Work, Power & Energy Resources
Energy Resources & Energy Transfers
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 9% of your exam marks.
W = Fd, P = W/t and comparisons of renewable vs non-renewable energy sources tested regularly.
Definition
- Power is the rate at which work is done, or equivalently the rate at which energy is transferred
- The SI unit is the watt (W), where 1 W = 1 J/s (one joule transferred per second)
- Common multiples: 1 kW = 1000 W; 1 MW = 1 000 000 W
Why time matters
- Two machines that do the same total work over different times have different powers. A weightlifter and a forklift may both lift a 200 kg crate onto a shelf, but if the forklift does it in 1 s while the weightlifter takes 30 s, the forklift's power is 30 times larger
- High-power devices either deliver large amounts of useful work very quickly (a rocket engine) or low amounts of useful work continuously over long periods (a wall-mounted clock running for years)
The power equation
P = W / t
- where:
- P = power (W)
- W = work done, or energy transferred (J)
- t = time taken for the transfer (s)
- Useful rearrangements:
- W = P × t (the energy a device transfers in a given time)
- t = W / P (how long a device of fixed power needs to deliver a target amount of energy)
Typical power ratings
| Device | Approximate power |
|---|---|
| Pocket torch | ~1 W |
| Incandescent light bulb | ~60–100 W |
| Hair dryer | ~1500 W |
| Electric oven | ~3 kW |
| Family car at cruise | ~25 kW |
| Large coal-fired power station | ~2 GW (2000 MW) |
| Saturn V rocket at launch | ~60 GW |
Defining power
What comes up: the exam asks you to "state what is meant by the term power" (1 mark).
Write: power is the rate at which energy is transferred, or equivalently the rate of doing work.
Watch out: the mark scheme rejects "energy transferred in a unit time" (the word "in" instead of "per" loses the mark) and also rejects defining power as current × voltage — that formula is not the definition.
Calculating time from power and energy
A pump transfers 1 350 000 J of energy at a constant power of 4500 W. Calculate the time taken.
Solution:
- Write the equation: P = W ÷ t
- Rearrange: t = W ÷ P
- Substitute: t = 1 350 000 ÷ 4500
- t = 300 s
Calculating efficiency
A small electric motor receives 600 J of electrical energy and delivers 480 J of useful mechanical energy. Calculate the efficiency of the motor.
Solution:
- Write the equation: efficiency = (useful energy output ÷ total energy input) × 100%
- Substitute: efficiency = (480 ÷ 600) × 100%
- efficiency = 80%