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Power ratings
Teaching notes
These activities are designed to teach the concept of ‘power’ to younger pupils, but could be used as
an introduction or reminder about power to older pupils as well. The evaluation tasks of ‘Activity one’
and ‘Activity three’ are designed to be more challenging.
Guidelines and answers
Activity one
Task one
Provide 10 appliances around the room as a ‘circus’. Ask students to find the power rating of each.
Suitable examples would be hairdryer, toaster, kettle, hair straighteners, mobile phone charger, iron,
desk lamp, printer, radio and fan heater. For safety ensure that none of the devices are plugged in.
Task two
Students must fill in the name of the appliance and note down its ‘power rating’, e.g. 20 W. Students
should then try to categorise the appliances into high/medium/low – this will help the students to think
about which use more power than others.
Task three
More able students could then complete task three – the idea is for them to evaluate their findings –
some of the powers might surprise them and this could be used as a discussion point. For example,
hair straighteners only use about 1/10 of a hairdryer, because once heated up initially they don’t
require much energy to ‘keep them going’.
Activity two
1. 2.5 A x 3 V = 7.5 W
2. 230 V x 12 A = 2760 W (NB: more able students could be encouraged to convert high power ratings
into kW, 2.76 kW)
3. 230 V x 18 A = 4140 W (4.14 kW)
4. 230 V x 1 A = 230 W (230 W)
5. 230 V x 15 A = 3450 W (3.45 kW)
Activity three
1. Complete the gaps in the table. You might want to use the formula triangle to help you.
appliance
current (A)
voltage (V)
power (W)
toaster
4.5
230
1035
hairdryer
2.6
230
600
television
0.65
230
150
tumble dryer
21.74
230
5000
2. 115W ÷ 230V = 0.5 A
3. 120 W ÷ 10 A = 12 V
4. 440 W ÷ 4 A = 110 V
Page 1 of 4
© 2015 Oxford International AQA Examinations.
Created by Teachit for Oxford International AQA Examinations.
Power ratings
Power: a measure of how quickly energy is transferred.
Activity one – power ratings
Task one – observation
Your teacher will show you a number of electrical appliances. You need to find the ‘power
rating’ on each. You are looking for a number in watts (W).
Task two – analysis
Complete the table to show the power ratings of the appliances.
Categorise each as ‘high’, ‘medium’ or ‘low’ power rating.
appliance
power rating (W)
high/medium/low
Task three – evaluation
Do any of the power ratings surprise you?
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Page 2 of 4
© 2015 Oxford International AQA Examinations.
Created by Teachit for Oxford International AQA Examinations.
Power ratings
Activity two – power calculations
Power can be calculated if you know the voltage and the current that the appliance uses.
Example:
power (watt, W) = voltage (volt, V) x current (ampere, A)
An electric fire uses a voltage of 230 V and a current of 10 A. What is its power?
power = voltage x current
= 230 V x 10 A
= 2300 W
Questions
Answer the following questions assuming that the mains electricity is 230 V.
1. The current through a lamp is 2.5 A and the voltage across it is 3 V. What is the power of
the lamp?
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2. A kettle is using mains electricity and has a current of 12 A flowing through it. What is the
power of the kettle?
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3. An oven is using mains electricity and has a current of 18 A flowing through it. What is
the power of the oven?
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4. Some hair straighteners are using mains electricity and have a current of 1 A flowing
through them. What is the power of the hair straighteners?
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5. A dishwasher is using mains electricity and has a current of 15 A flowing through it. What
is the power of the dishwasher?
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Page 3 of 4
© 2015 Oxford International AQA Examinations.
Created by Teachit for Oxford International AQA Examinations.
Power ratings
Activity three – rearranging the power equation
Voltage or current can also be calculated from the power equation by rearranging it. It is
often useful to use a triangle to rearrange an equation. In this example, I stands for current.
Example:
power (watt, W) = voltage (volt, V) x current (ampere, A)
An appliance has a power of 460 W and uses mains electricity. What current travels through
it?
current = power ÷ voltage
= 460 W ÷ 230 V
=2A
1. Complete the gaps in the table. You might want to use the formula triangle to help you.
appliance
current (A)
voltage (V)
toaster
4.5
230
hairdryer
2.6
power (W)
600
television
230
150
tumble dryer
230
5000
2. A DVD player is plugged into the mains. It has a power of 115 W. What current is flowing
through the DVD player?
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3. The current flowing through a laptop is 10 A and the power of it is 120 W. What is the
voltage across it?
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4. The current flowing through a toaster is 4 A and its power is 440 W. What is the voltage
of the power supply?
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Page 4 of 4
© 2015 Oxford International AQA Examinations.
Created by Teachit for Oxford International AQA Examinations.