Electric Charge, Conductors and Insulators

What electric charge really is

Electric charge is a basic property of matter, just like mass. There are two kinds — positive and negative — and they obey one golden rule:

Like charges repel; opposite charges attract.

Two positives push apart, two negatives push apart, but a positive and a negative pull together. Almost everything about static electricity, circuits and even lightning comes back to this single rule.

If you have met charge through rubbing balloons, this lesson goes deeper: where the charge comes from, why some materials hold it and others let it flow, and how you can charge something without even touching it. For the everyday version of these ideas, see static electricity.

Inside the atom: where charge lives

Every atom has a tiny central nucleus containing protons (positive) and neutrons (no charge). Around the nucleus sit electrons (negative). In a normal atom the number of protons and electrons is equal, so the charges cancel and the atom is neutral.

Here is the key idea that explains everything:

• Protons are trapped in the nucleus and essentially never move in ordinary situations.
• Electrons are light and live on the outside, so they can be pulled away or added.

So when an object becomes charged, it is always because electrons moved:

• Gain extra electrons → more negatives than positives → negatively charged.
• Lose some electrons → fewer negatives than positives → positively charged.

This is why we never talk about "moving protons" — only electrons travel. To learn how the same particles carry energy when they flow, see electricity basics.

Conductors and insulators

Whether charge can move through a material depends on whether its electrons are free to roam.

• A conductor has electrons that can move easily. Metals such as copper, aluminium and gold are excellent conductors because each metal atom releases one or more electrons into a shared "sea" that drifts through the whole material. Graphite and salty water also conduct.
• An insulator holds its electrons tightly, so charge cannot flow through it. Plastic, rubber, glass, dry wood and air are insulators.

This single difference explains a huge amount:

• Wires use a copper core (conductor) wrapped in a plastic coating (insulator) so current goes where we want and not into your hand.
• You can build static charge on a balloon or plastic comb because they are insulators — the charge stays put.
• You cannot build static on a metal rod held in your bare hand, because the charge flows straight through the metal and through you to the ground.

Charging by friction

The simplest way to charge an insulator is by friction — rubbing. When two different materials rub together, electrons transfer from one to the other.

For example, rub a polythene rod with a dry cloth and electrons move onto the rod, leaving it negative. Rub an acetate (perspex) rod with cloth and electrons move off it, leaving it positive. Crucially, charge is never created or destroyed — it is only transferred. If the rod gained electrons, the cloth lost exactly the same number, so the cloth is left with the opposite charge.

Charging by induction — no contact needed

You can rearrange the charge in an object without touching it at all. This is called charging by induction, and it works because charges inside an object respond to a nearby charge.

Bring a negatively charged rod near a neutral metal can (sitting on an insulating stand). The rod's negative charge repels the electrons in the can, pushing them to the far side. Now:

• the near side of the can becomes positive (electrons fled),
• the far side becomes negative (electrons crowded there).

The object is still neutral overall, but its charge has separated. If you now briefly connect the far side to the ground (earth it) while the rod is still near, electrons escape to the ground. Remove the earth, then remove the rod, and the can is left positively charged — and you never touched it with the rod. Note the can ends up with the opposite sign to the rod that charged it.

Induction also explains why a charged balloon sticks to a neutral wall: the balloon induces an opposite charge on the wall's surface, and opposite charges attract.

Worked example: counting the charge transfer

Charge is measured in coulombs (C), and one electron carries a tiny charge of about 1.6 × 10⁻¹⁹ C.

Problem: A rubbed rod gains a charge of −3.2 × 10⁻⁹ C. How many extra electrons does it now hold?

Solution: Number of electrons = total charge ÷ charge per electron = (3.2 × 10⁻⁹) ÷ (1.6 × 10⁻¹⁹) = 2 × 10¹⁰ electrons (twenty billion).

Even a tiny static charge involves an astonishing number of electrons — yet that is still a microscopic fraction of all the electrons in the rod.

Try it yourself! 🧪

Build a simple electroscope to detect charge. This is completely safe — it uses only static charge, no batteries and no mains.

You need: a clean glass jar, a strip of thin aluminium foil, a piece of stiff copper wire, a plastic comb or balloon, and a dry cloth or your hair.

1. Bend the copper wire into a hook at the bottom and poke the other end through a card lid so the hook hangs inside the jar.
2. Drape a small "V" of aluminium foil over the hook so two leaves hang down.
3. Charge the comb by rubbing it on dry hair, then touch it to the top of the wire.
4. Watch the two foil leaves spring apart — they now carry the same charge, and like charges repel.
5. Wait, or breathe on the foil, and the leaves slowly fall as the charge leaks away through the damp air.

You have built a real charge detector and watched "like charges repel" with your own eyes.

⚠️ Safety: This experiment uses harmless static only. Never try to detect or play with mains electricity — wall sockets and household wiring carry voltages that can kill. Static from a comb is safe; mains is not.

What we learned

Charge comes in two kinds, and like charges repel while opposites attract. Objects become charged when electrons move — gaining them makes something negative, losing them makes it positive. Conductors let charge flow; insulators hold it still. You can charge an object by friction (rubbing) or by induction (without touching), and charge is only ever transferred, never created or destroyed.