Electric Charges and FieldsElectric Charge

Around 600 BC, Thales of Miletus in Greece noticed that a piece of amber rubbed with wool or silk could pick up light things. The Greek word for amber, elektron, gave us the word electricity. Many other pairs of materials behave the same way: rub them together and they attract straws, pith balls and bits of paper.

Careful experiments with rubbed rods, repeated over many years, showed something more. Here are the experiments of Fig. 1.1 on a virtual lab bench.

Rub each pair with the red buttons. Then tap two charged things: the first one hangs from a thread, and the second is brought close to it.

The glass rod and the fur pushed each other away, so they carry the same kindopposite kinds of charge. The glass rod and the silk it was rubbed with pulled together, so they carry opposite kindsthe same kind of charge.

The property that tells the two kinds apart is called the polarity of charge. Rubbing always charges both bodies, one with each kind. Touch the rubbed glass rod back against its silk and the two stop attracting each other, and neither picks up bits of paper any more: the two kinds of charge have cancelled each other out. That is why the American scientist Benjamin Franklin named them positive and negative. By convention, the charge on a glass rod rubbed with silk (and on fur rubbed with plastic) is positive; the charge on the plastic rod (and on the silk) is negative. A body that has charge is electrified or charged; one with no charge is electrically neutral.

The Gold-Leaf Electroscope

A simple instrument for detecting charge is the gold-leaf electroscope (Fig. 1.2). A vertical metal rod passes through an insulating plug into a glass box. A metal knob sits on top of the rod, and two very thin gold leaves hang from its bottom end. When a charged body touches the knob, some of its charge flows down the rod to the leaves.

Charge on the knob: ${n} units

The two leaves share the charge, so they repel each other and diverge. The more charge, the wider they open: the degree of divergence is a measure of the amount of charge.

Where Does the Charge Come From?

All matter is made of atoms and molecules, and atoms are full of charge: positive protons in the nucleus, negative electrons around it. In a neutral body the two balance exactly. The forces that hold atoms together in a molecule and in a solid, the stickiness of glue, even surface tension: all of them are electrical at heart, forces between charged particles. The electric force is everywhere, which is why it is worth studying closely.

To charge a neutral body you must add or remove one kind of charge, and "charged" always means this excess or deficit. In solids, the charges that move are some of the outer electrons, which are less tightly bound. Rub a glass rod with silk and some electrons move from the rod to the silk: the rod is left positive (it lost electrons), the silk becomes negative (it gained them).

Sort each object after rubbing.

In every pair, what one body gains the other loses. No new charge is created by rubbing: it only moves from one body to the other. And the electrons that move are a very tiny fraction of all the electrons in the body.