# A garden irrigation pump is connected to a mains power supply. The pump operates at a potential difference of 230 V and draws a current of 2 A. After one hour of continuous use, the pump stops working. A technician finds that the resistance of the pump motor has increased significantly compared to when it was first switched on. Explain why the resistance of the pump motor increases during use, and describe how this affects the power transferred by the pump.

> OCR A-Level Physics B: Advancing Physics (H557) — 3.3 Work, energy and power · Explain · 5 marks

> A garden irrigation pump is connected to a mains power supply. The pump operates at a potential difference of 230 V and draws a current of 2 A. The pump runs continuously for one hour before the technician inspects it.

## Mark scheme (5 marks)

1. As current flows through the motor, electrons collide with atoms in the resistor/motor windings
2. These collisions transfer energy to the atoms, causing them to vibrate more / the motor gets hotter
3. Increased vibrations make it more difficult for electrons to flow, so resistance increases
4. Power is equal to current multiplied by potential difference (P = IV), so if resistance increases and potential difference stays the same, current decreases
5. Therefore power transferred decreases because power depends on current and a lower current means less power is transferred

## Key terms

- [resistance](https://www.gradenine.co.uk/glossary/resistance)
- [potential difference](https://www.gradenine.co.uk/glossary/potential-difference)
- [power](https://www.gradenine.co.uk/glossary/power)
- [current](https://www.gradenine.co.uk/glossary/current)

## Related

- [Revision notes for OCR A-Level Physics B: Advancing Physics (H557)](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-garden-irrigation-pump-is-connected-5f118bbb) · Published by Druglandscape Ltd.