Energy in systems: before and after

To track energy, compare the start and the end of a change. The total is always the same, but some is usually wasted.

Practise this lesson free (7 questions)

Comparing start and end

Pick a system (the objects you are interested in) and compare its start and end.
Ask what has changed: the speed (kinetic store), the height (gravitational store), the temperature (thermal store), a stretch or squash (elastic store), or the chemicals (chemical store).
Example: a cyclist rides up a hill. The chemical store in her muscles decreases; the gravitational store increases; some energy heats her and the surroundings.

Conservation and waste

Energy is conserved: the total at the end equals the total at the start. It is never made or destroyed.
energy supplied = useful energy + wasted energy
Wasted energy usually spreads into the surroundings as heat and sound: it is dissipated.
A bouncing ball never gets back to its starting height, because some energy is dissipated by heating and sound at each bounce.

Explaining the steps

Energy accounts show how much changes. To explain how it happens, describe the physical process: a force doing work, an electric current, heating, or a wave such as light or sound.
Example: in a torch, the battery drives a current through the bulb; the hot filament gives out light and heats the surroundings.
Worked example

An electric motor is supplied with 500 J. It lifts a load, giving 350 J to its gravitational store. How much energy is wasted?

  1. energy supplied = useful + wasted
  2. wasted = 500 − 350

Answer: 150 J

Key idea

Compare the start and the end of a change to see which stores increase and decrease. Total energy is conserved: energy supplied = useful + wasted. Wasted energy is dissipated, mostly as heat.

Key words
Speed
How far something travels each second: speed = distance ÷ time, often in m/s.
Force
A push or a pull, measured in newtons (N). It is an interaction between two objects.
Current
The flow of electric charge round a circuit, measured in amperes (A).

All KS3 Science key terms and definitions →

Common mistakes
Quick check

Try two quick questions

See if it has sunk in. Answers are marked straight away.

Extra practice

2 more questions

Try each one, then tap to check.

  1. A kettle is supplied with 150 000 J. 135 000 J heats the water. How much energy is wasted?

    Show the answer

    Answer: 15000 J

    150 000 − 135 000 = 15 000 J wasted

  2. A filament lamp is supplied with 100 J each second and wastes 90 J as heat. How much energy is transferred usefully as light each second?

    Show the answer

    Answer: 10 J

    100 − 90 = 10 J

Check you have got it

Answer 7 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.

Practise this lesson free (7 questions)All KS3 Science lessons

Video for this lesson coming soon. Subscribe on YouTube

Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.