to point charge E = 120783
120786120645120634120782
119954
119941120784
(b) Electric field intensity on the equatorial line of a dipole Eeq = 120783
120786120645120634120782
119927
119941120785 (d gtgt a)
(c) Field due to middle positive charge E1 = 1
41205871205760
119902
1198892 (vertically up)
field due to dipole E2 = 1
41205871205760
119875
1198893 (horizontal from +ve charge to negative charge left)
resultant field E = 11986412 + 1198642
2 = 120783
120786120645120634120782
120783
119941120785 119954120784119941120784 + 119927120784
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
74 Two particles carrying charges ndashq and +q and having equal masses m each are fixed at the ends of a light rod of
length lsquoarsquo to form a dipole The rod is clamped at an end and is placed in a uniform electric field E with the axis of the
dipole along the electric field The rod is slightly titled and then released Neglecting gravity find the time period of
small oscillations
Solution
Let the rod be clamped at the end of negative charge
Force acting on the positive charge F = Eq (in the direction of the field)
Let the angular displacement of the rod be lsquoθrsquo
Torque on the rod due to electrostatic force τ = F (perpendicular distance)
τ = Eq (a sin θ) = Eqa θ [for small angles sin θ = θ]
Iα = Eqa θ rArr α = 119864119902119886
119868θ =
119864119902119886
1198981198862 θ = 119864119902
119898119886θ [α = ω2 θ]
Angular frequency of the system ω = 119864119902
119898119886rArr
2120587
119879=
119864119902
119898119886rArr T = 120784120645
119950119938
119916119954
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential
75 Assume that each atom in a copper wire contributes one free electron Estimate the number of free electrons in a
copper wire having a mass of 64 g (take the atomic weight of copper to be 64 g molminus1)
Solution
Mass of copper wire m = 64 g
Atomic weight of copper M = 64 gmol
Number of moles n = 119898
119872=
64
64= 01
Number of copper atoms N = nNA = 01 x 6023 x 1023 = 6023 x 1022
Given Each copper atoms contributes one free electron
Number of free electrons Ne = 6 x 1022
Electric Field amp Potential