GCSE · Physics · AQA · Spec 8463
Power (P = E/t and P = W/t)
Power tells you how fast energy moves — the faster, the more powerful.
What you need to know
- Power is the rate of energy transfer or the rate of doing work.
- Use P = E / t when energy transferred is given; use P = W / t when work done is given.
- One watt equals one joule per second (1 W = 1 J/s).
- Two devices can transfer the same total energy but have different powers if they take different times.
The big picture
Power is the rate at which energy is transferred or work is done. A high-power device does the same job in less time than a low-power one. Power is measured in watts, where one watt means one joule is transferred every second. Understanding power helps you compare devices — a 2000 W kettle and a 200 W lamp may both run on electricity, but they move energy at very different rates.
TONIGHT'S REVISION
Power (P = E/t and P = W/t)
How fast energy is transferred — and how to calculate it
What is power? — the concept unfolded
Follow one unit of energy as it moves through a device. Notice where the 'rate' idea appears.
Stop and think
Commit to an answer before you reveal — this is the exact thinking the exam tests.
A 60 W lamp and a 600 W microwave both run for exactly 10 seconds. The microwave transfers how many times more energy than the lamp?
Power — the rate idea
Reason it through
Why does a higher-power device finish the same job in less time?
First link · your turn
What does a higher power rating actually tell you about each second of operation?
Relationship matrix
Tap any cell to reveal it. Tap a column header to read one property down every item.
Each cell hides a short answer and the reason behind it. Predict before you tap.
Cross-subject · Exam skill
AQA command words — Power questions
What AQA actually wants when it uses each command word on a power question
01Definition
Use a numerical method to produce an answer. Always show the equation used, the substitution, and the final value with its unit.
02Mark-band signal
Full marks require: equation stated or clearly implied, correct substitution, correct answer with unit (W, J, or s).
03Sentence stems
- Calculate the power of the device.
- Calculate the energy transferred in 30 seconds.
- Calculate the time taken to transfer 6000 J at a power of 200 W.
04Common mistake
Forgetting to convert minutes to seconds before dividing — always check the time unit is seconds before substituting.
Equations you need
Taken directly from the exam-board specification.
P = power (W) · E = energy transferred (J) · t = time (s)
Learn it — you must recall this in the exam
P = power (W) · W = work done (J) · t = time (s)
Learn it — you must recall this in the exam
Key points
Worked example
Problem
An electric motor transfers 4500 J of energy in 15 seconds. Calculate the power of the motor.
Memory hook
Think of power as the 'speed of energy' — just as speed = distance ÷ time, power = energy ÷ time. Fast energy transfer = high power.
★ Exam tip
AQA questions often give you energy and time and ask for power — always check whether the question gives 'energy transferred' or 'work done', then pick the matching equation. Both give the same power value when units are consistent. Always include the unit (W) in your answer.
⚠ Watch out
Confusing power (W) with energy (J) — power is the RATE of energy transfer, not the total amount. A 100 W bulb running for 1 hour transfers far more energy than a 100 W bulb on for 1 second.
Check yourself
Without looking — a 60 W lamp and a 600 W microwave both run for 10 seconds. Which transfers more energy, and how much more?
Flashcards
(23)What is power?
What is the equation linking power, energy and time?
What is the equation linking power, work done and time?
What is the unit of power?
What does 1 watt equal in terms of joules and seconds?
A device transfers 600 J in 20 s. What is its power?
A motor does 9000 J of work in 3 minutes. What is its power?
Two kettles transfer 180 000 J. Kettle A takes 90 s; Kettle B takes 180 s. Which is more powerful?
If a device has a power of 500 W and runs for 40 s, how much energy does it transfer?
What is the difference between using P = E / t and P = W / t?
What does a higher power rating mean for two devices doing the same job?
A 1500 W hairdryer runs for 3 minutes. How much energy does it transfer?
State the unit of energy transferred and of work done.
If power stays constant and time doubles, what happens to energy transferred?
If power stays constant and time halves, what happens to energy transferred?
A 200 W television runs for 5 hours. How much energy does it transfer? (Give answer in joules.)
What physical quantity does the symbol P represent in these equations?
What physical quantity does the symbol t represent in the power equations?
What physical quantity does the symbol E represent in P = E / t?
What physical quantity does the symbol W represent in P = W / t?
A pump does 24 000 J of work in 2 minutes. Calculate its power.
Can two devices have the same power but transfer different amounts of energy? Explain.
Can two devices transfer the same energy but have different powers? Explain.
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
Learn Power (P = E/t and P = W/t) properly — interactive practice, marked questions and flashcards.
Start this lesson freeMore AQA GCSE Physics topics
- Acceleration (a = Δv/t)
- Current, resistance and potential difference (V = I R)
- Density of materials (ρ = m/V)
- Distance and displacement
- Distance–time graphs
- Efficiency
- Energy stores and systems
- Gravitational potential energy (Ep = m g h)
- Kinetic energy calculation (Ek = 1/2 m v^2)
- Newton's First Law
- Newton's Second Law (F = m a)
- Resultant forces and resolving forces
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