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General Physics (PHY 2140)

IntroductiIntroductionon Syllabus and teaching strategy

Electricity and Magnetism• Properties of electric charges• Insulators and conductors• Coulomb’s law

Lecture 1. Chapter 15

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Lecturer: Prof. Alexey A. Petrov, Room 260 Physics Building, Phone: 313-577-2739, or 313-577-2720 (for messages)e-mail: apetrov@physics.wayne.edu, Web: http://www.physics.wayne.edu/~apetrov/

Office Hours: MWF 10:40 AM -11:35 AM, General Lectures, Room 150Tuesday 1:00-2:00 PM, on main campus, Physics Building, Room 260, or by appointment. 

Grading: Reading Quizzes (bonus) 5%Quiz section performance/Homework 10%Best Hour Exam 25%Second Best Hour Exam 25%Final 40%

Reading Quizzes: It is important for you to come to class prepared! BONUS POINTS: Reading Summaries

Homework and QUIZ Sessions: The quiz sessions meet once a week; 5 quizzes will count towards your grade.

BONUS POINTS: will be assigned by your quiz instructor.

Hour Exams and Final Exam: There will be THREE (3) Hour Exams and one Final Exam.Additional BONUS POINTS will be given out for class activity.

Online Content: Lecture Online will be made available to you as a supplemental reference. 

Syllabus and teaching strategy

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IntroductionIntroduction

Knowledge of electricity dates back to Greek antiquity Knowledge of electricity dates back to Greek antiquity (700 BC).(700 BC).Began with the realization that amber (fossil) when Began with the realization that amber (fossil) when rubbed with wool, attracts small objects.rubbed with wool, attracts small objects.This phenomenon is not restricted to amber/wool but may This phenomenon is not restricted to amber/wool but may occur whenever two non-conducting substances are occur whenever two non-conducting substances are rubbed together.rubbed together.

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15.1 Properties of Electric Charges - 15.1 Properties of Electric Charges - DiscoveryDiscovery

Observation of “Static Electricity”Observation of “Static Electricity” A comb passed though hair attracts small pieces of paper.A comb passed though hair attracts small pieces of paper. An inflated balloon rubbed with wool.An inflated balloon rubbed with wool.

““Electrically charged”Electrically charged” Rub shoes against carpet/car seat to charge your body.Rub shoes against carpet/car seat to charge your body. Remove this charge by touching another person/a piece of Remove this charge by touching another person/a piece of

metal.metal.

Two kinds of chargesTwo kinds of charges Named by Benjamin Franklin (1706-1790) as Named by Benjamin Franklin (1706-1790) as positivepositive and and

negativenegative..

Like charges repel one another and unlike charges Like charges repel one another and unlike charges attract one anotherattract one another..

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15.1 Properties of Electric Charges15.1 Properties of Electric ChargesNature of Electrical ChargeNature of Electrical Charge

Origin of charge is at the atomic level.Origin of charge is at the atomic level. Nucleus : “robust”, Nucleus : “robust”, positivepositive.. Electrons : mobile, Electrons : mobile, negativenegative..

Usual state of the atom is Usual state of the atom is neutralneutral..Charge has natural tendency to be transferred between Charge has natural tendency to be transferred between unlike materials.unlike materials.Electric charge is however always Electric charge is however always conservedconserved in the in the process.process.

Charge is Charge is not creatednot created.. Usually, Usually, negative chargenegative charge is transferred from one object to the is transferred from one object to the

other.other.

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15.1 Properties of Electric Charges15.1 Properties of Electric ChargesQuantizationQuantization

Robert Millikan found, in 1909, that charged objects may only have Robert Millikan found, in 1909, that charged objects may only have an integer multiple of a fundamental unit of charge.an integer multiple of a fundamental unit of charge.

Charge is Charge is quantizedquantized.. An object may have a charge An object may have a charge e, or e, or 2e, or 2e, or 3e, etc but not say 3e, etc but not say

1.5e.1.5e. Proton has a charge Proton has a charge +1e+1e.. Electron has a charge Electron has a charge –1e–1e.. Some particles such a Some particles such a neutronneutron have no (zero) charge. have no (zero) charge. A neutral atom has as many positive and negative charges.A neutral atom has as many positive and negative charges.

UnitsUnits In SI, electrical charge is measured in coulomb ( C).In SI, electrical charge is measured in coulomb ( C). The value of |The value of |e| = 1.602 19 x 10e| = 1.602 19 x 10-19-19 C C..

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15.2 Insulators and Conductors –Material 15.2 Insulators and Conductors –Material classificationclassification

Materials/substances may be classified according to their capacity Materials/substances may be classified according to their capacity to carry or to carry or conductconduct electric charge electric charge ConductorsConductors are material in which electric charges move freely. are material in which electric charges move freely.Insulator Insulator are materials in which electrical charge do not move freely.are materials in which electrical charge do not move freely.

Glass, Rubber are good insulators.Glass, Rubber are good insulators. Copper, aluminum, and silver are good conductors.Copper, aluminum, and silver are good conductors.

Semiconductors are a third class of materials with electrical Semiconductors are a third class of materials with electrical properties somewhere between those of insulators and conductors.properties somewhere between those of insulators and conductors.

Silicon and germanium are semiconductors used widely in the Silicon and germanium are semiconductors used widely in the fabrication of electronic devices.fabrication of electronic devices.

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Mini-quiz:Mini-quiz:

Identify substances or materials that can be classified asIdentify substances or materials that can be classified as Conductors ?Conductors ? Insulators?Insulators?

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15.2 Insulators and Conductors – Charging 15.2 Insulators and Conductors – Charging by Conduction.by Conduction.

Consider negatively charge rubber rod brought into contact Consider negatively charge rubber rod brought into contact with a neutral conducting but insulated sphere.with a neutral conducting but insulated sphere.Some electrons located on the rubber move to the sphere.Some electrons located on the rubber move to the sphere.Remove the rubber rod.Remove the rubber rod.Excess electrons left on the sphere. It is Excess electrons left on the sphere. It is negatively chargednegatively charged..This process is referred as This process is referred as charging by conductioncharging by conduction..

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15.2 Insulators and Conductors – 15.2 Insulators and Conductors – Earth/Ground.Earth/Ground.

When a conductor is connected to Earth with a When a conductor is connected to Earth with a conducting wire or pipe, it is said to be conducting wire or pipe, it is said to be groundedgrounded..Earth provides a Earth provides a quasi infinite reservoirquasi infinite reservoir of electrons: can of electrons: can accept or supply an unlimited number of electrons.accept or supply an unlimited number of electrons.

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15.2 Insulators and Conductors – Charging 15.2 Insulators and Conductors – Charging by Induction.by Induction.

Consider a negatively charged rubber rod Consider a negatively charged rubber rod brought brought nearnear a neutral conducting sphere a neutral conducting sphere insulated from the ground.insulated from the ground.Repulsive force between electrons causes Repulsive force between electrons causes redistribution of charges on the sphere.redistribution of charges on the sphere.Electrons move away from the rod leaving an Electrons move away from the rod leaving an excess of positive charges near the rod. excess of positive charges near the rod. Connect a wire between sphere and Earth on Connect a wire between sphere and Earth on the far side of the sphere.the far side of the sphere.Repulsion between electrons cause electrons Repulsion between electrons cause electrons to move from sphere to Earth.to move from sphere to Earth.Disconnect the wire.Disconnect the wire.The sphere now has a positive net charge.The sphere now has a positive net charge.This process is referred as This process is referred as charging by charging by inductioninduction..Charging by induction requires no contact Charging by induction requires no contact with the object inducing the charge.with the object inducing the charge.

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15.2 Insulators and Conductors – Charging 15.2 Insulators and Conductors – Charging by Induction.by Induction.

Consider a negatively charged rubber rod Consider a negatively charged rubber rod brought brought nearnear a neutral conducting sphere a neutral conducting sphere insulated from the ground.insulated from the ground.Repulsive force between electrons causes Repulsive force between electrons causes redistribution of charges on the sphere.redistribution of charges on the sphere.Electrons move away from the rod leaving an Electrons move away from the rod leaving an excess of positive charges near the rod. excess of positive charges near the rod. Connect a wire between sphere and Earth on Connect a wire between sphere and Earth on the far side of the sphere.the far side of the sphere.Repulsion between electrons cause electrons Repulsion between electrons cause electrons to move from sphere to Earth.to move from sphere to Earth.Disconnect the wire.Disconnect the wire.The sphere now has a positive net charge.The sphere now has a positive net charge.This process is referred as This process is referred as charging by charging by inductioninduction..Charging by induction requires no contact Charging by induction requires no contact with the object inducing the charge.with the object inducing the charge.

Q: How does this mechanism work is we bring is a positively charged glass rod instead?

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15.2 Insulators and Conductors – 15.2 Insulators and Conductors – Polarization.Polarization.

Polarization is realignment of charge within individual Polarization is realignment of charge within individual molecules.molecules.Produces induced charge on the surface of insulators.Produces induced charge on the surface of insulators.how e.g. rubber or glass can be used to supply how e.g. rubber or glass can be used to supply electrons.electrons.

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Mini-quizMini-quizA positively charged object hanging from a string is brought near a non A positively charged object hanging from a string is brought near a non conducting object (ball). The ball is seen to be attracted to the object. conducting object (ball). The ball is seen to be attracted to the object.

1.1.Explain why it is not possible to determine whether the object is Explain why it is not possible to determine whether the object is negatively charged or neutral.negatively charged or neutral.2.2.What additional experiment is needed to reveal the electrical charge What additional experiment is needed to reveal the electrical charge state of the object?state of the object?

? +

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Explain why it is not possible to determine whether Explain why it is not possible to determine whether the object is negatively charged or neutral.the object is negatively charged or neutral.

Two possibilities: Two possibilities: Attraction between objects of unlike charges.Attraction between objects of unlike charges.

Attraction between a charged object and a neutral object subject Attraction between a charged object and a neutral object subject to polarization.to polarization.

- +

+----

++

++

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What additional experiment is needed to reveal the What additional experiment is needed to reveal the electrical charge state of the object?electrical charge state of the object?

Two Experiments:Two Experiments: Bring known neutral ball near the object and observe whether there is Bring known neutral ball near the object and observe whether there is

an attraction.an attraction.

Bring a known negatively charge object near the first one. If there is an Bring a known negatively charge object near the first one. If there is an attraction, the object is neutral, and the attraction is achieved by attraction, the object is neutral, and the attraction is achieved by polarization.polarization.

? 0

-++++

+--+-

+-

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15.3 Coulomb’s Law - Observation15.3 Coulomb’s Law - Observation

Charles Coulomb discovered in 1785 the fundamental law of Charles Coulomb discovered in 1785 the fundamental law of electrical force between two stationary charged particles.electrical force between two stationary charged particles.An electric force has the following properties:An electric force has the following properties:

Inversely proportional to the square of the separation, r, between the Inversely proportional to the square of the separation, r, between the particles, and is along a line joining them.particles, and is along a line joining them.

Proportional to the product of the magnitudes of the charges |qProportional to the product of the magnitudes of the charges |q11| and |q| and |q22| | on the two particles. on the two particles.

Attractive if the charges are of opposite sign and repulsive if the charges Attractive if the charges are of opposite sign and repulsive if the charges have the same sign.have the same sign.

q1 q2

r

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15.3 Coulomb’s Law – Mathematical 15.3 Coulomb’s Law – Mathematical FormulationFormulation

kkee known as the Coulomb constant. known as the Coulomb constant.

Value of Value of kkee depends on the choice of units. depends on the choice of units.

SI unitsSI units Force: the Newton (N)Force: the Newton (N) Charge: the coulomb ( C).Charge: the coulomb ( C). Current: the ampere (A =1 C/s).Current: the ampere (A =1 C/s). Distance: the meter (m).Distance: the meter (m).

EExperimentally measurement: xperimentally measurement: kkee = 8.9875 = 8.9875101099 Nm Nm22/C/C22..

Reasonable approximate value: kReasonable approximate value: kee = 8.99 = 8.99101099 Nm Nm22/C/C22..

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Charge and Mass of the Electron, Proton Charge and Mass of the Electron, Proton and Neutron.and Neutron.ParticleParticle Charge ( C)Charge ( C) Mass (kg)Mass (kg)

ElectronElectron -1.60 -1.60 1010+19+19 9.11 9.11 1010+31+31

ProtonProton +1.60 +1.60 1010+19+19 1.67 1.67 1010+27+27

NeutronNeutron 00 1.67 1.67 1010+27+27

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ExampleExample

1e = -1.60 1e = -1.60 1010-19-19 c cTakes 1/e=6.6 Takes 1/e=6.6 10101818 protons to create a total charge of 1C protons to create a total charge of 1CNumber of free electrons in 1 cmNumber of free electrons in 1 cm33 copper ~ 10 copper ~ 102323

Charge obtained in typical electrostatic experiments with Charge obtained in typical electrostatic experiments with rubber or glass 10rubber or glass 10-6-6 C = 1 C = 1 ccA very small fraction of the total available chargeA very small fraction of the total available charge

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15.3 Coulomb’s Law – Remarks15.3 Coulomb’s Law – Remarks

The electrostatic force is often called Coulomb force.The electrostatic force is often called Coulomb force.It is a force: It is a force:

a magnitude a magnitude a direction.a direction.

Second example of action at a distance.Second example of action at a distance.

++

r

q1

q2F21

F21 +-

r

q1

q2F21F21

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Mini-QuizMini-Quiz

Name the first action at a distance force you have Name the first action at a distance force you have encountered in physics so far.encountered in physics so far.

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Example: Electrical ForceExample: Electrical Force

Question:Question:The electron and proton of a hydrogen atom are separated (on the The electron and proton of a hydrogen atom are separated (on the average) by a distance of about 5.3x10average) by a distance of about 5.3x10-11-11 m. Find the magnitude of the m. Find the magnitude of the electric force that each particle exerts on the other.electric force that each particle exerts on the other.

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Question:Question:The electron and proton of a hydrogen atom are separated (on the average) by The electron and proton of a hydrogen atom are separated (on the average) by

a distance of about 5.3x10a distance of about 5.3x10-11-11 m. Find the magnitude of the electric force that m. Find the magnitude of the electric force that each particle exerts on the other.each particle exerts on the other.

Observations:Observations:We are interested in finding the magnitude of the force between two particles of known charge, and a given distance of each other.We are interested in finding the magnitude of the force between two particles of known charge, and a given distance of each other.The magnitude is given by Coulomb’s law.The magnitude is given by Coulomb’s law.

qq11 =-1.60x10 =-1.60x10+19+19 C C

qq22 =1.60x10 =1.60x10+19+19 C C

r = 5.3x10r = 5.3x10+11+11 m m

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Question:Question:The electron and proton of a hydrogen atom are separated (on the average) by The electron and proton of a hydrogen atom are separated (on the average) by a distance of about 5.3x10a distance of about 5.3x10-11-11 m. Find the magnitude of the electric force that m. Find the magnitude of the electric force that each particle exerts on the other.each particle exerts on the other.

Observations:Observations:We are interested in finding the magnitude of the force between two We are interested in finding the magnitude of the force between two particles of known charge, and a given distance of each other.particles of known charge, and a given distance of each other.The magnitude is given by Coulomb’s law.The magnitude is given by Coulomb’s law.qq11 =-1.60x10 =-1.60x10-19-19 C Cqq22 =1.60x10 =1.60x10-19-19 C Cr = 5.3x10r = 5.3x10-11-11 m m

Solution:Solution:

Attractive force with a magnitude of 8.2x10Attractive force with a magnitude of 8.2x10-8-8 N. N.

2

2

22 199 8

22 11

1.6 108.99 10 8.2 10

5.3 10Nm

e e C

CeF k N

r m

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Superposition PrincipleSuperposition Principle

From observations: one finds that whenever multiple From observations: one finds that whenever multiple charges are present, the net force on a given charge is charges are present, the net force on a given charge is the the vectorvector sum of all forces exerted by other charges. sum of all forces exerted by other charges.Electric force obeys a Electric force obeys a superposition principlesuperposition principle..

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Example: Using the Superposition PrincipleExample: Using the Superposition Principle

Consider three point charges at the corners of a triangle, as shown Consider three point charges at the corners of a triangle, as shown below. Find the resultant force on qbelow. Find the resultant force on q33 if if

qq11 = 6.00 x 10 = 6.00 x 10-9-9 C C

qq2 2 = -2.00 x 10= -2.00 x 10-9-9 C C

qq3 3 = 5.00 x 10= 5.00 x 10-9-9 C C

+ x

y

- +q2

q1

3.00 m

4.00 mq3

F32

F31

37.0o

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Consider three point charges at the corners of a triangle, as shown Consider three point charges at the corners of a triangle, as shown below. Find the resultant force on qbelow. Find the resultant force on q33..

Observations:Observations:The superposition principle tells us that the net force on qThe superposition principle tells us that the net force on q33 is the vector sum is the vector sum of the forces Fof the forces F3232 and F and F3131..The magnitude of the forces FThe magnitude of the forces F3232 and F and F3131 can calculated using Coulomb’s law. can calculated using Coulomb’s law.

+ x

y

- +q2

q1

3.00 m

4.00 mq3

F32

F31

37.0o

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Consider three point charges at the corners of a triangle, as shown Consider three point charges at the corners of a triangle, as shown below. Find the resultant force on qbelow. Find the resultant force on q33..

5.00 m

2

2

2

2

9 93 2 9 9

32 22

9 93 1 9 8

31 22

932 31

931

2 2 9

5.00 10 2.00 108.99 10 5.62 10

4.00

5.00 10 6.00 108.99 10 1.08 10

5.00

cos37.0 3.01 10

sin 37.0 6.50 10

7.16 10

Nme C

Nme C

ox

oy

x y

C Cq qF k N

r m

C Cq qF k N

r m

F F F N

F F N

F F F N

65.2o

Solution:Solution:

+ x

y- +q2

q1

3.00 m

4.00 mq3

F32

F31

37.0o