Conductors vs insulators

It comes down to electrons

Every material is made of atoms, and every atom has electrons. What decides whether a material carries electricity is how tightly its atoms hold on to those electrons. The tighter the grip, the harder it is to get charge moving.

Conductors: loose electrons

In a metal such as copper, the outermost electrons aren't tied to any one atom. These free electrons wander from atom to atom all the time. Give them a small push, such as a battery's voltage, and they drift along together. That steady drift is a current, and a material that lets it happen easily is a conductor. Silver conducts slightly better than copper, but copper is far cheaper, so it's what wires are made of.

Insulators: locked electrons

In plastic, rubber or glass, the electrons are locked into the bonds between atoms. Push on them and almost nothing moves, so almost no current flows. A material like that is an insulator.

Why a wire uses both

A wire puts the two side by side. The copper core carries the current exactly where you want it to go. The plastic jacket around it keeps that current off your fingers and away from the next wire, so two wires can touch without the current jumping between them.

Push hard enough and anything gives way

Insulators have a limit. Push hard enough and even air breaks down and starts to conduct: dry air at sea level gives way at roughly 3,000,000 volts per meter. That's what happens in lightning. It's also why insulation on real cables carries a voltage rating.

Try it yourself

Look at a phone charger cable, a lamp cord or an old extension lead. The metal shows only at the plug's pins, where it has to touch the socket; everywhere you hold it, it's wrapped in plastic or rubber. Tool handles for electricians are wrapped in insulating rubber for the same reason.

Good to know

• Simplified: conductors have many free (conduction) electrons and insulators have almost none, so insulators pass extremely little current rather than literally none.
• Copper is used for its high conductivity and low cost; silver conducts slightly better.
• Electrons in metals aren't "loose" in a simple classical sense; the full picture (band theory) and semiconductors are skipped here.
• Pure water is a poor conductor, but tap water with dissolved ions conducts.
• Air breaks down at roughly three million volts per meter (dry air at sea level); the figure varies with humidity, pressure and the shape of the electrodes. In lightning the breakdown actually happens locally at the tip of the growing leader, so "push hard enough" is a simplification.
• Plastic insulation also has a voltage rating and can fail above it.