A wood-burning stove is used to heat a room. During this process, 18 000 J of chemical energy is released by burning the wood. Of this, 12 600 J is transferred as useful thermal energy to the room. Describe the useful energy transfer and the waste energy transfer that occur, and explain what is meant by the conservation of energy in this context.

Pearson Edexcel International GCSE Physics (4PH1) — 4.1 Energy transfers · Describe · 4 marks · View as Markdown

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

A wood-burning stove releases 18 000 J of chemical energy when wood is burned. Of this, 12 600 J is transferred as useful thermal energy to the room.

Model answer (4 marks)

The useful energy transfer is the conversion of 12 600 J of the wood’s chemical energy into thermal energy that heats the room.
The waste energy transfer is the remaining 5 400 J, which leaves the stove as thermal energy to the surroundings (e.g. through the chimney or as light from the flame).
Conservation of energy means that energy cannot be created or destroyed; the total energy before the reaction (18 000 J of chemical energy) equals the total energy after (12 600 J useful + 5 400 J waste = 18 000 J).

Examiner tips

  • Use the exact figures (12 600 J, 5 400 J) and state the direction of each transfer.
  • Explain conservation in terms of ‘energy cannot be created or destroyed’ and show the equality of input and output.

Common mistakes

  • Mixing up useful and waste energy amounts.
  • Failing to state that the total energy remains 18 000 J, i.e. not showing conservation.

Mark scheme (4 marks)

  1. The useful energy transfer is from chemical energy (in the wood) to thermal energy (heating the room).
  2. The waste energy transfer is the remaining 5 400 J transferred to the surroundings as thermal energy (e.g. up the chimney / to the outside air) or as light energy from the flames.
  3. Conservation of energy means energy cannot be created or destroyed — the total energy before equals the total energy after.
  4. The total energy output (useful + waste = 12 600 + 5 400 = 18 000 J) equals the chemical energy input of 18 000 J, demonstrating conservation of energy.

Key terms in this question

chemical energy · useful energy transfer · waste energy transfer · conservation of energy · thermal energy

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