Electrical Engineering - Inductors - Discussion

Discussion :: Inductors - General Questions (Q.No.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

Answer: Option B


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)  

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.
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)  

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

Current through inductor is I = V/R

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

Post your comments here:

Name *:

Email   : (optional)

» Your comments will be displayed only after manual approval.