Electricity and Magnetism

Appliances with different power outputs

Teaching Guidance for 11-14 Supporting Physics Teaching

Linking numbers to effects

It is a very useful exercise for pupils to investigate the power outputs of electrical appliances in and around the home. By collecting the figures and grouping them according to size, pupils can find out that appliances with the biggest power outputs are those which have some kind of heating function.

For example an electric fan heater might have a power output of 2000 watt or 2 kilowatt (2 kilowatt). The heater shifts 2000 J of energy per second.

Microwave ovens often have high and low power output settings:

Microwave low power 650 watt

Microwave high power 750 watt

The directions on the microwave food packet provide details for cooking times. For example, for a bolognese sauce:

  • A microwave with a power output of 650 watt cooks the contents in 4.5 minutes
  • A microwave with a power output of 750 watt cooks the contents in 3.5 minutes

Use sets of figures such as these to help pupils grasp the idea that high power electrical devices can shift a lot of energy in a shorter period of time:

Teacher: Just look at these figures on the packet. What can you say about the different cooking times compared with the high and low microwave power?

Janice: On higher power the cooking time is shorter.

Teacher: Exactly right! Why does that make sense?

Esme: Because on higher power you're putting more energy in.

Teacher: That's right. On higher power the microwave is shifting more joules of energy each second.

appears in the relation P=VI P=I^2R P=V^2/R ΔQ=PΔt
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