RLC Parallel Circuit Analysis

Thread Starter

zorro_phu

Joined Jun 14, 2018
20
I am confused why inductor current at t=0- is 5uA(in the solution)Screenshot_20180628-104526_1.jpg . But I think its 0A because no current flow because switch is open.I solved it as below.can someone help me understand the circuit diagram.Thank you
Screenshot_20180628-105319_1.jpg
 

Thread Starter

zorro_phu

Joined Jun 14, 2018
20
I forgot to mention, why capacitor voltage is 0v at t=0-. Is not the voltage of resistor = voltage of capacitor(t<0) since they are in parallel?
 

WBahn

Joined Mar 31, 2012
24,695
The solution is probably either for an earlier version of the the problem or the person doing it didn't read the schematic closely enough.
 

MrAl

Joined Jun 17, 2014
6,489
I forgot to mention, why capacitor voltage is 0v at t=0-. Is not the voltage of resistor = voltage of capacitor(t<0) since they are in parallel?
Hello there,

Are you using the schematic drawn there?
If so, where are you getting -5ua from?
The cap voltage is not 0v at t=0.
 

WBahn

Joined Mar 31, 2012
24,695
Hello there,

Are you using the schematic drawn there?
If so, where are you getting -5ua from?
The cap voltage is not 0v at t=0.
If I'm reading his original post correctly, he is saying that the solution provided is claiming that the initial inductor current (at t = 0-) is 5 uA. That is the main thing he is questioning. Similarly, I think the solution is saying that the initial cap voltage is zero and he is questioning that.

I haven't looked at his entire solution, but I took a quick look at his initial conditions and I think he got them right.
 

MrAl

Joined Jun 17, 2014
6,489
If I'm reading his original post correctly, he is saying that the solution provided is claiming that the initial inductor current (at t = 0-) is 5 uA. That is the main thing he is questioning. Similarly, I think the solution is saying that the initial cap voltage is zero and he is questioning that.

I haven't looked at his entire solution, but I took a quick look at his initial conditions and I think he got them right.
Hi,

Oh i thought i saw that he found the current source to be -5ua but maybe i read that wrong.
Yes i agree it is 5ua up until t=0 and then it goes to zero, but before that the cap must have charged up to some degree which puts a voltage across it.
[LATER]
I went back and looked at his work and he has -5ua written and then that produces what he wrote as Vc=-1uv, which is not correct.

Just for a point of verification, i got a peak current in the inductor of between 0.5 and 0.6 amps but i used R=1, C=1, L=1, and Vc(0)=1. If he wants to redo the problem with those values, he should be able to find that result within those bounds.
 

WBahn

Joined Mar 31, 2012
24,695
I went back and looked at his work and he has -5ua written and then that produces what he wrote as Vc=-1uv, which is not correct.
I agree with you. His cap voltage is consistent with the current he has in his diagram, but that current is not correct relative to the problem statement.
 
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