Core · Practice questions · Electrical power and energy

Watts and joules.

Six original Cambridge-style questions on P = IV and E = IVt: finding power, finding energy, rearranging for current or voltage, and converting time before calculating.

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.

How to handle power and energy questions

P = IV, then E = Pt.

Power: P = IV, in watts (one watt is one joule per second).
Energy: E = IVt = Pt, in joules, with t in seconds.
Rearrange: I = P ÷ V and V = P ÷ I when the power is given.

Calculation

[2 marks]

A heater draws a current of 2.0 A from the 230 V mains. Calculate its power.

P = IV = 2.0 × 230

P = 460 W

Calculation

[2 marks]

The same 460 W heater runs for 10 s. Calculate the energy it transfers.

E = Pt = 460 × 10

E = 4600 J

Calculation

[2 marks]

A 12 V motor has a power of 36 W. Calculate the current it draws.

I = P ÷ V = 36 ÷ 12

I = 3.0 A

Calculation

[3 marks]

A 9.0 V lamp draws a current of 0.40 A. Calculate the energy it transfers in 5.0 minutes, giving your answer in joules.

t = 5.0 × 60 = 300 s E = IVt = 0.40 × 9.0 × 300

E = 1080 J

convert minutes to seconds first

Calculation

[2 marks]

A device transfers 6000 J of energy in 30 s. Calculate its power.

P = E ÷ t = 6000 ÷ 30

P = 200 W

Analysis

[3 marks]

Lamp X has a power of 60 W. Lamp Y has a power of 100 W. Lamp X is left on for 5.0 minutes and lamp Y for 3.0 minutes. Calculate the energy transferred by each, and state which uses more.

X: E = Pt = 60 × 300 = 18 000 J Y: E = Pt = 100 × 180 = 18 000 J

Both transfer the same energy, 18 000 J. ✓

higher power but less time can give the same energy

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.