# A student is comparing four measurements taken in a school laboratory: a time of 0.002 s, an electrical resistance of 4 700 000 Ω, a distance of 0.000 045 m, and a voltage of 0.031 V. Describe how each measurement can be expressed using an appropriate SI prefix, and explain why using prefixes is useful when recording scientific measurements.

> AQA A-Level Physics (7408) — 3.1.1 Use of SI Units and Their Prefixes · Describe · 5 marks

## Mark scheme (5 marks)

1. 0.002 s expressed as 2 ms (milliseconds)
2. 4 700 000 Ω expressed as 4.7 MΩ (megaohms)
3. 0.000 045 m expressed as 45 μm (micrometres)
4. 0.031 V expressed as 31 mV (millivolts)
5. Prefixes make measurements easier to read / write / compare by avoiding large numbers of zeros (or very small / very large numbers)

## Key terms

- [SI prefix](https://www.gradenine.co.uk/glossary/si-prefix)

## Related

- [Revision notes for AQA A-Level Physics (7408)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-comparing-four-measurements-cc32e469) · Published by Druglandscape Ltd.