Electrical Circuit
Electricity and Magnetism

Paying the electricity bill

Teaching Guidance for 11-14 Supporting Physics Teaching

Making instructive connections, and finding out what you pay for

It is also interesting and instructive to talk about the relative costs involved in running different electrical appliances. The first thing to establish here is what it is that you are actually paying the electricity board for. The straight answer is dissipating energy.

For example, a two bar electric fire operates at 2 kilowatt and shifts:

2000 joule of energy each second

20,000 joule of energy every 10 second

200,000 joule of energy every 100 second

As soon as the fire is switched on, the electricity meter starts spinning around more quickly as the fire heats up the surroundings. The meter keeps a record of the total amount of energy shifted by all of the electrical appliances in the house.

It is instructive to make some direct comparisons between the costs of using different electrical appliances. It's not too difficult to bring these matters close to the pupils' own interests!

Teacher: I remember when I was a young lad (!), I used to play my hi-fi record player all of the time. My mother would get fed up with this, and when the electricity bill came would blame me for the size of the bill, saying, Look at this bill! No wonder we have to pay so much for electricity with you listening to that record player all the time. Now, my mother didn't know too much about science. Was she right about the electricity bill? Do you think I was really to blame? Has anybody here had a similar sort of experience?

The bill from your electricity company is not set out in terms of joules of energy shifted. As you can see from the example of the two bar electric fire, this would very quickly give rise to some rather big numbers!

Instead the unit of energy is taken as the kilowatt hour.

1 kilowatt hour is the amount of energy shifted when a

1 kilowatt device is left running for 1 hour.

The electricity companies refer to 1 kilowatt hour as a unit of energy, and charge you for the accumulated energy shifted, measuring your accumulation in these units. (The kilowatt hour (kW h) is a unit of energy, just like the joule.)

For example:

A 2 kilowatt electric fire left running for 3 hours shifts 6 kilowatt hour of energy (or 6 units of energy).

A 100 watt light bulb (0.1 kilowatt) left on for 60 hours shifts 6 kilowatt hour hours of energy (or 6 units of energy).

Introducing the kilowatt hour offers useful extension work for some pupils.

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