Electric charge

An electron's charge has a magnitude of 1.602176634 × 10⁻¹⁹ coulombs, so a single coulomb corresponds to the charge magnitude of about 6.24 billion billion electrons.

Why a rubbed balloon sticks to the wall

Rub a balloon on a wool jumper and it will hang on a wall by itself. Plastic wrap clings to a bowl, a sock comes out of the dryer stuck to a shirt, and a comb run through dry hair picks up scraps of paper. People noticed the same thing thousands of years ago with amber, a fossil tree resin: polished amber pulls bits of straw toward it. The word electric comes from the Greek word for amber, elektron. All of these are effects of electric charge, and in every case the charge got there by rubbing two materials together.

Two kinds of charge

Experiments with rubbed rods show that there are exactly two kinds of charge. Rub two glass rods with silk and hang them near each other: they push apart. The two silk cloths also push apart. But a glass rod and a silk cloth pull together. Benjamin Franklin called the two kinds positive and negative. The rule is short: like charges repel, unlike charges attract. The force gets weaker as the charges move apart, which is why the balloon must touch or nearly touch the wall.

Where does the charge come from? Every atom has a nucleus of protons, which are positive, and, in most atoms, neutrons, which have no charge, surrounded by negative electrons. A normal atom has as many electrons as protons, so its total charge is zero and we call it neutral. Protons are locked inside the nucleus, but some electrons are held loosely. Rubbing drags electrons from one surface to the other. When amber is rubbed with silk, the amber gains electrons and becomes negative; the silk loses them and becomes positive. Nothing is created: the charge only moves.

How much charge? The coulomb

Charge is measured in coulombs, written C. The smallest free charge found in ordinary matter is the charge of one proton, called the elementary charge e. Since 2019 its value has been fixed exactly: e = 1.602176634 × 10⁻¹⁹ C. An electron carries −e and a proton +e. Because each electron is so tiny a share of a coulomb, one coulomb is a huge number of electrons: 1 ÷ (1.602176634 × 10⁻¹⁹) ≈ 6.24 × 10¹⁸. Charge on any object always comes in whole numbers of e. You can have 3e or 1,000,000e, but never 2.5e. Physicists say charge is quantized.

The second rule is conservation of charge: the total charge of an isolated system never changes. When you rub the balloon on your hair, the balloon gains exactly as much negative charge as your hair gains positive charge. Add them up and you still have zero. This rule has never been seen to fail, from rubbed balloons to particle collisions.

Conductors and insulators

In metals some electrons are not tied to any one atom. These free electrons can wander through the whole piece of metal, so charge moves through it easily. Such materials are conductors: copper, aluminium, silver and gold are good ones. In plastic, rubber, glass and dry air almost every electron stays with its atom, so charge hardly moves. These are insulators. A household cable uses both: copper inside to carry the charge, plastic outside to keep it from reaching your hand. Rubbing works best on insulators, because the charge you move stays where you put it instead of spreading away.

From charge that sits still to charge that flows

A rubbed balloon holds its charge until it touches something. A battery is different: it keeps pushing charge around a closed loop of wire. In the circuit lab below, the blue dots stand for electrons moving through the wire, and the readout counts how many coulombs pass through the battery every second and turns that into a number of electrons.