## Section28.1Electrification by Friction

Try this: dry your hair, and comb it with a plastic comb. Then, bring the comb near bits of paper. You will find that the rubbed comb attracts pieces of paper shown in Figure 28.1.1.

The attraction of light pieces of materials is observed whenever two dissimilar materials are rubbed against one another and the rubbed materials brought near light objects such as bits of paper or dry leaves. You can see it not only when you rub comb with hair but also when you rub amber with cotton, or glass with wool, or rubber shoes with the nylon carpet, etc.

Ancient Greeks knew of this property, as recorded by Thales of Miletus (circa. 635 BC - 543 BC), when it was found that an amber rubbed with silk attracted light objects. A serious study of these effects began with William Gilbert of England (1544AD – 1603AD), who is regarded as the “father of the science of electricity and magnetism”. Gilbert coined the word electrification, after the Greek word $\eta\lambda\epsilon\kappa\tau\rho o\nu$ (elektron), meaning amber. A rubbed body, which attracts light objects, is said to be electrified, or electrically charged.

### Subsection28.1.1Two types of charges

When you rub two dissimilar objects, both objects become electrically charged. The charges on the two, however, are not of the same type as you can easily demonstrate by the following experiments with a cellophane tape that has one sticky side as shown in Figure 28.1.2.

Take two pieces of a cellophane tape and tape one on the non-sticky side of the other. Now, quickly pull them apart. If you bring them closer with their non-sticky sides facing each other (so that they do not get stuck again), you will notice that they attract each other. Tear one of the tapes, and then bring the two new pieces closer, you will notice that they repel each other.

You can explain these observations based on the hypothesis that there are two types of charges, and they are balanced in any uncharged material. But when the tapes are pulled apart, some charges of one type move from one tape to the other. As a result an imbalance of charges develops in both tapes. The attraction of the two tapes and the repulsion among parts of the same tape tells us that the unlike charges attract and the like charges repel.

Any name could be given to the charges. Traditionally we call the two types of charges as positive and negative charges and represent them by positive and negative real numbers respectively.