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The side PQ of a rectangular loop first slides outward from x = 0 to x = 2b and then back inward from x = 2b to x = 0, as shown. A uniform magnetic field directed perpendicular to the plane of the loop exists only in the region from x = 0 to x = b. Choose the option that correctly identifies the three graphs A, B and C with the quantities plotted against the position x.

1. A-Flux, B-Power dissipated, C-EMF
2. A-Power dissipated, B-Flux, C-EMF
3. A-Flux, B-EMF, C-Power dissipated
4. A-EMF, B-Power dissipated, C-Flux

Correct answer: A-Flux, B-EMF, C-Power dissipated

Solution

While PQ moves from 0 to b, the enclosed area in the field increases, so flux rises linearly; from b to 2b the loop is fully out of the field-occupied span so flux is constant. EMF = |dphi/dt| is a constant non-zero value while flux changes (0 to b) and zero while flux is constant (b to 2b), reversing sign on the return trip. Power = EMF²/R is non-zero (positive) only when EMF is non-zero. The triangular/trapezoidal linear-then-flat curve is Flux (A), the rectangular pulse that flips sign is EMF (B), and the always-positive rectangular pulse is Power (C).

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