# Linear Systems question...

Discussion in 'Homework Help' started by Vim0314, Mar 27, 2010.

1. ### Vim0314 Thread Starter New Member

Sep 1, 2009
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Hey guys. First time posting, but been reading stuff on here for a while.

I have attached a scan from my old test and I'm having trouble figuring out part b. Could someone help me figure it out?

I understand the cap is replaced by a voltage source and the inductor is replaced by a current source. But I am having trouble figuring out the current in the voltage source and the voltage across the current source.

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2. ### Ryoshima New Member

Mar 19, 2009
1
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I believe that:

Ic = -20 A
Vl = 120 V

Don't think those are right, but they are the results that I obtained. I'll keep an eye on this thread to see if anyone can confirm or deny my calculations.

3. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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Ryoshima, no one will deny your calculation or Vim0314 for that matter.
Some one will just confirm the calculations and will let you know if you are wrong or right.

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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I have VL(0+)=-54V and IC(0+)=-9A

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
You need to write a loop voltage equation that satisfies the loop conditions at t=0+. Keep in mind that VC(0+)=16V and IL(0+)=4A

Assume a current I is flowing in the 6Ω resistor - from this and the above information you can write a suitable loop voltage equation with I as the unknown.

Last edited: Mar 27, 2010
6. ### Vim0314 Thread Starter New Member

Sep 1, 2009
3
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I found the answer in the solutions manual and here it is attached...

In blue I boxed in the correct answers I am looking for. But in yellow I have circled where I have a problem. Can someone explain to me how they figured out the Ir1? and how a 24 magically appears?

thanks

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7. ### t_n_k AAC Fanatic!

Mar 6, 2009
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One could write ..... somewhat laboriously ......

VR1+VR2+VR3+48=0

IR1*2+16+IR3*6+48=0

IR1*2+16+(IR1-4)*6+48=0

IR1*2+16+IR1*6-4*6+48=0

IR1(2+6)+16-24+48=0

IR1(2+6)=-48-16+24

IR1=(-48-16+24)/(2+6)=-40/8=-5

8. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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Rather than hijack a thread it's better for all concerned if you start a new thread.

Keep in mind also the expectation that homework forum posts will show an attempt at solving the problem.

Last edited by a moderator: May 28, 2010