A electric kettle is used to boil water. Explain why the kettle is not 100% efficient, and describe what happens to the energy that is not usefully transferred.

WJEC GCSE Physics (Wales) — 1.6 Distance, speed and acceleration · Explain · 4 marks · View as Markdown

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

An electric kettle has a power rating of 2 kW. When in use, not all of the electrical energy is transferred usefully.

Model answer (4 marks)

The kettle’s power rating of 2 kW means 2000 J s⁻¹ of electrical energy is supplied. Only a part of this energy is converted into heat that raises the temperature of the water to its boiling point – this is the useful energy.

The remainder is lost as waste energy. It is carried away by the kettle’s metal body, the surrounding air, and to a lesser extent by sound and radiation. These losses mean the total energy output (useful + waste) equals the electrical input, in accordance with the conservation of energy.

Because some energy is always diverted to these waste channels, the efficiency, defined as useful energy divided by total input, can never reach 1 (100 %).

Examiner tips

  • Use the definition of efficiency and state the two energy streams (useful and waste).
  • Mention the conservation of energy to justify why the sum of outputs equals the input.
  • Explain that waste energy includes heating of the kettle body, air, and sound.
  • Keep the answer concise – 4 marks can be earned with 2–3 short sentences.

Common mistakes

  • Saying the kettle is 100 % efficient – ignores waste energy.
  • Failing to identify the waste energy channels (e.g. only mentioning heat loss).
  • Not linking the efficiency to the conservation of energy principle.

Mark scheme (4 marks)

  1. The useful energy transfer is heating the water (to boiling point)
  2. Some energy is transferred as waste energy (e.g. heating the surroundings / the kettle body / sound)
  3. Because energy cannot be created or destroyed (conservation of energy), the total energy input always equals useful plus waste output
  4. Efficiency cannot reach 1 (100%) because some energy is always transferred as waste energy transfer rather than useful energy transfer

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