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Homework Help: RL RC circuit

  1. Oct 9, 2007 #1
    1. The problem statement, all variables and given/known data
    The problem is:
    After being closed for along time, the switch shown in figure 1 opens at t=0. Find IA for all time.

    2. Relevant equations
    I believe it would be i(t)=Ioe^(-t/(CReq)) or i(t)=(Q/P)+Ae^(-Pt)
    Q/p=Vo/r and 1/p is time constant or t/(CReq)

    3. The attempt at a solution
    I find that if i label the inductor as Vc i find the voltage as 0 at time 0 because the capacitor becomes a short over a long period of time. I get what is in t<0 and when t>0 . I believe i need to find Req but the problem is how do i find that with the wire between the two resistors? Or is it just the 1k ohms anyway? After finding Req i know that I=if+in and in=Ke^(t/(CReq). t>0 and if=10/2000=1/200 so i=(1/200)+Ke^(t/(CReq) t>0 now i think i need to find out the current so that i can have whatever current i get say B so B=1/200 + K the K = B-1/200
    so that it means i=(1/200)+(K)e^(t/CReq) Then using B (current) for all t i=B+B(1-e^(-t/CReq)u(t)

    So how do i find what i called B and how do i find the original Req?
    Thanks to whoever can help me. Also i placed the original diagram, the t>0 and t<0 diagrams as well. Thanks again

    Attached Files:

  2. jcsd
  3. Oct 9, 2007 #2
    well the t<0 t>0 are my own....so that is what i think is right.
  4. Oct 13, 2007 #3
    Well, you can simply remove the voltage source in your tgreater diagram to find Req. It's just two parallel resistors in series with a third. This and the cap value will give you your time constant. From there you just need Ia initial and Ia final. You know that eventually the cap is going to be an open circuit, so all of the 10v will be traveling through the left two resistors. What does this tell you about Ia final?

    Ia initial is a tad trickier, but not bad. You know that the cap was an open circuit at t=0- and that it's set across a short circuit with no voltage drop. So what does this tell you about the cap's voltage? At time t=0+ the cap acts as a short circuit, so it should be a simple task to find the voltage across the 3k resistor and the corresponding current. With these values in hand, you should be able to answer this question.

    Hope it helps.

    I wouldn't put your diagrams in a zip next time. You'll probably get more and faster help.
  5. Oct 16, 2007 #4
    Thanks, i will keep that in mind for next time, they were to big so i just put them in a zip to make them fit. Thanks again
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