A student uses an electric fan heater to warm a room. Explain why the efficiency of the fan heater can never reach 100%, and describe TWO ways in which the efficiency of a different appliance, such as a motor, could be improved.

Eduqas GCSE Physics — 1.3 Making use of energy · Explain · 4 marks · View as Markdown

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

A fan heater is designed to transfer electrical energy into useful thermal energy and kinetic energy of the air. In practice, some energy is always transferred in ways that do not directly serve this purpose.

Model answer (4 marks)

A fan heater cannot reach 100 % efficiency because, by the law of conservation of energy, the electrical energy supplied must be divided between useful energy (heat to the room and kinetic energy of the air) and waste energy (heat lost to the surroundings, sound, vibration, etc.). The useful output energy can never exceed the total input energy, so the efficiency, defined as useful output divided by input, can never be 1 (or 100 %).

For a motor, efficiency could be improved by:
1. Applying lubrication to reduce friction between moving parts, which lowers the energy lost as heat.
2. Adding thermal insulation to the motor housing, which reduces heat loss to the environment and keeps more energy available for useful work.

Examiner tips

  • Define efficiency as useful output ÷ input and state the conservation of energy principle; explain why output cannot exceed input; give two specific, realistic improvements for a motor; use correct terminology (friction, lubrication, thermal insulation).

Common mistakes

  • Claiming the heater can reach 100 % efficiency; confusing efficiency with power; not specifying the two improvements for the motor or using vague terms like "better design".

Mark scheme (4 marks)

  1. Energy cannot be created or destroyed (conservation of energy), so some energy will always be transferred as waste energy transfer rather than useful energy transfer.
  2. Efficiency can never exceed 1 (or 100%) because the useful output energy transfer can never be greater than the total energy input.
  3. Lubrication — applying a lubricant such as oil to reduce friction between moving surfaces, which reduces waste energy transfer.
  4. Thermal insulation — adding an insulating material to reduce the transfer of heat from the appliance to its surroundings, so more energy goes to the useful output.

Key terms in this question

efficiency

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