# How to calculate the voltage across the inductor in a DC RL circuit at a time constant t=0?

Joined Feb 3, 2016
3
Let us consider a DC RL circuit with time constant t. Now when the time constant t=0 and how should we find the voltage across the inductor

#### dl324

Joined Mar 30, 2015
12,871
Is this homework?

Joined Feb 3, 2016
3
No sir it was a concept given in a book. But i didnot understand this. Because he just given this in a shortcut form.

#### ErnieM

Joined Apr 24, 2011
8,150
One must first ask what was happening at time = t0- or the instant before you ask.

If at t0 a switch is thrown to complete the circuit then you know the current before: simply zero. Then as the current in an inductor cannot change instantaneously you know the inductor current is zero, also the resistor current is also zero.

Ohms law tells you the voltage across the resistor is zero, hence the source voltage is completely across the inductor.