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 · View as Markdown

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

Model answer (5 marks)

0.002 s can be written as 2 ms (milliseconds). 4 700 000 Ω can be written as 4.7 MΩ (megaohms). 0.000 045 m can be written as 45 µm (micrometres). 0.031 V can be written as 31 mV (millivolts). Using SI prefixes makes measurements easier to read, write and compare because it removes long strings of zeros and keeps numbers within a convenient range.

Examiner tips

  • Use the correct prefix for each value (m, M, µ, m).
  • Show the conversion step (e.g. 0.002 s = 2×10⁻³ s = 2 ms).
  • Explain the benefit: easier comparison and fewer zeros.

Common mistakes

  • Using the wrong prefix (e.g. 0.002 s as 2 µs).
  • Leaving out the unit symbol after the prefix (e.g. 2 ms without ‘s’).
  • Not explaining why prefixes are useful.

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 in this question

SI prefix

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