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Inductors - Engineering

Q1:

A switch, a 220  resistor, and a 60 mH inductor are in series across a 20 V battery. What is the current through the inductor two time constants after the switch is closed?

A 91 mA

B 78 mA

C 57 mA

D 400 mA

ANS:B - 78 mA

In RL circuit V=v(1-e^R/L t)
Given time constant L/R =2
V=20*(1-e^-2)
=17.29v.

Current through inductor is I = V/R
=17.29/220=78mA.