A student measures the voltage across a battery using a voltmeter. He takes five readings: 9.1 V, 9.0 V, 8.6 V, 9.1 V, 9.0 V. Explain how the student should process these results to obtain the most reliable value for the voltage, and explain why one of the readings should be treated differently from the others.

AQA A-Level Physics (7408) — 3.1.2 Limitations of Physical Measurements · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

1. Identify 8.6 V as an anomalous result.
2. Exclude 8.6 V from further calculations.
3. Calculate the mean of the remaining four readings: (9.1+9.0+9.1+9.0)/4 = 9.05 V.
4. Explain that repeated measurements reduce random error by averaging.
5. Explain that 8.6 V may have arisen from a random error such as a sudden fluctuation or misreading, making it unrepresentative.

Examiner tips

  • Use the word ‘anomalous’ or ‘outlier’ to show you recognise the odd value. Show the arithmetic for the mean to gain full marks. Mention that averaging reduces random error and that the outlier is likely due to a random error.

Common mistakes

  • Including the 8.6 V reading in the mean. Failing to show the calculation of the mean. Not explaining why the outlier is unrepresentative.

Mark scheme (5 marks)

  1. Identifies 8.6 V as an anomalous result / outlier
  2. States that the anomalous result should be excluded / not included in the mean calculation
  3. States that the mean (average) of the remaining values should be calculated
  4. Explains that taking repeated measurements reduces the effect of random error
  5. Explains that the anomalous result may have been caused by a random error (e.g. a sudden fluctuation or misreading) making it unrepresentative

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