A2 Daily A Level Physics question
In a lab, a bar magnet’s north pole is pushed straight towards a flat coil. With a 200-turn coil and speed v, the peak induced emf is 3.0 V. The coil is then replaced by a 400-turn coil and the magnet is pushed at 0.5v. Which outcome must occur?
Answer
The correct answer is C.
Correct: C — 3.0 V; the face nearer the magnet becomes a north pole. The induced emf scales with number of turns and with how quickly flux changes, so doubling turns and halving speed gives the same peak (factor 2 × 0.5 = 1), and Lenz’s law means the near face becomes north to oppose the approaching north pole. A is wrong because it halves the emf by noticing the slower motion but ignores the doubling of turns. B is wrong because it overestimates the emf (doubling only) and also has the wrong polarity: a south face would attract the approaching north and reinforce the change, not oppose it. C is correct as explained. D is wrong because it assumes emf depends only on the number of turns, ignoring the halved speed, though the polarity is stated correctly.