Engineering Board Exam Reviewer · Applications: Electric Circuits and Vibrations · medium

An RL circuit has L=0.5 H and R=20 . The current is 4 A when the source is removed and the coil discharges through the resistor. How many milliseconds until the current falls to 1 A?

  1. 17.33 ms
  2. 69.31 ms
  3. 34.66 ms
  4. 138.63 ms

Answer

C. 34.66 ms

Solution

Ldi/dt+Ri=0 i=4e-(R/L)t=4e-40t. Setting i=1: e-40t= 14 t= 4/40 0.03466 s =34.66 ms.

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