# 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.

> Edexcel A-Level Physics (9PH0) — 8.1 Particle accelerators and detectors · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. The motor is not 100% efficient because some energy is dissipated / wasted
2. Energy is dissipated as heat / thermal energy (due to friction in the motor's moving parts)
3. This causes a rise in temperature of the motor / surroundings
4. The wasted energy is spread out into the surroundings (so it is stored in a less useful way)
5. 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

- [gravitational potential energy](https://www.gradenine.co.uk/glossary/gravitational-potential-energy)

## Related

- [Revision notes for Edexcel A-Level Physics (9PH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-motor-is-used-to-lift-8195f250) · Published by Druglandscape Ltd.