AQA GCSE Combined Science Foundation (8464), Foundation tier · Physics › Energy › Energy changes in a system, and the ways energy is stored
Practise Power. 9 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Power is how quickly energy is transferred or work is done. You need to calculate power, energy and time with P = E ÷ t and P = W ÷ t, and explain what a greater power means. Questions are usually 2 to 4 marks, often joined to work done, gravitational potential energy or efficiency.
Key facts
Power = rate of energy transfer = rate of doing work
P = E ÷ t
P = W ÷ t
Units: P in W, E and W in J, t in s
1 W = 1 J/s
1 kW = 1000 W; 1 MW = 1 000 000 W
E = P × t and t = E ÷ P
Notes
What power means
Power is the rate at which energy is transferred, or the rate at which work is done.
The unit of power is the watt (W). An energy transfer of 1 joule per second is a power of 1 watt.
Example: a 40 W lamp transfers 40 J of energy every second.
The equations
P = E ÷ t and P = W ÷ t (recall both).
P = power (W), E = energy transferred (J), W = work done (J), t = time (s).
Rearranged: E = P × t and t = E ÷ P.
When a motor lifts a load at a steady speed, the useful work it does equals the gain in the load's gravitational potential energy (m g h).
Comparing powers
diagram
Two motors lift the same weight through the same height. They do the same amount of work, but the one that does it in less time has the greater power.
Same work done, less time, so motor A has the greater power.
A 3 kW kettle heats the same amount of water faster than a 2 kW kettle, because it transfers energy at a greater rate.
A more powerful device does not always transfer more energy in total: that also depends on how long it is switched on.
How to answer each type of question
Calculate the power
2 marksGrade 4
Write P = E ÷ t (or P = W ÷ t).
Check the energy is in J and the time is in s.
Substitute and give the unit W.
Example. An electric motor transfers 9000 J of energy in 30 s. Calculate the power of the motor.
Show the model answerHide the model answer
P = 9000 ÷ 30 (1) P = 300 W (1)
Explain which device has the greater power
2 marksGrade 5
Compare the energy transferred (or work done) by each device.
Compare the times taken.
Link to power: the same energy in less time means a greater power.
Example. Motor A and motor B each lift a weight of 5.0 N through a height of 2.0 m. Motor A takes 4.0 s and motor B takes 10 s. Explain which motor has the greater power.
Show the model answerHide the model answer
Both motors do the same amount of work (10 J) (1). Motor A does it in less time, so it transfers energy at a greater rate and has the greater power (1).
Don’t lose marks
Leaving the time in minutes or hours instead of seconds.
Not converting kW (or MW) to W.
Defining power as 'energy' or 'how strong something is'. Power is the rate of energy transfer.
Mixing up W for the watt (the unit) with W for work done (in the equation).
More tips
Memory tricks
Power = how fast; energy = how much.
Triangle: E on top, P and t underneath. Cover the one you want.
Sense check: household devices range from a few watts (a phone charger) to about 3 kW (a kettle).
If you keep the power in kW and the time in seconds, the energy comes out in kJ.
Exam technique
For a definition, use the word 'rate': 'power is the rate of energy transfer'.
In compare questions, mention both the energy (or work done) and the time.
Give the final unit: W, or kW if the question asks for it.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State the unit of powerPower is measured in watts (W); 1 W is 1 joule transferred per second.
Grade 4
Define powerPower is the rate at which energy is transferred, or the rate at which work is done.
Grade 4
Calculate power from energy and timeDivide the energy transferred (J) by the time taken (s).
Grade 5
Compare the power of two machinesIf two motors do the same work, the one that takes less time has the greater power.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Write down the equation that links power (P), time (t) and work done (W).
P = W ÷ t
Write down the equation that links power (P), energy transferred (E) and time (t).
P = E ÷ t
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Power is the rate at which energy is transferred or the rate at which work is done.
(a) Which unit is the same as one watt? Tick (✓) one box.[1]
joule per second
joule second
kilogram per second
newton per metre
(b) An electric motor transfers 3600 J of energy in 45 s. Calculate the power of the motor. Use the equation: power = energy transferred ÷ time[2]
(c) A crane does 72 000 J of work in 60 s. Calculate the power of the crane. Use the equation: power = work done ÷ time[2]
Show the answer and mark scheme
(a)Answer: joule per second
(b)Answer: 80 W
P = 3600 ÷ 45
P = 80 (W)
(c)Answer: 1200 W
P = 72 000 ÷ 60
P = 1200 (W)
Question 2Medium6 marks
Two electric motors, A and B, are used to lift identical loads. Each load has a weight of 400 N and is lifted vertically through 3.0 m. Motor A takes 8.0 s to lift its load. Motor B takes 5.0 s to lift its load.
(a) Calculate the work done on each load.[2]
(b) Calculate the useful power output of motor B.[2]
(c) Explain why motor B has a greater useful power output than motor A.[2]
Show the answer and mark scheme
(a)Answer: 1200 J
W = 400 × 3.0
W = 1200 (J)
(b)Answer: 240 W
P = 1200 ÷ 5.0
P = 240 (W)
(c)Answer: Both motors do the same work, but motor B does it in less time, so it transfers energy at a greater rate.
both motors do the same (amount of) work / transfer the same (amount of) energy
motor B does it in less time / transfers energy at a higher rate