A student uses an electric toaster to make toast. The toaster has a power-rating of 900 W. Explain why the toaster cannot transfer all of the electrical energy input as useful energy, and describe what happens to the energy that is not usefully transferred.

Eduqas GCSE Physics — 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).

Model answer (4 marks)

The toaster converts electrical energy into heat energy to toast bread, but not all of the electrical energy is used for this purpose. Some of the energy is lost as waste heat that is transferred to the surrounding air and the toaster’s metal parts. This waste heat is a form of thermal energy that is not useful for cooking. According to the law of conservation of energy, energy cannot be created or destroyed; it can only change form. Because of these unavoidable waste energy transfers, the efficiency of the toaster can never reach 100 % – the useful output energy is always less than the total electrical energy input.

Examiner tips

  • Use the term ‘waste heat’ or ‘thermal energy’ to show understanding of energy transfer.
  • Show the conservation of energy principle and link it to efficiency < 1.
  • Explain that the toaster’s heat is partly lost to the surroundings, not all goes to the bread.

Common mistakes

  • Confusing electrical energy with the energy that actually cooks the bread.
  • Claiming the toaster is 100 % efficient or that all energy becomes toast.
  • Ignoring the role of heat loss to the environment.

Mark scheme (4 marks)

  1. The toaster produces waste energy transfer(s) / not all energy goes to the bread/toast
  2. Identifies a named form of waste energy transfer (e.g. heat/thermal energy transferred to the surroundings / light energy)
  3. States that energy cannot be created or destroyed / conservation of energy
  4. States that efficiency can never reach 1 (100%) because of waste energy transfers / the useful output energy is always less than the total energy input

Key terms in this question

power-rating

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