# 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

> 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.

## 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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