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A2 Daily A Level Physics question

2026-03-13 OCR A Materials: practical determination of Young modulus (M3.4) OCR A Level Physics A (H556) Module 3.4 Materials: stress, strain, Young modulus OCR A Level Physics A (H556) Module 3 Practical skills: determination of Young modulus (wire) and treatment of systematic errors

In a lab to determine the Young modulus of a steel wire, a micrometer used for the diameter reading has a +3% systematic error (it reads the diameter 3% too large). All other measurements are accurate. What is the resulting percentage error in the calculated Young modulus, and in which direction?

  1. A About 6% too low, because area depends on diameter squared. (correct)
  2. B About 3% too low, assuming area is proportional to diameter.
  3. C About 3% too high, since a larger diameter reduces stress a little.
  4. D About 6% too high, as the wire appears stiffer with a larger diameter.

Answer

The correct answer is A.

Correct: A — About 6% too low, because area depends on diameter squared. With diameter reading +3%, the cross-sectional area is about (+3%)×2 = +6%, and since the calculated modulus varies inversely with area, the result is about 6% lower. B is wrong because it assumes area scales linearly with diameter, giving only 3% rather than the squared effect. C is wrong in both sign and size: overestimating the diameter lowers the calculated modulus, not raises it, and the magnitude would be about 6%, not 3%. D is wrong because the direction is incorrect: a larger measured area lowers stress and thus lowers, not raises, the calculated modulus.

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