Here are two circuits. One with a voltage source and other with a voltage source. I see in my solution manual that :

For the first circuit (voltage source) ,only one resistor (100 ohm) is used to calculate the value of T (Tau). i.e L/R.

However in the second circuit (current source), resistors are first added in series and then their equivalent value is used.

why is this contradiction?

 

Because you have an ideal voltage source. Hence the voltage across left 100 ohms resistor is always equal to the voltage of a ideal voltage source. So, no effect on the rest of the circuit .
But this is not the case in terms of a current source becouse now you have current divider ( the current will split between R and LR network).

And a tip. If you want to find an equivalent resistance seen from any component point of view you should always replace all voltage source with a short circuit and all current source with an open circuit. Just like we normally do when we are trying to find Thevenin's equivalent resistance Rth.

 

Because you have an ideal voltage source. Hence the voltage across left 100 ohms resistor is always equal to the voltage of a ideal voltage source. So, no effect on the rest of the circuit .
But this is not the case in terms of a current source becouse now you have current divider ( the current will split between R and LR network).

And a tip. If you want to find an equivalent resistance seen from any component point of view you should always replace all voltage source with a short circuit and all current source with an open circuit. Just like we normally do when we are trying to find Thevenin's equivalent resistance Rth.

So , If I talk about this circuit , The Resistance to be used in R/L , will the parallel equivalent resistance ? (60 || 60 || 60)
Considering that we kill the sources.

 

will the parallel equivalent resistance ? (60 || 60 || 60)
Considering that we kill the sources.

No, wrong
You need to find this resistance

 

No, wrong
You need to find this resistance

View attachment 153294

so its 60 || 60 =30 ohm.
am I correct ?

 

No, wrong
You need to find this resistance

View attachment 153294

Its 90 ohms actually . 60 || 60 =30 , then 30 + 60 ohms = 90 ohms
am I right this time?

 

90 ohms is the correct answer.

 

90 ohms is the correct answer.

Thanks alot for your time !

 

I think 60 || 60 =30 , then 30 + 60 ohms = 90 ohms too