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An L-R circuit connected to a battery is switched on at t = 0. Immediately after the switch is closed, which of the following quantities is NOT zero?
- current in the circuit
- magnetic energy stored in the inductor
- power delivered by the battery
- emf induced in the inductor
Correct answer: emf induced in the inductor
Solution
Just after switching on, the inductor prevents an instantaneous jump in current, so the current is zero at t = 0+. Therefore the magnetic energy (1/2 L i²) is zero, and power delivered by the battery (V*i) is zero. However, the inductor develops a back-emf equal to the full battery emf at that instant (since L*di/dt is maximum and equals V), so the induced emf is non-zero.
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