Explain why a central heating system that pumps hot water through pipes to radiators is able to warm all the water in a large tank, even though the heating element is only at the bottom of the tank.

Cambridge International IGCSE Physics (0625) — 2.3 Transfer of thermal energy · Explain · 4 marks · View as Markdown

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

A large water storage tank is heated by a pipe carrying hot water that enters near the base of the tank. After some time, the water throughout the entire tank is warm.

Model answer (4 marks)

The water near the heating pipe absorbs heat and its temperature rises.
The heated water expands and becomes less dense, so it rises.
Cooler, denser water sinks to replace it, creating a convection current.
The convection current continuously circulates, transferring thermal energy throughout the whole volume of water in the tank.

Examiner tips

  • Use the word "convection" to show understanding of the process.
  • Show the sequence: heating → expansion → rise → replacement by cooler water → circulation.
  • Mention that the current keeps repeating, so all water eventually warms.

Common mistakes

  • Confusing conduction with convection; forgetting to mention density change.
  • Not explaining that the current keeps repeating, so the whole tank warms.
  • Using vague terms like "mixing" instead of "convection current".

Mark scheme (4 marks)

  1. The water near the heating pipe gains thermal energy and its temperature increases.
  2. The heated water expands and becomes less dense, so it rises.
  3. Cooler, denser water sinks to replace it, creating a convection current.
  4. This convection current continuously circulates, transferring thermal energy throughout the whole volume of water in the tank.

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