An 8.0-mH inductor and a 2.0-$\Omega$ resistor are wired in series to an ideal battery. A switch in the circuit is closed at time 0, at which time the current is 0. The current reaches half its final value at a time of:
A) 2.8 ms
B) 4.0 ms
C) 3 s
D) 170 s
E) 250 s

Question

Quizplus · Accepted Answer

The time constant for an LR circuit is given by τ = L/R. For the current to reach half its final value, it takes approximately 0.693 times the time constant. With L = 8.0 mH and R = 2.0 Ω, τ = 4 ms, so 0.693τ ≈ 2.8 ms.

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