www.gradeup.co

1

www.gradeup.co

2

Electrostatics is an important topic from JEE Main / JEE Advanced Exam Point of view. Every year

there are 1-3 questions asked from this topic. This short notes on Electrostatics will help you in revising

the topic before the JEE Main & IIT JEE Advanced Exam.

Electric Potential and Capacitance

Electric Potential Energy- It is defined as the work done in moving a charge from one point to the

other.

www.gradeup.co

3

Electric Potential- It is defined as the change in the Electric potential energy per unit charge.

Electric potential due to a point charge- The Electric potential due to a point charge at any distance r

is

Electric potential due to a system of charges- For a discrete system of charges the electric potential

will use the same principle of superposition. The net potential due to the system of charges is equal to

the sum of the sum of the individual potentials.

www.gradeup.co

4

Electric potential due to an Electric Dipole- Potential due to electric dipole depends on r and on the

angle between position vector and dipole moment.

The Potential at any point form the centre of the dipole is

Equipotential Surface- If the charge is uniformly distributed over the surface then the potential at each

and every point is same then that surface is considered as an Equipotential Surface.

So the work done to move a charge from one place to other on an equipotential surface is zero.

Potential energy of a system of two-point charge- The energy that result from the collection of

charges when the charge will employ a force on any other charge. It is the total amount of work done in

moving the charges from an infinite distance to their corresponding positions in the system.

www.gradeup.co

5

is the position vector between the charges .

Potential energy of electric dipole in an electric field

As the electric field implies torque in the dipole, the work done in rotating it against the torque is stored

in the form of its potential energy.

www.gradeup.co

6

Conductor, Insulator and Dielectric

Conductors and Insulators- The materials that allow the free movement if electrons or charge carriers

are known as Conductors and the material that avoid any sort of flow of the charged carriers is known

as Inductors.

Points to remember

1. The electrostatic field inside an inductor is zero.

2. At the surface of a charged conductor at every point, the electrostatic field is normal to the surface.

3. In the static situation, the interior of a conductor has no excess charge.

4. At the surface of the charged conductor electric field is

5. The electrostatic field inside a metal cavity is always zero.

Free Charges- Free charges that move throughout the conductor without any constraint. E.g. Electron

or ion moving inside a vacuum.

Bound Charges- Bound charge cannot move in response to an external electromagnetic. E.g. Atoms in

an insulator undergoes charge separation when subjected to an electric field, resulting in the formation

of dipoles that align themselves to counteract the subjected field.

Dielectric Materials- Dielectric materials are non-conducting substances where the free movement of

charges is not possible. The external field induces a dipole moment by re-orienting molecules of the

dielectric which in return produces a field that opposes the external field.

www.gradeup.co

7

Polarisation- Polarization is a phenomenon occurs when an external electric field alters the negative

cloud of electrons around positive atomic nuclei in the opposite direction of the applied field. This

insignificant separation of charge makes one side of the atom slightly positive and the opposite side

somewhat negative.

Thus, Polarization is a comparative shift of positive and negative in opposite directions inside an

insulator or dielectric stimulated by an external field.

Polarisation is expressed as , where is the constant of dielectric and known as

Electrical susceptibility, and is the applied Electric field.

The Electrical susceptibility is also defined the degree of allowance for electric field lines to penetrate

into the dielectric.

It can also be expressed in terms relative permittivity .

Capacitor and Capacitance

Capacitor- Capacitor is a two-terminal electrical passive component that used to store the energy in the

electrical field.

www.gradeup.co

8

Capacitance (C) - Capacitance is the ability of a capacitor to store electrical energy is known as its

Capacitance.

, where Q is the charge, and V is the voltage.

The constant C is called the capacitance of the capacitor. It is independent of charge or potential and

depends only on the geometrical alignment of the system of two conductors.

Parallel Plate Capacitor: It consists of two large plane parallel conducting plates separated by a small

distance and charge is distributed along the conducting surface of the plates.

The capacitance of a parallel plate capacitor is

Combination of Capacitors

Series Combination- If two capacitor C1 and C2 are connected in series then the net capacitance

is

www.gradeup.co

9

If n capacitors are connected in series, then the net capacitance is

Parallel combination- If two capacitor C1 and C2 are connected in parallel then the net capacitance

is

If n capacitor are connected in parallel then the net capacitance is

www.gradeup.co

10

Effect of dielectric on capacitance- If a dielectric medium of dielectric constant k is inserted between

the plates of the capacitor then the net capacitance of the capacitor is,

where is the dielectric constant of the material, Co is the capacitance without the dielectric.

Energy stored inside the capacitor- In a capacitor, the work done on moving a positive charge from a

negative conductor to a positive conductor against the repulsive forces is stored as energy stored in the

plates of the Capacitor.

, where Q is the charge, C is the capacitance of the capacitor.

Van De Graf Generator

Van De Graf Generator is an electrostatic generator that is capable of generating a large static electric

potential. It creates a very high electric potential in the order of a few million volts which results in a

very large electric field at least until breakdown field of air is reached.

It uses a moving belt that stores charge on a hollow metal structure. The hollow metal structure has a

design of a globe which is placed on the top of a column that is insulating in nature.

www.gradeup.co

11

Working- A large spherical shell is placed at some height above the ground. An insulating column is

fixed which holds it. Two pulleys are coiled with a belt-like insulating material, with one being at

ground level and the other one at the centre of the shell. The belt carries out a nonstop motion, thus

carrying a positive charge constantly from the ground to the top. This belt is kept moving continuously

by a motor driving the lower pulley. The positive charge is transferred to the larger shell by a carbon

brush, thus coating the outer shell with a very high potential over the time.

Subscribe to YouTube Channel for JEE Main

All the best!

Team Gradeup

All About JEE Main Examination: https://gradeup.co/engineering-entrance-exams/jee-main

Download Gradeup, the best IIT JEE Preparation App

www.gradeup.co

19