Power — AQA GCSE Physics
Test yourself on Power with AQA GCSE practice questions.
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Power explained
Power tells you how fast energy moves, not how much moves.
Read the full explanation
It is defined as the energy transferred per second, or equally the work done per second, and both give P = E ÷ t and P = W ÷ t. One watt is one joule per second, so a 2000 W kettle transfers 2000 J every second. Rearranged, E = P t, which is how energy transferred by an appliance is found from its rating and running time. Two devices can transfer identical amounts of energy and still have very different powers: the faster one has the higher power. Mega and kilo prefixes are common in this content, so a 696 MW wind farm transfers 696 000 000 J each second. Power reappears in the electricity topic through P = V I and P = I² R, which reach the same watts by a different route.
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.
The clearest illustration of power is two devices doing identical jobs at different speeds. Two motors lift the same weight through the same height, so both do the same work and transfer the same energy; the motor that finishes in half the time has twice the power. The same logic runs through the specification: two kettles that boil identical volumes of water transfer similar energy, but the quicker one has the greater power, and a student running upstairs has a higher power output than one walking the same stairs. Turn it into numbers whenever you can: work done = force × distance, then divide by the time, and the comparison becomes a pair of values in watts.
Students should be able to explain how the power transfer in any circuit device is related to the potential difference across it and the current through it, and to the energy changes over time: power = potential di f f erence × current P = V I power = current × resistance P = I R power, P, in watts, W potential difference, V, in volts, V current, I, in amperes, A (amp is acceptable for ampere) resistance, R, in ohms, Ω
Power is the energy transferred each second. Each coulomb passing through a device transfers V joules, and I coulombs pass every second, so the energy per second is P = V I. Substituting V = I R for a resistor gives the second form, P = I² R. Note that the current is squared, so doubling the current quadruples the power dissipated, which is why thick cables are used for large currents. Choose the equation that matches the data: P = V I when you are given potential difference and current, P = I² R when you are given current and resistance. A 230 V kettle drawing 8.7 A transfers 230 × 8.7 = 2001 W, about 2 kW, and a 3 A current in a 5 Ω element gives P = 3² × 5 = 45 W.
Your focus
- Define power as the energy transferred or work done each second and state its unit.
- Calculate the energy transferred by an appliance from its power rating and running time in seconds.
- Convert a power quoted in megawatts or kilowatts into watts before use in a calculation.
Show all 11 objectives
- Determine the power of a device from a measured energy transfer and the time it took.
- Explain why two motors that lift the same load to the same height can have different powers.
- Calculate and compare the power of two appliances from the energy each transfers and the time each takes.
- Suggest why a measured power output for a person climbing stairs exceeds the value found from height alone.
- Rank everyday devices by power using data on energy transferred and time taken.
- Calculate the power of a device from its potential difference and current using P = V I.
- Calculate the power dissipated in a resistor from the current and resistance using P = I² R.
- Explain why doubling the current in a cable quadruples the power wasted heating it.
Power exam tips
Marking Points
- one mark for a definition in terms of energy transferred per unit time or work done per unit time
- one mark for the equation E = P t or P = E ÷ t written correctly
- one mark for a correct substitution with time in seconds
- one mark for an answer in watts, or in joules when energy is the unknown
- one mark for stating that the same energy is transferred or the same work is done by both devices
- one mark for identifying the shorter time as the reason for the greater power
- one mark for supporting the comparison with a calculation using P = E ÷ t
- one mark for a conclusion naming which device is more powerful with its value in watts
- one mark for selecting P = V I or P = I² R to match the quantities given
- one mark for correct substitution of the values into the chosen equation
- one mark for the calculated power with the unit watts, W
- one mark for a correct rearrangement, such as I = P ÷ V, when the power is the quantity supplied
Examiner Tips
- 💡Write the equation in words as well as symbols when the question says write down the equation which links the quantities.
- 💡Convert MW and kW to W before substituting, then convert back only if the answer line asks for it.
- 💡Quote the unit watt or joule per second, since a bare number loses the final mark.
- 💡Anchor the comparison on what is equal first, then name the one thing that differs.
- 💡If the question gives a weight in newtons, work done is weight × height with no extra step for g.
- 💡For a stairs question, remember the runner also gains a kinetic store, so the total power output exceeds the value from height alone.
- 💡Write the equation, substitute, then evaluate; the substitution mark is awarded even when the arithmetic slips.
- 💡Check the equation sheet for the squared term, because the printed specification line is easy to misread.
- 💡Convert kilowatts to watts before substituting, and state the unit with every answer.
Common Mistakes
- defining power as the energy transferred, with the per second left out
- leaving time in minutes or hours when the equation needs seconds
- mixing up the symbols P and W, so writing work done where power was asked for
- treating a more powerful device as one that transfers more energy in total rather than more energy each second
- claiming the faster motor transfers more energy, when it transfers the same energy in less time
- comparing powers without first checking that the weight lifted and the height are the same
- giving the answer as more powerful because it is bigger or uses more electricity, with no reference to time
- calculating work done with the distance moved sideways rather than the height lifted
- using P = I R instead of P = I² R because the square is easy to miss in printed text
- forgetting to square the current, so a 3 A current in 5 Ω is given as 15 W
- mixing kilowatts and watts in the same calculation without converting
- quoting the answer in joules because power and energy have been confused