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In a circuit, an ideal cell is connected in parallel with a coil (L = 4 H, R = 0) and a fuse F (R = 0) in series with a resistor. The fuse blows when current through it reaches 5 A. The switch is closed at t = 0. When does the fuse blow?

1. after 5 sec
2. after 2 sec
3. after 10 sec
4. almost at once

Correct answer: almost at once

Solution

With an ideal cell (say 10 V) and a 2-ohm resistor in parallel with the inductor: at t = 0+, the inductor carries zero current (open circuit), so the full current V/R = 5 A flows through the fuse — it blows almost at once.

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