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A2 Daily A Level Physics question

2026-01-29 OCR A Electromagnetic induction — generators (qual.) (M6.3) OCR-A Module 6.3 Electromagnetic induction — Faraday’s and Lenz’s laws; flux linkage; generators (qualitative)

A bicycle dynamo acts as a simple AC generator. At 5.0 revolutions per second, the oscilloscope shows a peak emf of 3.0 V. With the same coil and magnetic field, and negligible load on the dynamo, what is the peak emf at 8.0 revolutions per second?

  1. A 1.9 V
  2. B 4.8 V (correct)
  3. C 3.0 V
  4. D 7.7 V

Answer

The correct answer is B.

Correct: B — 4.8 V. For a simple generator with unchanged coil and field and negligible load, the peak emf is proportional to rotation speed; increasing from 5.0 to 8.0 rps multiplies the emf by 8/5, so 3.0 × 8/5 = 4.8 V. A assumes emf decreases with speed (∝ 1/f), which is the opposite of the correct proportionality. B matches the proportional dependence on speed and the 8/5 ratio. C assumes no change with speed, ignoring that faster rotation increases the rate of flux change. D implies a squared dependence on speed (or an extra spurious factor), overestimating the increase.

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