A student measures the resistance of a resistor five times using an ohmmeter. His results are: 47 Ω, 52 Ω, 49 Ω, 48 Ω, and 51 Ω. Describe how the student should process these results to obtain a reliable value for the resistance, and explain what the range of results tells him about the precision of his measurements.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Calculate the mean of the five readings: (47+52+49+48+51)/5 = 247/5 = 49.4 Ω. All five readings are used – none is an outlier. The range is 52−47 = 5 Ω, showing the spread of the data. A smaller range would indicate higher precision; a 5 Ω range means the measurements are not very precise. Precision refers to repeatability of the readings, not to how close they are to the true value.
Examiner tips
- Show the calculation of the mean and range; include all five readings. Explain that a small range = high precision. Mention precision vs accuracy. Use correct units (Ω).
Common mistakes
- Leaving out one reading as an outlier without justification; calculating the mean incorrectly; confusing precision with accuracy; omitting the range calculation.
Mark scheme (5 marks)
- Identifies that the student should calculate a mean (average) of the five readings
- States that all five readings should be included in the mean (no reading is anomalous / there is no outlier to exclude)
- Links a smaller range to greater precision (or states the range indicates how spread out / consistent the readings are)
- States or calculates that the range here is 5 Ω (52 − 47) indicating the measurements are not very precise
- Distinguishes precision from accuracy — precision is about repeatability / consistency of readings, not about closeness to the true value
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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