Power — Work, Power & Energy Resources | IGCSE Physics

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 9% of your exam marks.

stable

Medium

Stable9%

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
Saturn V rocket at launch	~100 MW
Large coal-fired power station

Exam tip

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.

Worked example

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

Worked example

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%