A tumble dryer uses electrical energy to dry clothes. The tumble dryer transfers 600 J of electrical energy in one cycle. Of this, 450 J is transferred as useful thermal energy to the clothes. Explain what is meant by waste energy transfer in this context and describe TWO ways in which the efficiency of the tumble dryer could be increased.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A tumble dryer contains a rotating drum driven by an electric motor. The drum spins the clothes whilst hot air is blown through them. The motor and the drum both produce unwanted energy transfers during operation.
Model answer (5 marks)
Waste energy transfer is energy that the tumble dryer gives off in a form that does not help dry the clothes. For example, the motor and the drum generate heat and sound that are not used for drying.
1. Lubricate the motor and drum bearings. This reduces friction, so less electrical energy is turned into heat that is not used for drying.
2. Add thermal insulation to the drum casing. This keeps more of the heat inside the drum, so a larger fraction of the input energy becomes useful thermal energy for the clothes.
1. Lubricate the motor and drum bearings. This reduces friction, so less electrical energy is turned into heat that is not used for drying.
2. Add thermal insulation to the drum casing. This keeps more of the heat inside the drum, so a larger fraction of the input energy becomes useful thermal energy for the clothes.
Examiner tips
- Define waste energy transfer clearly. Explain each improvement and link it to a reduction in wasted energy and a rise in efficiency. Use terms such as ‘friction’, ‘heat loss’, ‘insulation’ and ‘efficiency’ exactly as in the mark scheme.
Common mistakes
- Failing to identify a specific waste form (e.g. sound or motor heat). Describing improvements that do not reduce waste energy, such as changing the motor speed. Using vague phrases like ‘make it better’ without explaining how efficiency rises.
Mark scheme (5 marks)
- Waste energy transfer is energy transferred by the tumble dryer to a form that does not directly serve the purpose of drying clothes
- Identifies a correct form of waste energy in context, e.g. heat from the motor / sound from the rotating drum
- First method: lubrication of the drum / motor to reduce friction, so less energy is wasted as heat
- First method linked to efficiency: less energy is wasted so a greater proportion of the input energy is transferred usefully, increasing efficiency
- Second method: thermal insulation of the drum casing to reduce heat lost to the surroundings, so more energy is available as useful output
Key terms in this question
waste energy transfer · efficiency
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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