CHAPTER 01

ELECTRIC CHARGES AND FIELDS

Electrostatics is the study of electric charges at rest. It is also called static electricity.

Quantization of Charge

The Quantization of electric charge is the property of virtue of which any charge exists in discrete lumps or packets of certain minimum charge,+ or -e is the charge of the electron and +e is the charge of the proton. 

Thus according to the quantization of charge, the charge on the body is an integral multiple of the charge of electron ie Q=+-ne where n=1,2,3

examples of charge : +-1e, +-2e, +-3e

The cause of quantization is that only an integral number of electrons can be transferred from one body to another. this property establishes the discrete nature of charge, ruling out the continuous nature

Electric Charge is additive.

The total charge on  a body is algebraic sum of all the charges, located anywhere on the body. While taking algebraic sums, the sign of the charge must be taken into account. Two charges +3q and +2q will get added up to give +5q whereas two charges +5q and -4q will get up to give +q. Hence Electric charge is Additive.

Conservation of Electric Charge

Comparison of electric Charge

Coulomb's Law

Unit of Charge

Dielectric constant or Relative  Permittivity or specific inductive capacity 

Charging of Body

Methods of Charging: Abody can be charged by

The force between two charges in terms of their position vectors

The superposition Principle

Electrostatic Field

Relation between electric field strength and force

Electric Field intensity due to a point charge

electric  field intensity due to a group of charges

Electric line of Force(Field Lines)

properties of electric lines of force

Electric dipole

Dipole Moment

Dipole Field

1.  Field intensity on the Axial Line of the dipole

2. Field Intensity on the Equatorial line of dipole

Dipole in a uniform electric field 

Potential Energy of a Dipole in a Uniform Electric Field

Flux of an electric field

electric flux

Gauss's theorem or Gauss's Law in Electrostatics

Applications of Gauss's Theorem

1. Electric field Intensity due to a line charge (Cylindrical Symmetry)

2. Electric Field Intensity due to a uniformly charged shell

3. Electric Field intensity of  a solid nonconducting sphere

4. Electric field intensity due to a thin infinite sheet of charge

5. Electric field intensity due to two thin parallel sheets of charge

1. Electric flux through a sphere

2. Electric Flux through a cylinder

Electrostatic Shielding

Electrostatic shielding is a phenomenon seen when a Faraday cage is used to block the effects of an electric field. The effects of external fields on the internal contents are blocked using the cage. 

Electrostatic shielding applications

• During a lightning thunderstorm, it is advised to stay inside the car and not under trees or in the open ground because the outer metallic body of the car acts as an electromagnetic shield from the lightning.
• In a coaxial cable, the central conductor is protected by an electrostatic shield by connecting the outer conductor to the ground

What is Faraday cage?

The Faraday cage is an enclosure that blocks the external electric fields in conductive materials. It is also known as the Faraday shield.

Michael Faraday, in the year 1800 invented Faraday’s cage. He found that when the metal cage, which acts as an electrical conductor, was charged, the charges appeared only on the surface with no effect on the interiors. When this was conducted on a large scale, he lined a room in a metal foil, allowing high-voltage discharges from an electrostatic generator. He used a device to detect the electric charges called an electrostatics to prove his hypothesis that the outer surface of the metal foil was conducted by the current. In contrast, the interior of the room was devoid of electric charges.

How does Faraday cage work?

Faraday cage has a cage-like appearance made of fine metal mesh and chain-like fences, which vary in size and design. The cage can hold the electrostatic charges and electromagnetic fields such that they get distributed throughout the exterior of the material it is used on. The distribution of charges on the exterior surface happens so that the charges cancel with the charges in the interior.

Faraday cage applications

• MRI scanning rooms: The rooms are built so that they prevent mixing the external radio frequency signals with the data from the MRI machine.
• Microwave ovens: The mechanism used is based on the Faraday cage such that the electromagnetic energy within the oven is contained and the exterior is a shield from radiation.
• They find application in analytical chemistry to reduce noise while making sensitive measurements.

Grounding Concept

When a conductor is grounded.

The potential of the conductor becomes zero.

Earth is considered to be an infinite source as well as sink of electrons, hence as many electrons can flow in either direction as required by the conductor to make its potential equal to zero.