Ans. A comb rubbed with hair and amber rubbed with silk attract small pieces of paper. This is due to electric charges, they acquire during rubbing.
The following experiments show that we can produce two types of electric charges through the process of rubbing.
Experiment 1: Take a plastic rod. Rub it with fur and
suspend it horizontally by a silk thread. Now take another plastic rod and rub it with fur and bring near to the suspended rod. We will observe that both the rods will repel each other (Fig. 13.1). It means during the process of rubbing both the rods were charged.
Experiment 2: Now take a glass rod and rub it with silk and suspend it horizontally. When we bring the plastic rod rubbed with fur near to the suspended glass rod, we observe that both the rods attract each other (Fig. 13.2).
In the first experiment, both rods are of plastic and both of them have been rubbed with fur. Therefore, we assume that charge on both rods would be of the same kind. *
In the second experiment, rods are unlike and their attraction implies that charge on the two rods are not of the same kind but of opposite nature.