A student uses a hot water bottle to keep warm in bed. The hot water bottle starts at a high temperature and gradually cools down until it reaches room temperature. Explain what happens to the energy stored in the hot water bottle during this process, and why the total energy of the closed system does not change.

AQA GCSE Physics (8463) — 4.1.2 Conservation and dissipation 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 hot water bottle loses thermal energy to the surrounding air and bedding.
The energy is transferred as heat and spreads out into the surroundings.
Energy cannot be created or destroyed – the first law of thermodynamics holds.
The amount of energy lost by the bottle equals the amount gained by the surroundings, so the total energy of the closed system remains constant.

Examiner tips

  • Use the word ‘heat’ to describe the transfer, not just ‘energy’.
  • Show the energy balance: lost = gained.
  • Mention the first law explicitly.
  • Keep each point short and to the point.

Common mistakes

  • Confusing the bottle with the surroundings as separate systems; the system is the bottle + surroundings.
  • Using vague terms like ‘warm up’ instead of ‘heat transfer’.
  • Forgetting to state that the total energy stays the same.

Mark scheme (4 marks)

  1. Thermal energy is transferred from the hot water bottle to the surroundings
  2. The energy is dissipated / spread out into the surroundings
  3. Energy cannot be created or destroyed (conservation of energy)
  4. The energy lost by the hot water bottle equals the energy gained by the surroundings, so the total energy of the closed system stays the same

Key terms in this question

closed system

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