# A delivery drone uses an electric motor to hover in place, holding a parcel at a fixed height. During hovering, the drone's motor transfers 2400 J of electrical energy every 10 seconds. Of this, 600 J per 10 seconds is dissipated as heat due to resistive forces in the motor. Explain why the drone cannot be 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 delivery drone hovers at a fixed height while carrying a parcel. Its electric motor transfers 2400 J of electrical energy every 10 seconds. Of this, 600 J per 10 seconds is dissipated as heat.

## Mark scheme (5 marks)

1. Some energy is always dissipated / lost to the surroundings
2. Resistive forces in the motor cause energy to be transferred as heat / thermal energy
3. The wasted energy is spread out into the surroundings (dissipation of energy)
4. Efficiency cannot exceed 1 (or 100%) because of conservation of energy — useful output energy must always be less than total input energy when dissipation occurs
5. Correct identification of useful output energy and correct efficiency value — 1800 J out of 2400 J is usefully transferred, giving an efficiency of 0.75 (or 75%)

## Key terms

- [resistive force](https://www.gradenine.co.uk/glossary/resistive-force)

## 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-delivery-drone-uses-an-electric-149006fa) · Published by Druglandscape Ltd.