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AQA · GCSE · Combined Science · Higher

Constant water temperature during heating

Q04.4 2 marks

Question 04 is about a wood-fired hot tub in which the fire continues burning after the water reaches 42 °C.

Diagram of a wood-fired hot tub. A pipe carries water from the hot tub through a heater where burning wood heats the pipe, then back into the hot tub.

Figure 5

Enlarged question image

Diagram of a wood-fired hot tub. A pipe carries water from the hot tub through a heater where burning wood heats the pipe, then back into the hot tub.
Figure 5

The temperature of the water reaches 42 °C. The temperature then stays constant even though the fire continues to burn. Explain why the temperature of the water stays constant.

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Marking points

  1. 1 State that energy is dissipated or lost to the surroundings.
  2. 2 Explain that this happens at the same rate as energy is transferred to the water.

Full-mark answer

Energy is being dissipated to the surroundings at the same rate as energy is transferred to the water, so the water’s internal energy and temperature do not increase.

Why this answer loses marks

The water cannot get any hotter than 42 °C.

This does not explain the balance between energy being supplied and energy being dissipated.

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