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A square conducting loop of side 10.0 cm lies perpendicular to a uniform magnetic field of 20 T. A 10 V bulb is connected in series with the loop. The field is reduced steadily to zero over a time interval deltaₜ. For what deltaₜ will the bulb shine at full brightness (i.e. the induced emf equals 10 V)?

1. 20 ms
2. 0.02 ms
3. 2 ms
4. 0.2 ms

Correct answer: 20 ms

Solution

The flux through the loop is B*A. As B falls to zero uniformly, the induced emf is A*B/deltaₜ. Setting this equal to the bulb's 10 V rating and solving gives deltaₜ.

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