Solve the problem.
-How many seconds after the switch in an RL circuit is closed will it take the current i to reach 20% of its steady state value? Express answer in terms of R and L and round coefficient to the nearest hundredth.
A) 0.22 L/R seconds
B) 1.81 L/R seconds
C) 1.61 L/R seconds
D) 0.42 L/R seconds

Question

Quizplus · Accepted Answer

The time constant for an RL circuit is L/R. To reach 20% of the steady state value, we use the formula i(t) = I(1 - e^(-Rt/L)). Solving for t when i(t) = 0.2I gives t ≈ 0.22 L/R.

Solve the Problem. -How Many Seconds After the Switch in an RL Circuit