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AQA GCSE Physics · Topic 4.1 Energy
Power
In this lesson you'll learn to: define power as the energy transferred per second,
use P = E ÷ t and P = W ÷ t, and explain why two machines can do exactly the same
job but have very different powers.
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Prefer to watch? Scroll to the bottom for the video on this topic
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Start here — have a play
Run up the stairs, then change only the time and run again. Watch two things at once: the runner visibly speeds up, and the power gauge swings much further round. The energy transferred never changes — it's the same stairs — but the power does.
The working
g = 9.8 N/kg · the climb plays in real time · answers to 2 s.f.
▲ The climb plays in real time, so a 10 s climb really does take 10 seconds. That's the point.
Same energy, different power — why?
Because power isn't about how much energy you transfer. It's about how fast you transfer it. The notes below give you the definition, the equation and the unit, plus the exam questions that come up every year.
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Revision notes
What power means
Two students carry identical boxes of books up the same stairs. One takes 10 seconds, the other takes 20. They transfer exactly the same energy — but the quicker one is twice as powerful.
Learn this definition word for word
Power is the rate at which energy is transferred, or the rate at which work is done. In other words, it is the energy transferred each second.
The equations
There are two, and they are the same equation wearing different hats. Use whichever matches the words in the question.
P = E ÷ t
power = energy transferred ÷ time taken
| Symbol | Quantity | Unit |
|---|---|---|
| P | power | W (watts) |
| E | energy transferred | J (joules) |
| t | time taken | s (seconds) |
P = W ÷ t
power = work done ÷ time taken
⭐ Key fact — what a watt actually is
1 watt = 1 joule per second. A 60 W bulb transfers 60 joules every second it is on. So "watts" already contains the "per second" — you never divide by time twice.
Kilowatts and megawatts
Watts are small, so real appliances are quoted in kilowatts. Convert to watts before you substitute, exactly as you would with joules:
- 1 kW = 1000 W (2.4 kW = 2400 W)
- 1 MW = 1 000 000 W (50 MW = 50 000 000 W)
| Thing | Typical power | Meaning |
|---|---|---|
| LED bulb | 10 W | 10 J every second |
| Old filament bulb | 60 W | 60 J every second |
| Person running upstairs | ≈ 350 W | 350 J every second |
| Microwave | 800 W | 800 J every second |
| Kettle | 3 kW = 3000 W | 3000 J every second |
| Family car engine | 75 kW | 75 000 J every second |
| Wind turbine | 3 MW | 3 000 000 J every second |
💡 Exam tip — the two-machines question
"Two motors lift the same load through the same height. Motor A takes 4 s, motor B takes 10 s. Compare their powers." The answer: they transfer the same energy, but A does it in less time, so A is more powerful. Never say A "does more work" — it doesn't.
💡 Exam tip — minutes into seconds
Power is joules per second, so any time in minutes or hours must be converted first: 1.5 min × 60 = 90 s. This is the single most common mistake on this spec point.
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Worked examples
EXAMPLE 1A student of mass 60 kg runs up a flight of stairs 3.0 m high in 5.0 s. Calculate their useful power output.
Energy transferred to the gravitational potential store
E = m g h = 60 kg ×
9.8 N/kg ×
3.0 m = 1764 J
Power is that energy divided by the time
P = E ÷ t = 1764 J ÷
5.0 s = 352.8 W
P = 350 W
To 2 s.f. That's 350 joules transferred every second — about six 60 W bulbs.
EXAMPLE 2A 2.4 kW kettle is switched on for 90 s. Calculate the energy it transfers.
Convert the power into watts first
2.4 kW × 1000 = 2400 W
Rearrange P = E ÷ t for E
E = P t = 2400 W ×
90 s = 216 000 J
E = 220 000 J (220 kJ)
To 2 s.f. Either form is fine — but always write the unit.
EXAMPLE 3A 60 W lamp transfers 18 000 J of energy. For how long was it switched on?
Rearrange P = E ÷ t for t
t = E ÷ P = 18 000 J ÷
60 W = 300 s
The question may want it in minutes
300 s ÷ 60 = 5.0 min
t = 300 s (5.0 minutes)
EXAMPLE 4Two cranes lift identical loads through the same height. Crane A takes 4.0 s, crane B takes 10 s. Each does 12 000 J of work. Compare their powers.
Crane A
P = W ÷ t = 12 000 ÷ 4.0
= 3000 W = 3.0 kW
Crane B
P = W ÷ t = 12 000 ÷ 10
= 1200 W = 1.2 kW
A is 2.5 times as powerful as B
Both do the same work. A is more powerful only because it does it in less time.
📘 Now do it in your workbook
Exam-style power calculations — including the rearranging and the unit conversions — with full worked answers so you can mark your own work.
AQA GCSE Physics Workbook · ENERGY · page 11The full workbook covers the whole of Topic 1 Energy with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
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Prefer to watch? Here's the whole thing
Every calculation on this page worked through out loud, including the kW and minute conversions.
▲ In GHL you can also use the lesson's built-in video field instead of this embed.
Now put it into practice
Page 11 of the workbook is free. Power questions are short and very scoreable — as long as you convert the units first.
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