Download Power ratings Power: a measure of how quickly energy is transferred.

Power ratings
Power: a measure of how quickly energy is transferred.
Activity 1 – power ratings
Task 1 – 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 2 – 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 3 – evaluation
Do any of the power ratings surprise you?
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Power ratings
Activity 2 – 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
Remember that mains electricity in the UK 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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Power ratings
Activity 3 – 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. Use 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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Power ratings
Teaching notes
These activities are designed to teach the concept of power at Key Stage 3, but could be
used as an introduction or reminder about power at Key Stage 4 as well. The evaluation
task of ‘Activity 1’ and ‘Activity 3’ are designed to be more challenging.
Activity 1
Task 1
Provide ten 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 2
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 3
More able students could then complete task 3 – 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 2
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 3
1. Complete the gaps in the table. Use might want to use the formula triangle to help
you.
appliance
toaster
hairdryer
television
tumble dryer
current (A)
4.5
2.6
0.65
21.74
voltage (V)
230
230
230
230
power (W)
1035
600
150
5000
2. 115W ÷ 230V = 0.5 A
3. 120 W ÷ 10 A = 12 V
4. 440 W ÷ 4 A = 110 V
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