A washing machine transfers electrical energy when it is running. Explain how improving the efficiency of the washing machine reduces waste energy transfer.

WJEC GCSE Physics (Wales) — 1.3 Making use of energy · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A washing machine has a power rating of 2 kW. Much of the electrical energy input is lost as heat and sound rather than being used to wash the clothes.

Model answer (4 marks)

Efficiency is the ratio of useful output energy to total input energy. If the washing machine is made more efficient, a larger proportion of the electrical energy is converted into useful work (washing the clothes) and a smaller proportion is lost as heat and sound. Because energy cannot be created or destroyed, the reduction in useful energy output must be matched by a reduction in waste energy transfer, so less heat and sound is produced.

Examiner tips

  • Define efficiency as useful output ÷ total input
  • Explain that higher efficiency means more useful energy, less waste
  • Mention conservation of energy to justify why waste falls
  • Use terms ‘heat’ and ‘sound’ as examples of waste

Common mistakes

  • Failing to define efficiency correctly
  • Confusing ‘useful energy’ with ‘total energy’
  • Not linking higher efficiency to reduced waste energy

Mark scheme (4 marks)

  1. Efficiency is the ratio of useful output energy transfer to total energy input
  2. Waste energy transfer is energy transferred to a form that does not directly serve the purpose of the system (e.g. heat/sound in the washing machine)
  3. Improving efficiency means a greater proportion of the total input energy becomes useful energy transfer
  4. So the waste energy transfer (heat/sound) is reduced because energy cannot be created or destroyed (conservation of energy)

Key terms in this question

efficiency · waste energy transfer

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