A washing machine transfers electrical energy to clean clothes. Explain why the washing machine is not 100% efficient, and describe what happens to the energy that is not usefully transferred.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A washing machine has a power rating of 2 kW. During a wash cycle, only some of the electrical energy input is usefully transferred to the clothes and water.
Model answer (4 marks)
A washing machine is not 100 % efficient because not all the electrical energy supplied is converted into useful work on the clothes. Some of the energy is lost as waste energy.
The main losses are:
1. Heat – friction in the motor, bearings and drum causes thermal energy to be produced.
2. Sound – vibration of the drum and motor radiates acoustic energy.
These waste energies are transferred to the surroundings and dissipated, in accordance with the conservation of energy.
The main losses are:
1. Heat – friction in the motor, bearings and drum causes thermal energy to be produced.
2. Sound – vibration of the drum and motor radiates acoustic energy.
These waste energies are transferred to the surroundings and dissipated, in accordance with the conservation of energy.
Examiner tips
- Use the word "waste energy" or "energy losses" to show understanding of efficiency.
- Mention both heat and sound as specific forms of waste energy.
- Explain that the lost energy is transferred to the surroundings, not destroyed.
Common mistakes
- Saying the machine is 100 % efficient or ignoring heat and sound losses.
- Forgetting to state that the waste energy is transferred to the surroundings.
- Using vague terms like "other losses" without specifying heat or sound.
Mark scheme (4 marks)
- The washing machine produces waste energy transfer(s) / not all energy input is usefully transferred
- Energy is wasted as heat/thermal energy due to friction in moving parts (e.g. drum motor / bearings)
- Energy is also wasted as sound energy (e.g. vibration of the drum/motor)
- The waste energy is transferred to the surroundings / dissipated, consistent with the conservation of energy (energy cannot be created or destroyed)
Related
- All Eduqas GCSE Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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