AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Power: Students should be able to give examples that illustrate the definition of power eg comparing two electric motors that both lift the same weight through the same height but one does it faster than the other.
Power is the rate of energy transfer, or the rate of doing work: P = E ÷ t, where E is energy transferred in joules and t is time in seconds, giving power in watts.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Power is the rate of energy transfer, or the rate of doing work: P = E ÷ t, where E is energy transferred in joules and t is time in seconds, giving power in watts. Two motors lifting the same weight through the same height transfer the same energy, because energy transferred equals weight × height, so the slower motor takes longer and has lower power. The faster motor transfers the same energy in less time, so its power is greater. This shows power depends on both energy and time, not on energy alone.
For example: Lifting 600 N through 2 m transfers 1200 J; in 4 s that is 300 W, but in 2 s it is 600 W.
What earns the marks
- States that power is the rate of energy transfer or the rate of doing work, with the equation P = E ÷ t.
- Explains that both motors transfer the same energy because they lift the same weight through the same height.
- Compares the times taken, identifying that the faster motor transfers the same energy in less time.
- Concludes that the faster motor has the greater power because power is energy transferred per second.
- Uses a numerical example, such as 1200 J in 4 s = 300 W compared with 1200 J in 2 s = 600 W.
Common mistakes and how to put them right
Saying the faster motor transfers more energy.
Put it right by noting both transfer the same energy because weight and height are unchanged, and only the time differs.
Confusing power with force or energy.
Put it right by defining power as energy transferred per second, measured in watts.
Using P = E × t instead of P = E ÷ t.
Put it right by checking that a shorter time must give a larger power for the same energy.
Exam tips
- Write the equation, substitute the values with units, then state the conclusion in words.
- When comparing two situations, quote both calculated powers so the comparison is explicit.
- Check that the unit of power is the watt, where 1 W = 1 J/s.
Try the lesson
The start of the Power: Students should be able to give examples that illustrate the definition of power eg comparing two electric motors that both lift the same weight through the same height but one does it faster than the other. lesson, made from checked teaching notes for this specification point.
Have a go first
Two electric motors lift identical loads through the same height. Motor A takes 4 seconds and motor B takes 8 seconds. Which statement best illustrates the definition of power?
Key words
Say what each word means before you turn the card.
Power
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P = E ÷ t
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Watt
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Rate of energy transfer
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See every definition
- Power
- The rate of energy transfer, or the rate of doing work, given by P = E ÷ t with power in watts.
- P = E ÷ t
- The equation where E is energy transferred in joules and t is time in seconds, giving power in watts.
- Watt
- The unit of power, where 1 W = 1 J/s.
- Rate of energy transfer
- Energy transferred per second, so the faster motor transferring the same energy in less time has the greater power.
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