H1 Physics 8867 · Nov 2023PhysicsPaper 1Commented

Paper 1 Q2 — Uncertainty of a difference

Measurement — combining uncertainties when quantities are subtracted

What the examiners said

“The candidates who got this wrong chose the other three distractors equally which indicates a lack of understanding of how to combine uncertainties when quantities are being subtracted from each other.”

Try the question (reconstruction)

A student measures the outer diameter of a pipe as a = (52.0 ± 0.5) mm and the inner diameter as b = (28.0 ± 0.5) mm. What is the wall-related difference (a − b) with its uncertainty?

Examiner report: “Distractors were chosen equally — candidates did not know how to combine uncertainties under subtraction.”

Explore — why uncertainties add when you subtract

Slide the uncertainty of each measurement. The bars show every value (a − b) could take. Watch the true interval (green) against the ‘subtract the uncertainties’ misconception (red).

Δa (mm): 0.5 Δb (mm): 0.5

Key idea: the simulated dots regularly land outside the red interval — subtracting uncertainties underestimates the possible spread.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H1 Physics 8867 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

Made by lookang, using Claude Fable 5. More resources: iwant2study.org