A2 Daily A Level Physics question
A lab AC generator (coil of fixed area and N turns in a uniform magnetic field) drives a 24 Ω resistor. The generator’s internal resistance is negligible. Spun at 10 rev s−1 clockwise, the resistor dissipates 2.0 W on average. It is then spun at 20 rev s−1 in the opposite direction. Which statement must be true?
Answer
The correct answer is A.
Correct: A — The resistor’s average power increases to about 8 W; the current reverses direction twice as often but the heating effect ignores the reversal. A is correct because doubling rotation speed doubles the generated rms voltage, so power in a fixed resistor scales with the square of speed: 2.0 W × (2)^2 ≈ 8 W; reversing spin only flips the phase. B is wrong because power in a purely resistive load depends on the rms voltage amplitude, not directly on frequency, and a sign flip does not cancel the voltage. C is wrong because although heating ignores sign, the voltage amplitude increases with speed, so power cannot remain at 2.0 W (it quadruples). D is wrong because reversing spin does not reduce the emf amplitude; it inverts the waveform and increases frequency, so there is no reduction to 1.0 W; in the limit of zero speed the emf would be zero, consistent with amplitude scaling with speed.