A2 Daily A Level Physics question
In the standard Young modulus practical, a 1.0 m copper wire of diameter 0.40 mm extends by about 1.0 mm under a 30 N load. The dominant reading uncertainty in extension is ±0.10 mm. You must repeat the experiment but reduce the percentage uncertainty in the value of E while ensuring the maximum stress does not exceed that in the original setup. You may change only one of the options below. Which is the best choice?
Answer
The correct answer is A.
Correct: A — Use a 2.0 m length of the same 0.40 mm wire with the same 10–30 N load range. Doubling L doubles the extension (ΔL ∝ L), so the relative uncertainty falls from 0.10/1.0 ≈ 10% to 0.10/2.0 ≈ 5% while the stress F/A is unchanged, keeping within the original elastic limit. A — As shown, it both increases sensitivity and keeps maximum stress the same, satisfying the constraint. B — Thinner wire increases extension by about (0.40/0.28)^2 ≈ 2.0, but it also roughly doubles the stress for the same loads, risking exceeding the elastic region, which the question forbids. C — Increasing the load to 60 N also roughly doubles extension but doubles the stress too, again risking leaving the elastic region and violating the condition. D — Two wires in parallel double the area so extension halves (~0.5 mm), making the relative uncertainty worse (0.10/0.5 ≈ 20%); averaging does not fix resolution-limited uncertainty.