 # Electric Charge Definition and Properties

## What is an Electric Charge?

Electric Charge is a fundamental property of matter and never found free. There are two kinds of electric charges namely positive and negative charges. If a body has an excess of electrons, it is said to be negatively charged and if it is deficient in electrons, it is said to be positively charged.

### Introduction to Electric Charge

1. Benjamin Franklin introduced the concept of positive and negative charges.
2. Repulsion is the sure test for the detection of a charge.
3. In S.I. system the unit of charge is the coulomb.
4. Electric Charge is a scalar quantity.
5. Like charges repel and unlike charges attract.
6. Electric Charge is conserved. It can neither be created nor destroyed. It can only be transferred from one object to another.
7. The charge is quantized. The smallest charge is associated with an electron (-) and proton (+) is 1.610–19 coulomb.
8. All charges in nature exist as integral multiples of electron charge i.e. q = n.e.
9. A coulomb is equivalent to a charge of 6.2431018 electrons.

When a body is positively charged, its mass slightly decreases. When a body is negatively charged, its mass slightly increases.

Special Cases:

1. In the case of a conductor, its charge spreads over the entire outer surface and in the case of an insulator, its charge is localized
2. The electric Charge given to a conductor always resides on the outer surface of the conductor.

## Charging of Bodies

The process of making a neutral body into a charged body is known as electrification. Electrification is a universal phenomenon.

A body can be electrically charged by anyone of the following three ways:

• Friction
• Contact
• Electrostatic induction

### Charging by Friction

The electricity (i.e. transfer of electrons) that is produced due to friction is called frictional electricity.

#### Process of Charging

When we rub two neutral bodies, there will be some transfer of electrons from one body to the other due to structural modifications because of the frictional forces acting on them.

In this method, one of the bodies acquires a negative charge while the other gets a positive charge, both of which are equal in magnitude.

#### Examples

• When a glass rod is rubbed with a silk cloth, glass acquires a positive charge and silk cloth acquires a negative charge. Electrons are removed from glass rod and are added to silk cloth.
• When an ebonite rod is rubbed with fur cloth, ebonite rod acquires a negative charge and fur cloth acquires a positive charge. Electrons are transferred from fur cloth to ebonite rod.

### Electric Series

The list of substances is given below, called electric series, arranged in such a manner that if any two of them rubbed together, the one occurring earlier would be positively charged.

Glass, Flannel, Wool, Silk, Sealing wax, Hard metal, Hard rubber, Resin, Sulphur, etc.

Explanation: If we select glass and silk, the glass will acquire a positive charge while silk will get a negative charge when a glass rod is rubbed with silk.

### Charging by Contact

A neutral body can be charged by making contact with a charged body. Here the body will acquire a charge that is the same as that of the charging body. Thus, by contact, a similar charge is formed on both bodies. In this method, the electric charge on the first body decreases.

### Charging by Electrostatic Induction

Induction always precedes attraction. Polarisation of charges in a body when a charged body is present near that is called induction. In induction, a charged body is brought near an uncharged body. Then the uncharged body acquires a charge opposite in sign to that of the charged body.

Induced charge on dielectric slab of dielectric constant K is ${{q}^{1}}=-q\left[ 1-\frac{1}{K} \right]$.

#### For metals $K = \alpha\;\;Therefore, \;{q}’ = {-q}'$

Without a decrease in charge of the body, which induces by the method of induction, bodies can be charged continuously.