Energy stores and transfers

Energy is stored in different ways (kinetic, gravitational, elastic, chemical, thermal…) and transferred between stores. It is never made or destroyed. Efficiency tells you how much of the energy ends up being useful.

Key facts

  • Energy is measured in joules (J).
  • Energy can’t be created or destroyed, only transferred.
  • Efficiency = useful energy out ÷ total energy in (× 100 for a percentage).
  • Wasted energy usually ends up as heat in the surroundings.

Worked examples

Example 1

A light bulb takes in 200 J and gives out 50 J as light. How efficient is it?

  1. Useful out ÷ total in. 50 ÷ 200 = 0.25
  2. As a percentage. 0.25 × 100 = 25%

Answer: 25%

Example 2

Describe the energy transfers when a ball is dropped.

  1. At the top it has energy in its gravitational store.
  2. As it falls, that moves into its kinetic store: it speeds up.
  3. When it hits the floor, energy goes to thermal and sound.

Answer: Gravitational → kinetic (→ thermal and sound)