Copper is a good conductor because, like other metals, it contains free electrons. Free electrons are also known as conduction electrons. Each copper atom provides a single free electron, so there are as many free electrons as atoms.

Free electron concentration in copper n = 8.5 × 10²⁸ per m³

When a voltage is connected across a piece of copper, it pushes the free electrons so that they flow through the metal – that's an electric current.

It is because the free electrons are already spread through the wire. As soon as the switch is closed, there is a force on all the electrons, which gets them moving. It's a bit like a bicycle chain. As soon as you start pedalling, the back wheel starts to turn. The force on the back wheel is instantaneous even though the individual links are travelling at a visible speed. But because the links are already spread around the chain 'circuit' they all start to move at the same time.

In reality, even when no current flows through a piece of copper, the free electrons are moving rapidly about. Their speed is about 10⁶ m s^-1; that's 3000 times the speed of sound in air! However, since they are moving at random, there is no net flow of electrons in any particular direction and so there is no current.

When a voltage is applied, the electrons gain an additional velocity, so that there is a net flow along the wire. This extra velocity is called their drift velocity. Here's a way to picture this:

Think of a swarm of bees. They are all milling around the hive. Each bee is moving, but the swarm stays still. Now one sets off and the rest follow. The swarm is still a milling mass, but overall it moves away from the hive. The free electrons in a metal are like the bees; it takes a voltage to make the mass of electrons move through the wire.

Conventional current flows the other way - from positive to negative.