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In the circuit shown, E = 25 V, L = 2 H, C = 60 microF, R1 = 5 ohm, R2 = 10 ohm. Switch S is closed at t = 0. Find the current through R1 at t = 0.

1. 0
2. 5 A
3. 2.5 A
4. 4 A

Correct answer: 5 A

Solution

At t = 0+: inductor L has zero current (open circuit). Capacitor C is uncharged (short circuit). With L open and C short: the branch with L is open, so current flows through E -> R1 -> C (short) path. Current = E / R1 = 25 / 5 = 5 A. (R2 is in series with L which is open, so no current through R2 branch at t=0.)

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