A2 Daily A Level Physics question
A hand‑cranked AC generator uses a single rectangular coil rotating at constant speed in a uniform magnetic field, connected to a high‑resistance voltmeter. At the instant when the plane of the coil is parallel to the magnetic field lines, which statement must be true?
Answer
The correct answer is A.
Correct: A — The induced emf has its maximum magnitude because the flux through the coil is changing most rapidly, even though the instantaneous flux is zero. A rotating coil produces emf proportional to how fast the flux linking it changes; when the plane is parallel to the field, the flux is zero but changing fastest. B confuses emf with the value of flux: zero flux does not imply zero emf; it is the rate of change of flux that matters. C is wrong because the flux is not maximum when the plane is parallel to the field; it is actually zero then (flux is maximum when the plane is perpendicular to the field). D mixes generator and motor ideas: torque does not set the emf; with a high‑resistance voltmeter the current (and hence motor torque) is negligible, and in any case large torque does not imply zero emf.