Electricity and Magnetism

Electrical power - a demonstration

Classroom Activity for 11-14 Supporting Physics Teaching

What the Activity is for

The aim of this interactive demonstration is to introduce the idea of electrical power by examining a range of electrical appliances. In addition, for those pupils who are able to follow the line of argument, the link is made from basic ideas of current and voltage to that of power.

What to Prepare

  • collection of electrical appliances
  • joule and watt meter

Some joulemeters are designed to work at low voltages only (for example up to 16 volt), whilst others operate at the mains supply voltage (around 240 volt). You will need to select electrical appliances to demonstrate according to the type of joule and watt meter.

What Happens During this Activity

Use this part of the activity to get pupils talking about a range of familiar electrical appliances.

These might include:

  • A 40 watt bulb in a lamp holder
  • A 100 watt bulb in a lamp holder
  • A CD player
  • Hair tongs
  • A convector heater

Ask the pupils how the two lamps differ from one another and move the discussion on to considering how you would know which kind of bulb to buy in a supermarket:

Teacher: OK, so we have these two bulbs, one dim, one bright. How would you know which kind to get in a supermarket?

: Is it called the watts? Like a 100 watt?

Teacher: That's exactly right. In fact this here is a 100 watt bulb. What do you think the dim one is? How many watts?

Move towards the idea that this value in watts is the electrical power of the device and that the power measures the amount of energy shifted per second (see narrative and teaching and learning issues).

Different appliances: using a wattmeter

Look at each of the appliances in turn and ask the pupils to predict their power output. If you have access to a wattmeter, it is interesting at this point to connect it up to the different appliances.

Wattmeters measure the power dissipated by a device as time goes by. Most have some kind of flashing display, on which each flash indicates 100 joule of energy shifted. The faster the flashing, the greater the rate at which energy is shifted, the bigger the power output. With some meters the output can be connected up to an amplifier and speaker so that you can hear the rate of flashing.

It is helpful to make the link from the rate of flashing to the model of charged particles working in the device and shifting energy as they go:

Teacher: Wow! Just look at that. Can't you just see the charged particles working hard and shifting energy as they pass through the tongs! The power output is really huge here.

This kind of activity leads into the homework task.

For those classes which are able to cope with the ideas involved you might now return to the base ideas of current (as charged particles passing per second) and voltage (as energy shifted per coulomb) and develop the idea of power (as energy shifted per second).

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