A motor is used to lift boxes onto a shelf in a warehouse. For every 400 J of electrical energy supplied to the motor, only 100 J is transferred as useful gravitational potential energy. Explain why the motor 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 warehouse motor transfers electrical energy to lift boxes. For every 400 J of electrical energy input, 100 J is gained as gravitational potential energy by the boxes.
Model answer (5 marks)
The motor is not 100 % efficient because some of the electrical energy supplied is lost as heat. In the motor the electrical energy is converted into mechanical work by the magnetic field, but friction in the brushes, windage and internal resistance of the windings dissipate part of this energy as thermal energy. This heat raises the temperature of the motor and its surroundings, spreading the energy into the environment in a less useful form. The total energy supplied still equals the sum of the useful gravitational potential energy and the wasted heat, so the efficiency is the ratio of useful output to total input and can never be greater than 1 (or 100 %).
Examiner tips
- Use the term ‘heat’ or ‘thermal energy’ for the waste; mention friction or resistance in the motor; explain that the heat raises temperature and spreads into surroundings; state the efficiency formula and that it cannot exceed 1.
- Keep the answer concise – 5 marks can be earned with 3–4 short sentences.
Mark scheme (5 marks)
- The motor is not 100% efficient because some energy is dissipated / wasted
- Energy is dissipated as heat / thermal energy (due to friction in the motor's moving parts)
- This causes a rise in temperature of the motor / surroundings
- The wasted energy is spread out into the surroundings (so it is stored in a less useful way)
- The total energy input still equals the total energy output (conservation of energy), so efficiency is the ratio of useful output energy to total input energy and can never exceed 1 / 100%
Key terms in this question
gravitational potential energy
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
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