Electrical Circuit
Electricity and Magnetism

Do the electrical charges move instantly?

Teaching Guidance for 5-11 Supporting Physics Teaching

When the light comes on

Wrong Track: The bulb cannot light until the energy is carried to it.

Wrong Track: The charges travel from the battery to bulb, and light it when they get there.

Right Lines: The circuit starts working as soon as you throw the switch. The charges start moving everywhere and electrical working lights the bulb.

When do the charges start to move?

Thinking about the learning

It's possible that this kind of thinking is encouraged in children by donation models that suggest that everything comes from the battery, and is then carried round to the bulb.

Thinking about the teaching

It would appear that when we turn on a switch the electric charges move immediately in all parts of the circuit and instantly light a bulb. At home, there's no noticeable delay between your finger activating the switch and the LED bulb glowing – even though the switch and bulb may be a long way apart.

Actually it does take a very short time for the charges to start moving. The electric field that sets the charges in motion takes a finite time to pass through the wires. The field propagates (moves) at approximately the speed of light: 300,000 kilometre /second, that is 300 millimetre in a thousand-millionth of a second.

Does this delay matter? An electrical signal would take a mere hundredth of a second to pass from the UK to the USA through cables under the Atlantic Ocean. However, a modern computer will time itself on a signal changing many millions of times a second. With signals that are fractions of a millionth of a second long, even small delays in signals travelling along wires must be taken into account when designing circuit boards.

The bulb in our circuit does not turn on immediately, but the delay is so short that it is only significant in situations far removed from school laboratory benches and domestic lighting.

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