# Electrical Engineering - Inductors - Discussion

### Discussion :: Inductors - General Questions (Q.No.18)

18.

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

Explanation:

No answer description available for this question.

 Sumeet said: (Oct 4, 2011) In a RL circuit V=v(1-e^-t*R/L) as t = 2*L/R So e^-2*L/R*R/L would be e^-2 as a result V = .86*v. So current i =V/R i.e. 86*20/220 = 78 ma(approx.)

 Pravin Jadhav said: (Sep 10, 2013) Please explain that how l/r term comes?

 Giri said: (Mar 1, 2014) TIME CONSTANT AND CURRENT RELATIONSHIP: 0 0% of max current. 1Ts 63.2% of max current. 2Ts 86.5% of max current. 3Ts 95% of max current. 4Ts 98.2% of max current. 5Ts 100% of max current. First find the maximum possible current through the circuit. Inductor obeys completely a short circuit on DC(ignoring coil resistance). So the impedance of the circuit is only R. Max current is I = V/R. ie,. 20/220 = 0.090909. Question is asked for the two time constant that is '2Ts'.. So the Equivalent current is 86.5% of max current(0.090909) from the above data = (86.5/100)x0.090909 =0.0786363 or 78mA.

 Kavi said: (Apr 25, 2014) How comes time constant and current relationship?

 Adeel Awan said: (Jul 6, 2014) RL circuit V=v(1-e^-t*R/L). as t = 2*L/R. So e^-2*L/R*R/L. e^-2. as a result V = .86*v. So current I =V/R i.e. 86*20/220 = 78 mA(approx.).

 Roopesh said: (Jun 18, 2015) While calculating current, why inductive reactance doesn't come in picture?

 Vinay Raj said: (Nov 28, 2015) @Roopesh. Because it is battery, a DC source. So Xl won't come into a picture.

 Dhaval said: (May 4, 2017) Why taking t=2L/R? This time constant is for series RLC circuit. And there is no capacitor in the circuit. This circuit is RL circuit.for RL circuit time constant is t=L/R.

 Dhaval said: (May 4, 2017) Yes, you gave the right solution, Thanks @Giri.

 Jone said: (Apr 8, 2018) 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.

 Haris said: (Jan 11, 2020) Thanks @Giri.