Power
Learn Power for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Power is the rate of energy transfer or work done, measured in watts, where one watt is one joule per second.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Power is the rate at which energy is transferred, or the rate at which work is done. It is measured in watts, and one watt is one joule per second.
There are two equations: power = energy transferred ÷ time, and power = work done ÷ time. Both give the same thing, because work done and energy transferred are the same quantity measured in joules.
A more powerful device transfers the same amount of energy in less time. Two motors that lift the same load through the same height do the same work, but the more powerful one does it faster — the energy transferred is identical, only the time differs.
Revision notes
The equations
Power = energy transferred ÷ time. Power = work done ÷ time.
Energy and work are both in joules, time in seconds, power in watts. One watt equals one joule per second.
Comparing power
A more powerful device transfers the same energy in a shorter time.
If two motors raise the same load through the same height, both do the same work. The more powerful one simply completes it faster, so the energy transferred is the same but the time is less.
Rearranging
Energy = power × time. Time = energy ÷ power.
Check the units before substituting: time must be in seconds, so minutes must be multiplied by 60. Kilowatts must be converted to watts by multiplying by 1000.
Key points
- Power is the rate of energy transfer.
- Power = energy transferred ÷ time.
- Power = work done ÷ time.
- Power is measured in watts.
- One watt is one joule per second.
- A more powerful device transfers energy faster.
Worked examples
Example 1
A motor transfers 6000 J of energy in 20 seconds. Calculate its power. [2 marks]
Model answer
Power = energy transferred ÷ time = 6000 ÷ 20 (1 mark) = 300 W (1 mark).
Example 2
Define power. [2 marks]
Model answer
Power is the rate at which energy is transferred (1 mark), or the rate at which work is done, measured in watts (1 mark).
Example 3
Two motors lift the same load through the same height, one in 5 s and one in 10 s. Compare the work done and the power. [3 marks]
Model answer
Both motors do the same amount of work, because the load and height are the same (1 mark). The motor taking 5 s has twice the power (1 mark), because it transfers the same energy in half the time (1 mark).
Common mistakes
Confusing power with energy.
Energy is measured in joules; power is the rate of transfer, in watts.
Forgetting to convert minutes to seconds.
Time must be in seconds, so multiply minutes by 60.
Saying a more powerful device transfers more energy.
It transfers the same energy in less time.
Using kilowatts without converting.
Multiply kilowatts by 1000 to get watts.
Exam tips
- Convert time to seconds before substituting.
- State watts as the unit for power.
- Distinguish energy from the rate of transfer.
- Remember work done and energy transferred are the same quantity.
Key terms
- Power
- The rate of energy transfer, in watts.
- Watt
- One joule per second.
- Work done
- Energy transferred by a force, in joules.
- Rate
- How much of something per unit time.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.