A student is investigating the properties of a superconducting material. The student records the following measurements: a critical temperature of 0.000093 K above absolute zero, an electrical resistance of 0.000000047 Ω at room temperature, and a magnetic field strength of 0.0000003 T. Express each measurement using an appropriate SI prefix and explain why using SI prefixes is more useful than writing values in standard form for communicating measurements to other scientists.

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)

Critical temperature: 93 µK (micro‑kelvin) – 0.000093 K = 93 × 10⁻⁶ K.
Electrical resistance: 47 nΩ (nano‑ohm) – 0.000000047 Ω = 47 × 10⁻⁹ Ω.
Magnetic field strength: 300 nT (nano‑tesla) – 0.0000003 T = 300 × 10⁻⁹ T (or 0.3 µT).
Using SI prefixes makes the numbers shorter and easier to read, avoiding long strings of zeros or many decimal places. Prefixes are internationally standardised, so scientists everywhere interpret the same units without ambiguity.

Examiner tips

  • Show the conversion step for each value; use the correct prefix symbol (µ, n, etc.).
  • Explain both practical readability and international standardisation.
  • Use UK spelling (micro‑kelvin, nano‑ohm).

Common mistakes

  • Writing 93 µK as 93 × 10⁻⁶ K without the prefix symbol; "µ" is the accepted symbol. Using the wrong prefix (e.g. 93 mK instead of µK).
  • Confusing 0.3 µT with 300 nT – both are correct but the question expects the most convenient prefix.

Mark scheme (5 marks)

  1. Critical temperature correctly converted: 93 μK (micro-kelvin) or 93 × 10⁻⁶ K
  2. Electrical resistance correctly converted: 47 nΩ (nano-ohm) or 47 × 10⁻⁹ Ω
  3. Magnetic field strength correctly converted: 300 nT (nano-tesla) or 0.3 μT (micro-tesla)
  4. SI prefixes are more compact / easier to read / avoid writing many zeros or long decimal numbers
  5. SI prefixes are internationally recognised / standardised so scientists worldwide understand the same units without ambiguity

Key terms in this question

SI prefix

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