Energy and Thermal Physics

Paying for joules at home

Classroom Activity for 11-14 Supporting Physics Teaching

What the Activity is for

To establish that domestic appliances use energy at different rates and give rise to different financial costs.

What to Prepare

  • a collection of power ratings for domestic appliances.
  • a diary of a day's activity for the appliances, showing the number of seconds that it was used for during one 24-hour period.
  • a copy of a current electricity bill for you region, showing the cost for one unit (3.6 megajoule)

What Happens During this Activity

This activity could usefully be carried out with spreadsheets, using technology to help keep track of the calculations.

First explain that the power ratings on the appliances give the number of joules to be paid for one second of use. Then show how to find the total number of joules to be paid for, by multiplying the number of joules per second (the power) by the number of seconds. You'll need to take some care here, as many appliances will have a power in kilowatt, so thousands of joules a second.

Then compile a table of the number of joules to be paid for, and work out the costs for each appliance, using these rows headings for each appliance: appliance power/watt; time used/second ; energy cost/joule; money cost/pound.

Draw special attention to the way in which the common currency of joules allows the pupils to compare very different things such as the cost of ironing with the cost of running a hair-dryer, for example.

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