A student records four measurements during a medical physics investigation: a time interval of 0.000003 s, a frequency of 25 000 000 Hz, a length of 0.012 m and a mass of 0.00045 kg. Rewrite each measurement using an appropriate SI prefix, and name the prefix used in each case.

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

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

Model answer (5 marks)

0.000003 s = 3 µs (microseconds)
25 000 000 Hz = 25 MHz (megahertz)
0.012 m = 12 mm (millimetres)
0.00045 kg = 0.45 g (or 450 mg)
Micro, Mega, Milli, Milli

Examiner tips

  • Use the correct prefix symbol (µ, M, m) and write the full name in parentheses; this shows understanding of the conversion. Check that the numeric part is in the correct range for the chosen prefix (e.g. 3 µs, 25 MHz, 12 mm, 0.45 g).
  • common_mistakes
  • :
  • Writing 0.000003 s as 3 ms instead of 3 µs; Using the wrong prefix for the mass, e.g. 0.45 kg instead of 0.45 g or 450 mg.

Mark scheme (5 marks)

  1. 0.000003 s = 3 µs (microseconds) OR correct prefix name 'micro' stated
  2. 25 000 000 Hz = 25 MHz (megahertz) OR correct prefix name 'mega' stated
  3. 0.012 m = 12 mm (millimetres) OR correct prefix name 'milli' stated
  4. 0.00045 kg = 0.45 g OR 450 mg OR correct prefix name 'milli' stated for mg / recognition that 0.45 g uses no prefix on grams
  5. All four prefix names correctly matched to their respective measurements (award this mark only if at least three of the four converted values above are correct)

Key terms in this question

prefix

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