Consider the magnitudes and directions of the induced potential difference in the following situations: (a) A magnet moving with a speed of 1 m/s with its north pole towards a stationary coil. (b) The same magnet moving with a speed of 1 m/s with its south pole towards the same stationary coil. The induced potential differences are of ______ magnitude(s) and have ______ sign(s) in the above two cases.
- A.the same, the same
- B.the same, opposite
- C.different, opposite
- D.different, the same
Correct answer
Explanation
The correct answer is B, the same, opposite. The same magnet moves at the same speed in both cases, so the rate of change of magnetic flux through the coil is equal and the induced potential difference has the same magnitude. But turning the other pole towards the coil reverses the direction of the field through it, so the induced voltage takes the opposite sign. This follows from Faraday's law of electromagnetic induction, which says the induced emf equals the rate of change of flux, and from Lenz's law, which fixes the direction so that the induced current opposes the change causing it. A is wrong because the sign must flip once the pole facing the coil is reversed. C is wrong because the speed, and therefore the rate of flux change, is unchanged, so the two magnitudes match. D is wrong on both counts. Exam tip: speed decides how big the induced emf is, the field direction decides its sign - Faraday for magnitude, Lenz for direction.
Practice Questions
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