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

 

Is this homework?

 

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

 

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.