2.2 Charging of a body

  Electrostatics

Charging of a body can be achieved by three methods,

1. Charging by rubbing
2. Charging by contact
3. Charging by induction

2.2.1 Charging by rubbing

All material bodies contain a large number of electrons and an equal number of protons in their normal state. When rubbed against each other, electrons gains energy and some electrons are transferred from one body to another.

The body that donates the electron becomes positively charged while that which receives the electrons becomes negatively charged.

For example: When a glass rod is rubbed with a silk cloth, glass rod becomes positively charged because it donates the electrons while the silk cloth becomes negatively charged because it receives electrons.

Electricity so obtained by rubbing two objects is also known as frictional electricity.

2.2.2 Charging by contact

The process of giving one object a net electric charge by placing it in contact with another object that is already charged is known as charging by contact.

When a negatively charged ebonite rod is touched with a metal object, such as a sphere, some of the excess electrons from the rod are transferred to the sphere. Once the electrons are on the metal sphere, where then can move readily, they repel each other and spread out over the sphere's surface. The insulated stand prevents them from flowing to the earth. When the rod is removed, the sphere is left with a negative charge distributed over its surface.

2.2.3 Charging by induction

Induction is defined as the redistribution of electrical charge in an object caused by the influence of nearby charges.

Consider a negatively charged rod is brought close to a metal sphere without touching it. In the sphere, the free electron close to the rod moves to the other side by repulsion. As a result, the part of the sphere nearer to the rod becomes positively charged and the part farthest from the rod becomes negatively charged. However, the net on the rod is still conserved i.e. zero. Now if the rod is removed, the free electrons return to their original position and the charged regions disappear.

Now, when a metal wire is attached to the sphere and the ground as shown in figure (b), some of the free electrons leave the sphere and flows into the earth. Now if the earthing wire is removed, followed by the charged rod, then the sphere is left with a net positive charge as shown in figure (c).