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Final value of RL circuit

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s55

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I have an undergraduate level question about RL circuits.
If you visit the following link, there is a simple question about an RL circuit.


I have a solution of the question, but I don't understand it, and so I would like to ask anybody who knows about it.

The solution says
"For t<0, the switch is closed, and the inductors (both L1 and L2) behave like short circuits. The initial current \[I_0\] in the 3H inductor is 100/10=10A."

I pretty much understand this explanation. but the following solution does not make sense to me:
"For t>0, L1 has an initial voltage in s-domain, \[{LI}_{0}=3(10)=30 V\]"

For t<0, L1 becomes a short circuit that means the voltage across L1 is 0V, and the impedance of L1 is also 0 ohm. Then, how does L1 has 30V-s as an initial voltage? Isn't it 0v either?
 

I think it's a bad exercise problem, because v0 rises to infinity when the switch is opened, at least when using an ideal switch.
 

Hi,

I agree with FvM.
But i1 is simple to calculate.

Klaus
 

I think it's a bad exercise problem, because v0 rises to infinity when the switch is opened, at least when using an ideal switch.
Yes.

If we neglect that effect, we could say that the final value of I1 which is also the final value of I2 (at t>>0) is 100/(10 + 15) = 4A.
 

I think it's a bad exercise problem, because v0 rises to infinity when the switch is opened, at least when using an ideal switch.
I understand this exercise is not good, but I need to understand the answer of this question. I think I made everyone misunderstood my question. Sorry about that. My question is NOT how to get i1.

My real question is how we can get 30V as an initial voltage of L1 at t=0, even though L1 becomes short circuit for a long time for t<0. For reference, I attached the screen shot of the solution showing s-domain circuit of this exercise.
 

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First off, I have noticed that the solution doesn't really match the question. The question showed no info of frequency content in the voltage source. Why then is the solution showing such?
--- Updated ---

The circuit in the question shows straight DC. That is why we had to ignore the inductances. If there is frequency content in the voltage source, then the entire analysis would have to be reviewed.
 
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