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Switches

Electricity and magnetism · level 2, lesson 2 of 6

A question to start

When you press a light switch, nothing moves that you can see, yet the light comes on. What does a switch do?

The idea

A switch is a way to make and break a gap in a circuit.

  • Switch on (closed): two metal pieces touch. The loop is complete and the electricity flows.
  • Switch off (open): the metal pieces are pulled apart. There is a gap and the flow stops.

A switch can be anywhere in the loop, before or after the bulb. Either way, it opens or closes the whole loop.

open: a gap, offclosed: no gap, on

Worked examples

1. A paper clip switch. Push two drawing pins into card, wind the circuit wires round them, and lay a paper clip across the pins. Lift the clip: off. Lay it down: on.

2. A doorbell. The button is a switch that is only closed while you press it.

3. Where the switch goes. Put the switch on either side of the bulb. It still turns the bulb on and off.

Try it

  1. What does a switch do to a circuit?
  2. Is a switch open or closed when the light is on?
  3. Does it matter which side of the bulb the switch is on?
  4. Why does a doorbell stop ringing when you let go of the button?
Answers
  1. It makes or breaks a gap in it.
  2. Closed.
  3. No, it opens or closes the whole loop either way.
  4. The button springs back, opening the gap.

Common mistake

"Off means the switch is closed." On a circuit, a closed switch means no gap, so the bulb is on.

Where it leads