A student measures the width of a human hair as 0.00008 m. Explain how the student could express this measurement using an appropriate SI prefix, and state why using SI prefixes is useful in scientific work.

AQA A-Level Physics (7408) — 3.1.1 Use of SI Units and Their Prefixes · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

The width can be written as 80 µm (micrometres) or 0.08 mm (millimetres). The symbol µ represents the prefix micro‑, meaning 10⁻⁶, so 80 µm = 80×10⁻⁶ m = 0.00008 m. The symbol m for milli‑ means 10⁻³, so 0.08 mm = 0.08×10⁻³ m = 0.00008 m.

Using SI prefixes prevents the need to write many zeros, making the number shorter and easier to read. It also allows scientists to compare and communicate values quickly, because the same prefix system is used worldwide, giving a standardised and consistent way of expressing quantities.

Examiner tips

  • Show the conversion to 80 µm or 0.08 mm and state the prefix symbol and its factor; include the reason for using prefixes (shorter numbers, easier comparison, standardisation).
  • Use correct UK spelling and the exact symbol µ for micro‑.
  • Mention that the prefix system is universal for scientific communication.

Common mistakes

  • Writing 0.00008 m as 80 mm instead of 80 µm; confusing micro‑ and milli‑; not stating the factor (10⁻⁶ or 10⁻³).
  • Using a non‑SI unit such as inches or centimetres; not explaining why prefixes are useful.

Mark scheme (5 marks)

  1. Converts 0.00008 m correctly to 80 µm (micrometres) OR 0.08 mm (millimetres)
  2. Identifies the correct prefix symbol used (µ for micro OR m for milli) and its meaning
  3. States that SI prefixes avoid writing large numbers of zeros / very small or very large numbers
  4. States that SI prefixes make values easier to read, compare, or communicate
  5. States that SI prefixes are part of a universal/standardised system used by scientists worldwide, allowing consistency

Key terms in this question

SI prefix

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