BRIEF LESSONS OF

PHYSICS

I

© Copyright, 2020, Deepak Kumar Singh All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy recording, or any information storage or retrieval system, without permission in writing from the publisher. The opinions/ contents expressed in this book are solely of the author and do not represent the opinions/ standings/ thoughts of Publications Name. No responsibility or liability is assumed by the publisher for any injury damage or financial loss sustained to persons or property form the use of the information, personal or otherwise, either directly or indirectly. While every effort has been made to ensure reliability and accuracy of the information within, all liability, negligence or otherwise, form any use, misuses or abuse of the operation of any methods, strategies, instructions or ideas contained in the material herein is the sole responsibility of the reader. Any copyrights not held by publisher are owned by their respective authors. All information is generalized, presented informational purposes only and presented “as is” without warranty or guarantee of any kind. All trademarks and brands referred to in this book are for illustrative purposes only, are the property of their respective owners and not affiliated with this publication in any way. Any trademarks are being used without permission and the publication of the trademark is not authorized by associated with or sponsored by the trade mark owner.

ISBN: 978-93-89960-83-9 Publishing Year 2020

Published and Printed by: Rudra Publications Office Address: Kapil Nagar, New Sarkanda, Bilaspur, Chhattisgarh – 495001 Phones: +91 9522285558 +91 9522263336 Email: [email protected] Website: www.rudrapublications.com

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BRIEF LESSONS OF

PHYSICS

By Deepak Kumar Singh M.Sc. Electronics & Communication (Double Gold Medalist)

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Dedicated To

My Grandparents, Late Mrs. & Mr. D. Singh And My Parents, Mrs. Savitri Devi & Mr. Arun Kumar Singh

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Acknowledgements

This book has grown out of the notes developed by me over years of teaching this discipline to class 12 students of various CBSE school and postgraduate students of Electronics and Communication of physics department at Ranchi University, Ranchi. I would like to thank all those who directly or indirectly contributed in making this venture a success. I express my deep regards for Dr. Arun Kumar, a renowned physics associate professor and author under whose utmost care I have grown up as a student, a teacher and as an author. Sincere and specific mention must be made of Afzal Ansari. Thanks are also due to my students, past and present. Finally, I would like to thank Mr. N. N. Srivastava, principal of D.A.V Model School CFRI, Dhanbad and Mr. Sukhdev Singh for building keen interest of physics in me and for guiding me in all aspects when I was in school.

Deepak Kumar Singh

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Contents: Unit I: Electrostatics 1.

Electric Charges and Fields

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Electrostatic Potential and Capacitance

Unit II: Current Electricity 3.

Current Electricity

Unit III: Magnetic Effects of Current & Magnetism 4.

Moving Charge and Magnetism

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Magnetism and Matter

Unit IV: Electromagnetic Induction and Alternating Current 6.

Electromagnetic Induction

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Alternating Current

Unit V: Electromagnetic Waves 8.

Electromagnetic Waves

Unit VI: Optics 9.

Ray Optics and Optical Instruments

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Wave Optics

Unit VII: Dual Nature of Matter and Radiation 11.

Dual Nature of Matter and Radiation

Unit VIII: Atoms and Nuclei 12.

Atoms

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Nuclei

Unit IX: Electronic Devices 14.

Electronic Devices

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CHAPTER 1 Electric charges and fields Charge: Intrinsic property of all the elementary particles, which give rise to electric force between various objects. or Electric charge is a physical quantity due to which electrical and other related effects are produced in the matter. S.I Unit: coulomb or C. Methods of Charging: (1) Charging by Friction: when two insulating bodies are rubbed against each other, there is a transfer of electron from one body to the other. The body which had a lower work function, loses electron and becomes positively charged, while the body which gains e- becomes negatively charged. +Fur → glass → silk → human body → cotton → wool → sealing wax → amber→ resin → Sulphur → rubber → ebonite- (which appears earlier will positive) (2) Charging by induction: The process of charging, uncharged body without bringing a charged body in actual contact with it is known as charging by induction.

The maximum value of induced charge is given by  1 q ' = −q 1 −   k (3) Charging by Conduction: when a charged body is brought in contact with an uncharged body, e- are transferred from one body to the other. The body which loses e becomes positively charged and the body which gains e- becomes negatively charged. Properties of Charge: (i) Charges are additive in nature i.e. they are scalar and can be added by algebraic method. (ii) Charges are conserved i.e. they can neither be created nor be destroyed. (iii) Quantization of electric charge: It is a property of an electric charge which states that any charged body will have an integral multiple of the basic charged on an e - i.e. 1.6x10-19 C Q = ± ne , n = 1,2,3…….. (iv) Like charges repel each other whereas unlike charges attract each other. (v) Charge can't exist without mass where mass can exist without net charge. (vi) Charges are invariant in nature i.e. independent of velocity of charged particle (vii) Accelerated charges radiate energy. Coulomb's law:

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According to this law, the force between any two point charges at relative rest, is directly proportional to the product of the magnitude of charges and inversely proportional to the square of the distance of separation between them. (i) F  q1q1 1 F 2 (ii) r qq F  1 22 r 1 q1q2 F= air 4 o r 2 where  o constant = permittivity of free space 1 k= = 9 109 Nm2c −2 (SI Unit) 4 o in c.g.s. k = 1 1 q1q2 Fm = medium 4 r 2  = absolute permittivity of the medium Permittivity: The permittivity of a medium is a measure of the extent of difficulty with which the medium allows electric field lines to pass through it.  o = 8.854 10−12 C 2 N −1m−2 Relative permittivity (  r ) of the medium is equal to the ratio of the force between two charged in vacuum to that in the medium. 1 q1q2 4 o r 2 F  r = = = 1 q1q2  o Fm 4 r 2

r =

 =k o

Significance of coulomb's law: (i) Coulomb’s law holds over large range distance (10 -14 to few km). (ii) Coulomb's law is based on physical observation and cann't be derived. (iii) It is a universal law. (iv) Coulomb force, like gravitational force is conservative. (v) The magnitude of coulomb force doesn't depend on the polarity of the charges. (vi) Coulomb's law is valid only when the separation between charges are greater than 1015m. For distance, 10-15m (less than it) nuclear forces come into play and dominate all other forces. Drawbacks of coulomb's law: (i) The distance between the two point charges must remain constant. (ii) The charges should not radiate energy i.e. they should not be accelerated. Coulomb's law in vector form:

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Let the position vectors of two point charges q1 & q2 in the rectangular coordinates system be r1 & r2 respectively.

From fig r21 = r2 − r1 r12 = r1 − r2

(i) (ii)

r21 = −r12

(iii)

From eqn. (i) & (ii) Let F12 = force exerted on q1 by q2

F21 = force exerted on q2 by q1 From Coulomb’s law

&

F12 =

1 q1q2 rˆ12 4 o ( r12 )2

F12 =

1 q1q2 r12 4 o ( r12 )3

(iv)

F21 =

1 q1q2 r21 4 o ( r21 )3

(v)

1 q1q2 r12 4 o ( r12 )3

(vi)

Putting the value of eqn. (iii) in eqn. (v) F21 = −

from eqn.(iv) & (vi)

F12 = − F21 (vii) from eqn.(vii) it is clear that Coulomb force, therefore act in action reaction pairs. Principle of superposition: According to the principal of superposition, total force acting an a given charge due to number of charges around it is the vector sum of the individual forces acting on that charge due to all charges. F = F1 + F2 + F3 + ...... + Fn (i) According to Coulomb's law 1 qqi Fi = ri (ii) 4 o ri 3 From eqn. (i) & (ii) q n qi F=  ri 4 o i =1 ri 3 Linear charge density:

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This is defined as the amount of charge per unit length of a body and is denoted by . Its unit is Cm-1. Q = l Q = total Charge l = length Surface charge density: This is defined as the amount of charge per unit surface area of a body and is denoted by ‘’. Its unit is Cm-2. Q = A Volume charge density: This is defined as the amount of charge per unit volume of a body and is denoted by  . Its unit is Cm-3. Q = V Electric field: The space around a charge within which its electrical effect can be felt is called electric field. Electric field intensity ( E ): The electric field intensity at any point in an electric field is defined as the force experienced by a unit positive charge placed at that point. F E= qo kqq rˆ E= 2 o r qo

E=

kq rˆ r2

E= E =

kq r2

S.I Unit: NC-1 or Vm-1 The electric field intensity E at a point is obtained by using the principle of superposition. E = E1 + E2 + E3 + ....... + En Electric field Lines: Electric field lines is defined as the path followed by a unit positive charge when it is free to move in an electric field. Or It is an imaginary line use to represent the electric field around a charge. It is a vector quantity. The magnitude of the field is represented by the density of field lines. Electric field is strong near the charge, so the density of field line is more near the charge and the lines are closer. There are two types of field

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(i) Uniform field (ii) Non- Uniform field. Note: Field of an isolated charge is non-uniform. The field lines are radial and not parallel to one another.

Properties of electric field lines: (i) They start from a positive charge and end at a negative charge. (ii) The tangent drawn at any point on a line gives the direction of the electric field at that point. (iii) Two electric lines of force never intersect each other. (iv) Electric field lines never form closed loop i.e. they never start from and end at the same charge. (v) In a charge free region, electric field lines can be taken to be continuous curves without any breaks. Motion of a charged particle in an electric field: When charged particle initially at rest is placed in the uniform field (i) F = qE F qE a= = (ii) m m (i) Motion along direction electric field: If ‘u’ be its initial velocity ‘v’ be its final velocity after a time ‘t’,

v = u + at

 qE  v = u +  t  m

(a)

[From eqn. (ii)] The path of the particle will be straight Line. (ii) Motion perpendicular to direction of applied field: Suppose that the initial velocity of the particle is ‘u’ along the x-axis and that a uniform electric field exist along the y-axis. The acceleration of the particle along the x-axis will be zero, while is acceleration along the y-axis will be qE ay = = constant m 1 y = u yt + a yt 2 2 1  qE  2 y=  (a) t 2 m 

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