## Tuesday, October 23, 2007

### Study guide H C Verma JEE Physics Ch. 29 ELECTRIC FIELD AND POTENTIAL

Syllabus

Coulomb’s law; Electric field and potential; Electrical potential energy of a system of point charges and of electrical dipoles in a uniform electrostatic field; Electric field lines; Flux of electric field;
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Verma - Topics

29.1 What is electric charge?
29.2 Coulomb's law
29.3 electric field
29.4 Lines of electric force
29.5 Electric potential energy
29.6 Electric potential
29.7 Electric potential due to a point charge
29.8 Relation between electric field and potential
29.9 Electric dipole
29.10 Torque on an electric dipole placed in an lectric field
29.11 Potential energy of a diple placed in a uniform electric field
29.13 The electric field inside a conductor

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Study Plan

Day 1

29.1 What is electric charge?
29.2 Coulomb's law

Day 2
29.3 electric field
Ex. 29.1, 29.2
29.4 Lines of electric force
Worked out examples 1,2

Day 3

29.5 Electric potential energy
Ex. 29.3
29.6 Electric potential
Ex. 29.4
29.7 Electric potential due to a point charge
Ex. 29.4
WOE 3 to 5

Day 4

29.8 Relation between electric field and potential
Ex. 29.5
WOE 5 to 10

Day 5
29.9 Electric dipole
29.10 Torque on an electric dipole placed in an lectric field
29.11 Potential energy of a diple placed in a uniform electric field

Day 6
29.13 The electric field inside a conductor
WOE 11 to 15

Day 7
WOE 17 to 19

Day 8

Exercises 1 to 10

Day 9
Exercises 11 to 20

Day 10
Exercises 21 to 30

Day 11
Exercises 31 to 35

Day 12
Exercises 36 to 40

Day 13
Exercises 41 to 45

Day 14
Exercises 46 to 50

Day 15
Exercises 51 to 55

Day 16
Exercises 56 to 60

Day 17
Exercises 61 to 65

Day 18
Exercises 66 to 70

Day 19
Exercises 71 to 75

Day 20
Exercises Objectve I 1 to 9 and Objective II 1 to 8

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Introduction

The credit for the discovery of electricity goes to Thales of Miletus, regarded as one of the seven wise men of Greece. Around 600 B.C., Thales discovered that when amber is rubbed with wool, it acquires the property of attracting light objects such as straw, wood shavings etc. The investigation of this phenomenon remained in abeyance for over 2000 years.

Near the end of the 16th century A.D., william Gilbert, a physician to Queen Elizabeth I, found that like amber, there are several othe substances which, when rubbed with suitable substances, exhibit the property of attracting light objects. For example, a glass rod rubbed with silk attracts light objects such as paper, cork etc. Similarly an ebonite rod when rubbed with fur attracts light objects. As this phenomenon was first observed with amber and amber is called electron in Greek, Gilbert name the phenomenon as electricity.

He proposed that a body upon rubbing becomes eletrified or charged and acquires the property of attracting light objects. Such a body is said to have acquired an electric charge.

Subsequently, it was observed that (i) two ebonite rods rubbed with fur repel each other and; (ii) two glass rods rubbed with silk repel each other, while (iii) an ebonite rod rubbed with fur attracts a glass rod rubbed with silk.
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Concepts covered

The chapter may seem to be a difficult chapter for the first reading.

29.1 What is electric charge is a simple one.

29.2 Coulomb's law - this topic is also straight forward. Charges have forces of attraction or repulsion among them.

Through experimental data, Coulomb proposed the law that gave the formual for finding the force exerted by a charged particle on the other charged particle.

F = kq1q2/r² ... (29.1)

q1, q2 are the charges on particles
r is the separation or distance between them and
k is a constant.

k in SI units is 8.98755 * 10^9 N-m²/C²

29.3 Electric Field

A charge produces an electric field in the space around it and this electric field exerts a force on any other charge placed it.

The field takes finite time to propagate. If a charge is displaced from its position, the field at a distance r will change after a time t = r/c, where c is the speed of light.

Intensity of electric field: is defined in this way. If a charge q is brought into the electric field and if it experiences an electric Vector F (Vr(F)), we define the intensity of electric field at the given point where the charge q is there as

Vr(E) = Vr(F)/q

Vr(E) = Vector of electric field

29.4 Lines of force - concept is similar to magnetic lines of force

29.5 Electric potential energy - this topic may require some time to think over and grasp.

29.6 Electric Potential - Electric field is a vector quantity. Electric potential is a scalar quantity. This concept follows from the concept of electric potential energy.

29.7 This topics gives expressesions for electric potential for a point charge and a system having number of point charges. Follows from the topic above

29.8 This section gives the relationship between the scalar quantity potential and vector quantity field. spend some time to grasp the concepts once again.

29.9 Electric dipole: You know by now dipole. Some of the chemical compounds have dipole. In this section, electric dipole moment is defined. It is a vector quantity.

Vr(p) = q*Vr(d)
p = electric dipole moment
q = charge one positive and one negative
d = distance between charges which is small. vector gives the direction from negative charge to positive charge.

There is expression for electric potential due to a dipole. If you are already comfortable with the concept of electric potential (scalar quantity) this topic will be easy for you.

Then follows discussion of electric field (vector quantity)

29.10 gives the expression for torque experienced by dipole when placed in an electric field.

29.11 discusses potential energy of the dipole whne placed in a uniform electric field.

29.12 describes why some materials are conductors, insulators and semiconductors. The conductors have large number of free electrons.

29.13 informs us that inside a conductor the electric field zero. The electrons shift to one side of the conductor and therefore once face of it becomes negatively charged and the other face positively charged.

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Audiovisual lectures

Lesson 30: Electric Charges and Coulomb's Law www.curriki.org/nroc/Introductory_Physics_2/lesson30/Container.html

Lesson 31: Electric Fields
www.curriki.org/nroc/Introductory_Physics_2/lesson31/Container.html

Lesson 32: Electric Potential
www.curriki.org/nroc/Introductory_Physics_2/lesson32/Container.html

Chapter 11: Conductors and Capacitors
Lesson 33: Electrostatics with Conductors
www.curriki.org/nroc/Introductory_Physics_2/lesson33/Container.html

Websites

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JEE Question 2007 Paper II

Positive and negative point charges of equal magnitude are kept at (0,0,a/2)and (0,0,-a/2) respectively. The work done by the electric field when another positive point charge is moved form (-a,0,0) to (0,a,0) is

(A) positive
(B) negative
(C) zero
(D) depends on the path connecting the initial and final positions

Correct Choice: C
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For more questions and answers on the topics related to electricity visit
http://iit-jee-physics-atps.blogspot.com/2008/03/jee-past-objective-questions.html
Posting started on 23.3.2008