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Electric current Electrons and current
How current flows

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.

Interactive graphic of electrical conduction
Interactive graphic of electrical conduction
Picture 3.4 Close the switch to apply voltage: electrons start to flow through. Increase the voltage and they drift faster.
About current
Notice that the electrons start to flow as soon as the switch is closed. The message to get them moving is instantaneous (in fact it travels close to the speed of light). However, the electrons themselves travel much more slowly. So how does the current come on everywhere as soon as the switch is closed?

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.

Electron velocities
The animation above is slightly deceptive because it shows the electrons as being stationary when there is no voltage.

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.

Graphic of current
Picture 2.5 Conventional current flows in the opposite direction to the electrons.
Current direction
Electrons are negatively charged. They flow from negative to positive in a circuit.

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

Question 4
a) In a piece of metal, when no current is flowing, the conduction electrons are moving around randomly. What is their average velocity?

b) The drift velocity of electrons is very small - about a millimetre per second. However, when you switch on a light, it comes on as soon as you close the switch. Explain how this happens.

c) The conventional direction of an electric current is the opposite of the direction of motion of the electrons that carry it. Why do you think this is so?

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