If we rub a plastic comb on hair and bring it close to small bits of paper, they are pulled towards it: the comb is electrified, that is, charged. The Greeks called hardened resin «elektron», and the word «electricity» comes from it. When two bodies are rubbed together, both become electrified. There are two kinds of electrification: a glass rod rubbed with silk becomes positive (+), an ebonite rod rubbed with fur becomes negative (–). Bodies with the same sign repel each other, bodies with different signs attract. An electroscope (its leaves open) or an electrometer (its pointer turns) shows that a body is electrified. Substances that conduct electric charge (metals) are conductors; substances that do not (glass, rubber, plastic, air) are dielectrics, and an object made of a dielectric is an insulator.
Worked examples
A glass rod is rubbed with silk. The rod becomes positive and the silk becomes negative, because when two bodies are rubbed both become electrified and their charges have opposite signs.
A light ball hanging on a thread moves away from an electrified rod. So the ball and the rod carry charges of the same sign. Attraction alone proves nothing yet, because even uncharged light objects are pulled towards an electrified body.
Class activity
“Comb and paper”: rub a plastic comb on dry hair or a woollen cloth and bring it close to small paper scraps; then try the comb without rubbing and write down the difference. Safety: use dry objects only; no wall socket is used in this experiment.
Practice
1
What are the two kinds of electrification and in which bodies do they appear?
Positive (a glass rod rubbed with silk) and negative (an ebonite rod rubbed with fur).
2
Sort into conductors and dielectrics: copper wire, rubber, glass, aluminium foil, dry air.
Conductors: copper wire, aluminium foil. Dielectrics: rubber, glass, dry air.
3
Two light balls repel each other. What can we say about them?
Both are charged and their charges have the same sign.
4
Why do the leaves of an electroscope open when a charged rod touches it?
The charge spreads to both leaves, they get the same sign and repel each other.