physics - NotNul

PHYSICS

Volume Two: New syllabus

THIRD EDITION

REFRESHER COURSE

FOR PUC – II

by

Expert Publishing House, Mangaluru

Prof. HARISH BHAT HOD, Dept. of Physics

S. R. PU College Mangaluru

Prof. VADIRAJ RAO HOD, Dept. of Physics S. D. P. T. PU College

Kateel

Prof. VIJETH NAYAK S. R HOD, Dept. of Physics

Expert P. U. Science College Mangaluru

Prof. PRASHANTH RAO Dept. of Physics

Canara PU College Mangaluru

Physics VOLUME II REFRESHER COURSE book for PUC-II (as per NEW syllabus) Published by Expert Publishing House, Mangaluru-575 003 Edition Third: April 2015 Earlier Karnataka Syllabus Editions – Vol. II: First Published 2008; Seventh Edition 2012 Cover Design and Graphics: Expert Computer Systems & Mudra Printers, Mangaluru Copyright © Publisher All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, photocopying included, without permission in writing from the Publisher. DTP Typesetting & Production Team: Expert Publishing House, Mangaluru 575 003,

Ph: 0824-249 5796. [email protected]

Printed at: Prakash Offset Printers, Mangaluru

FOREWORD Dear readers, For me and EXEPRT, it is an eve of gaiety and splendor to bring to

stands the Second Edition of our book ‘A Refresher Course in Physics’ for the students of Second year PU (Karnataka Board). This pioneering work is the result of the collective and dedicated efforts by the best minds in the field of college education.

This book is an ensemble of pearls of wisdom picked carefully by

EXPERT from the various oceans of knowledge, unmatched in quality and style. It shall beyond doubt prove a shot in the arm as far as the scoring of students is considered.

The Third edition of this book has been put to print as hands on

approach to solve the difficulties faced by students while preparing for II PU Board examination. Utmost care is taken to ensure that their hard work gets translated into full success. Optimal use of experience and knowledge has been made to ensure that the book is an unfailing and indispensable companion to students as well as teachers. I am confident that any student who makes use of this book is most likely to get 100/100 in the Board Exam.

I hope that all P U teachers and in particular, the students who aim

high and earnestly desire to achieve cent percent success will make best use of this book and join the league of excellence.

I heartily thank Prof. Harish Bhat, Prof. Vadiraj Rao, Prof.

Vijeth Nayak S. R and Prof. Prashanth Rao whose names are synonymous with classes of quality; for the painstaking effort they have put in as authors.

NARENDRA L NAYAK; B. E (Mech) Managing Partner, Expert Publishing House &

Chairman, Expert Educational & Charitable Foundation, Mangaluru

ALTERNATING CURRENT UNIT – VI

EXPERT PHYSICS – II PUC 262

Q. Define period (T) of an ac Ans: It is the time taken by the ac to complete one cycle of variation. Q. Define frequency (f) of an ac. Ans: It is the number of cycles completed by the ac in one second.

∴ f=T1

Q. Define peak value or amplitude of an ac (I0 or V0). [Model paper] Ans: It is the maximum instantaneous value attained by an alternating voltage or

current in either half cycle of ac. Q. Give the relation between angular frequency and frequency of AC

Ans: We have ω = tθ . For one complete cycle of variations, θ = 2π and t = T

∴ ω =T2π or ω = 2πf (∵ f=

T1 )

ω is called angular frequency of AC. Q. Define Instantaneous value of AC Ans: It is the value of alternating voltage or current at a given instant of time. The

equations V = V0sinωt and I = I0sinωt are the expressions for instantaneous value of voltage and current respectively.

Q. Define mean value of AC (Vm or Vav and Im or Iav) Ans: It is the mean or average of instantaneous values of AC taken over one half

cycle. For a sinusoidal AC, Vm= ⎟⎠⎞

⎜⎝⎛π2 Vo and Im = ⎟

⎠⎞

⎜⎝⎛π2 Io

Q. What is the mean value of AC taken over one complete cycle Ans: Mean value of AC taken over one complete cycle is zero. Q. Define root mean square value of AC (Irms or Vrms) Ans: It is the square root of mean of squares of instantaneous values of AC taken

over one complete cycle.

For a sinusoidal AC, Vrms =2

V0 and Irms= 2

I0

OR

ALTERNATING CURRENT CHAPTER – 7



RMS value of AC is defined as that steady current which when flowing through a

given resistor produces the same quantity of heat in a certain time as that produced by AC flowing through the same resistor for the same time.

Q. Mention the expression for rms value of alternating voltage interms of peak

value. [March 2014]

Ans: Vrms =2

V0

Q. What is the relation between ‘rms’ and ‘average’ value of sinusoidal A.C. voltage?

Ans: Vav= π

22 Vrms

Q. The peak value of an ac voltage is 141.4 V. What is its rms value?

Ans: Vrms = ==414.1

4.1412

Vo 100V

Q. The rms value of current in a circuit is 1A. What is its peak value?

Ans: Irms = 2

Io or Io = Irms 2 = 1 × 1.414 = 1.414 A

Q. Alternating current is represented by the equation I = 10sin (314t). What is the

frequency of the ac? Ans: The given equation is of the form I = Iosin ωt

On comparing we get ω = 314 or 2πf = 314 or f = 2

10014.32

314=

×= 50 Hz

Q. Define phase of AC and explain Ans: It is defined as the fraction of time period that has elapsed since the voltage or

current last passed through zero value in positive direction. In the AC wave form of V = V0sinωt and I = I0sinωt Phase of AC at A = 0 ×T=0

Phase of AC at B = )T(41

Phase of AC at C = )T(21

Phase of AC at D = )T(43

Phase of AC at E = T

E D C

B A

V or I

t T

V I



V

ωt I

If the AC wave form is a graph showing the variation of V or I versus ωt. The phase of AC is expressed in terms of fraction of 2π.

Phase of AC at A = 0 = 0 × (2π)

Phase of AC at B =41

2=

π (2π)

Phase of AC at C = π =21 (2π)

Phase of AC at D = )2(43

23

π=π

Phase of AC at E = 2π =1 × (2π) Q. Obtain an expression for the current in an ac circuit containing a pure

resistance. [Model Paper] Ans: Consider a wire, of resistance R connected in series with a sinusoidal AC source. Let V = Vosinωt --- (1) be the p.d applied between the ends of the resistance wire at

any instant ‘t’. By Ohm’s law, I =RV

I = R

tsinV0 ω

∴I = Iosinωt ---- (2) From (1) and (2), we find that V and I are in phase with each other. Q. What are phasors? Ans:A phasor is a vector which rotates about the origin with angular speed ω as

shown. The magnitudes of the phasors V and I represent the

peak value or amplitude of the alternating quantities. The vertical components of phasors represent the instantaneous value of ac. The phasor diagram shown is for the case of ac applied to a resistor.

~

R

I

ωt1 ω t1

V

O

V or I ↑

ωt → T

V I

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