A student uses an electric fan heater to warm a room. Describe the useful energy transfer and the waste energy transfer that occur when the fan heater operates. Then explain, using the law of conservation of energy, why the efficiency of the fan heater can never exceed 100%.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electric fan heater is designed to heat a room. It takes in electrical energy and transfers it in several ways.
Model answer (5 marks)
The useful energy transfer is the conversion of electrical energy into thermal energy that heats the room.
The waste energy transfer is the conversion of electrical energy into sound energy by the fan motor, which does not contribute to heating.
The law of conservation of energy states that energy can be transferred, stored or dissipated but never created or destroyed.
Because the useful output energy can never exceed the total input energy, no extra energy can be produced.
Since some input energy is always lost as waste, the useful output energy is always less than the total input energy, so the efficiency is always less than 100%.
The waste energy transfer is the conversion of electrical energy into sound energy by the fan motor, which does not contribute to heating.
The law of conservation of energy states that energy can be transferred, stored or dissipated but never created or destroyed.
Because the useful output energy can never exceed the total input energy, no extra energy can be produced.
Since some input energy is always lost as waste, the useful output energy is always less than the total input energy, so the efficiency is always less than 100%.
Examiner tips
- Use the exact terms ‘useful energy transfer’ and ‘waste energy transfer’.
- Show the conservation law and link it to the inequality of useful vs total energy.
- Explain why efficiency <100% by stating that some energy is always lost.
Common mistakes
- Confusing the fan’s mechanical work with useful heating energy.
- Claiming the efficiency can reach 100% by ignoring waste sound energy.
Mark scheme (5 marks)
- The useful energy transfer is the transfer of electrical energy to thermal (heat) energy that warms the room
- The waste energy transfer is the transfer of electrical energy to sound energy (from the fan motor) that does not directly serve the purpose of heating the room
- The law of conservation of energy states that energy can be transferred, stored or dissipated but never created or destroyed
- The useful output energy can never be greater than the total input energy because no extra energy can be created
- Some input energy is always dissipated as waste energy transfer, so the useful output energy is always less than the total input energy, meaning efficiency is always less than 100%
Key terms in this question
useful energy transfer · waste energy transfer · conservation of energy · efficiency
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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