What is electric charge?

Rub a balloon on your hair, then lift it away slowly. Your hair stands up and follows the balloon, as if by magic! It is not magic, though. It is electric charge at work.

Plus and minus

Everything is made of tiny atoms. Inside every atom there are even tinier particles. Two kinds of them carry electric charge: protons and electrons.

• Protons sit in the middle of the atom. They carry a plus charge, also called positive.
• Electrons move around the outside. They carry a minus charge, also called negative.

You cannot hold a charge in your hand like a pebble. It is something these particles have, and it lets them pull or push on each other — even without touching.

Which charges pull and which push?

Charges follow one simple rule:

• A plus and a minus pull toward each other. We say they attract.
• Two pluses push each other away, and so do two minuses. We say they repel.

The closer two charges are, the harder they pull or push.

Go deeper: Closer means much stronger for grown-ups and the extra curious

The push or pull between two charges grows fast as they get closer. Halve the distance, and the force becomes four times as strong. This rule is called Coulomb’s law:

$$F = k \, \frac{q_1 \, q_2}{r^2}$$

• $F$ is the force in newtons (N). A positive $F$ means the charges push apart; a negative $F$ means they pull together.
• $q_1$ and $q_2$ are the two charges in coulombs (C).
• $r$ is the distance between the charges in metres (m).
• $k$ is a fixed number, about $9 \times 10^{9}$ N·m²/C².

One electron carries $-1.602 \times 10^{-19}$ C, so one coulomb is the charge of about 6.24 billion billion electrons ($6.24 \times 10^{18}$). That is a lot of charge. Two objects holding 1 C each, 1 metre apart, would push on each other as hard as the weight of about 600,000 cars. Everyday static charges are tiny in comparison: a rubbed balloon carries less than a few millionths of a coulomb.

Protons in a nucleus sit extremely close together, and they are all plus, so they push each other apart very hard. Another force holds them together: the strong nuclear force. It is even stronger than the electric push, but it only reaches about as far as the width of a nucleus.

Why isn’t everything pulling and pushing?

A normal atom has exactly as many electrons as protons. Every plus is matched by a minus, so the charges cancel out. We say the atom is neutral, or balanced.

Almost everything around you is balanced like this. That is why your chair and your pencil don’t jump toward each other or fly apart.

How does rubbing make a charge?

Protons are locked deep inside their atoms. But some electrons sit on the outside, and rubbing can move them from one thing to another.

When you rub a balloon on your hair, some electrons move from your hair onto the balloon. Now the balloon has extra electrons, so it has a minus charge. Your hair has lost electrons, so it has a plus charge.

Plus and minus attract, so your hair reaches toward the balloon. And every hair now has the same plus charge, so the hairs push each other apart and stand on end!

Charge that stays put on a balloon or a sweater like this is often called static electricity.

Go deeper: Where does the charge come from? for grown-ups and the extra curious

Rubbing does not make new charge. It only moves charge from one object to another. The balloon gains exactly as much minus charge as the hair gains plus charge, so the total stays the same. This rule is called conservation of charge: the total charge of a closed system never changes.

Exactly which particles jump across when two materials touch is still being studied. Often they are electrons, but sometimes they may be charged atoms (ions) or tiny specks of the material itself. Either way, one object ends up with extra minus charge and the other with extra plus charge.

Try it yourself

• Rub a balloon on your hair or on a woolly sweater. Hold it over tiny bits of paper. The bits jump up and cling to it!
• Rub the balloon again and press it gently against a wall. It stays there, held by its charge.
• Turn on a tap so that a thin, smooth stream of water runs out. Hold a rubbed balloon close to the stream, without touching it. The stream bends toward the balloon!

The paper and the wall have no charge of their own. But the balloon’s extra electrons push the electrons in the paper slightly away. So the side of the paper closest to the balloon becomes a little bit plus — and plus and minus attract.

These tricks work best on a dry day. In damp weather, the charge leaks away faster.

Sparks and lightning

Sometimes a lot of charge builds up in one place. Then electrons can jump through the air in a tiny spark. You may know the feeling: you walk across a carpet, touch a metal door handle and — zap!

Lightning is the same thing, only gigantic. Inside a storm cloud, strong winds toss bits of ice up and down, and they crash into each other. This separates the charges: the lower part of the cloud usually ends up with a huge minus charge.

When the charge gets big enough, a giant spark jumps down to the ground. Most lightning, though, never reaches the ground at all: it flashes inside the clouds or between them.

A lightning bolt heats the air to about 30,000 °C — hotter than the surface of the Sun. The air expands so suddenly that it bangs. That bang is thunder.

Go deeper: A small zap and a giant bolt for grown-ups and the extra curious

The zap from a door handle can start at several thousand volts. It only stings, because very little charge flows, and only for a tiny moment. An average lightning bolt is very different: it carries a current of about 30,000 amperes, moves about 15 coulombs of charge and releases around a billion joules of energy.

Charges on the move

When charges move, they make an electric current. In a wire, electrons flow along, pushed by the voltage of a battery. Some materials let charges move easily, and others hardly at all. Find out which in conductors and insulators.

Check yourself

You rub two balloons on your hair and hang them side by side. What happens?

They pull together They push each other apart Nothing at all

Show the answer

They push each other apart — Both balloons picked up extra electrons, so both have a minus charge. Two charges of the same kind push each other apart.