Home · IGCSE 0625 · Motion, forces, energy · Power · Practice
Practice questions · Power

Joules per second.

Six original Cambridge-style questions on calculating power from work or energy and time, rearranging the equation, and the difference between power and energy.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Per second

P = W ÷ t = ΔE ÷ t.

01
[2 marks]

Define power, write an equation for it, and state its unit.

  • Power is the rate of doing work, or rate of transferring energy. ✓
  • P = W ÷ t (= ΔE ÷ t); unit watt (W), 1 W = 1 J/s. ✓
02
Calculation
[2 marks]

A motor does 900 J of work in 15 s. Calculate its power.

P = W ÷ t = 900 ÷ 15

60 W

03
Calculation
[2 marks]

A 60 W lamp is left on for 120 s. Calculate the energy it transfers.

E = P × t = 60 × 120

7200 J

04
Calculation
[2 marks]

A crane transfers 6000 J of energy to a load in 12 s. Calculate the power of the crane.

P = ΔE ÷ t = 6000 ÷ 12

500 W

05
Analysis
[2 marks]

Two motors do the same amount of work. Motor A finishes in 5.0 s and motor B in 10 s. State which motor is more powerful and explain why.

  • Motor A is more powerful. ✓
  • It does the same work in less time, and P = W ÷ t, so a smaller time gives a greater power. ✓
06
Calculation
[2 marks]

A 200 W device transfers 5000 J of energy. Calculate the time this takes.

t = E ÷ P = 5000 ÷ 200

t = 25 s