AQA Combined Science (8464) 6.1.1.4 · Quantitative
Power
Combined Science · Foundation tier
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A 3000 W kettle and a 1500 W kettle each heat 0.5 kg of water from 20 °C to 100 °C.
If you had to guess, does the 3000 W kettle transfer more energy to the water, or the same energy, faster?
Key words
- power
- watt
- energy transferred
- work done
Key knowledge
Power is the rate at which energy is transferred or the rate at which work is done.
An energy transfer of 1 joule per second is equal to a power of 1 watt.
power = energy transferred ÷ time
P = E ÷ t
power = work done ÷ time
P = W ÷ t
- power, P, in watts (W)
- energy transferred, E, in joules (J)
- work done, W, in joules (J)
- time, t, in seconds (s)
You need to remember both equations.
Two motors lift the same weight through the same height.
They do the same amount of work.
The motor that does it in less time has the greater power.
Joules and seconds go in. Watts come out. Convert anything else first.
- 1 kW = 1000 W
- 1 MW = 1 000 000 W
- 1 kJ = 1000 J
- 1 minute = 60 s
- 1 hour = 3600 s
Wrong: It took 2 minutes, so divide by 2.
Right: Time must be in seconds. 2 minutes is 120 s.
Wrong: The more powerful motor can lift the heavier load.
Right: Power is how quickly energy is transferred. It is not a force.
Key fact
Power is the rate at which energy is transferred or the rate at which work is done. One watt is an energy transfer of one joule per second.
- Define: give the exact meaning in one sentence.
- Calculate: convert, write the equation, substitute, rearrange, then give the answer with its unit.
- Explain: link the work done and the time taken to the rate of energy transfer.
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