NCERT Solutions Curiosity Chapter 5 .4.2 Non-contact forces — Activity 5.7: Let us experiment

The two hanging balloons move away from each other, as if they are repelling each other (Fig. 5.9b), and the threads slant outwards.

Before rubbing they hung quietly side by side (Fig. 5.9a). Nothing has been added to them except the rubbing — and now they push each other apart across a gap. So rubbing has put something on both balloons that makes them repel.

The balloon and the woollen cloth move towards each other, as if they are attracting each other. The balloon swings out of its vertical position to meet the cloth.

This is the opposite of what the two balloons did to each other — and the cloth is the very object that was used to rub them.

Three conclusions follow, in order:

1. Both balloons were charged in the same way, so they must have acquired similar charges. Since they repelled each other, similar (like) charges repel each other.
2. The charged balloon and the woollen cloth attracted each other. Both the rubbing object and the rubbed object get charged, but they acquire the opposite kind of charge. Their attraction shows that opposite kind (unlike) charges attract each other.
3. Therefore there are exactly two kinds of static charges, and they are called ‘positive’ and ‘negative’.

The force exerted by a charged body on another charged body or an uncharged body is called the electrostatic force. It acts across a gap, so it is a non-contact force.