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Homework Help: First Order circuit

  1. Jun 3, 2014 #1
    the question is to find I(0)- and I(0)+
    the I is at the

    what I have attempted
    finding Req on the right side
    I eliminate the capacitor and Req = 12*6/(12+6) = 4
    so Vc (0)- = Req/(Req+12) *18 = 9/2

    T = Req*c= 4*1/12 = 1/3
    so V(t) = 18/4 e^(-3T)

    so I (0)- = V(t)/12 = 3/8 e^(-3t)

    but I don't know how to find the I(0)+

    sorry for bad English and bad picture
    thx for replying in advance

    Attached Files:

  2. jcsd
  3. Jun 3, 2014 #2


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    Homework Helper

    Be a bit more specific: What are I(0)+ and I(0)-?

  4. Jun 3, 2014 #3
    its the current that before and after time = 0
    a moment right after for +
    a moment right before for -
  5. Jun 3, 2014 #4
    Hello darwinharianto
    See the attachment.

    Attached Files:

  6. Jun 3, 2014 #5
    thanks a lot

    so no need for me to the e^x
    because the assignment before got e^x for the V
  7. Jun 4, 2014 #6
    Hello darwinharianto
    as you know before t(-) we assume that our circuit is in steady state so we don't have exponential form.however after t(0) capacitor tries to be discharge with RC period time so exponential form will appear.
    if you want exponential form of i after t(0) follow the attachment.

    Attached Files:

  8. Jun 5, 2014 #7
    thanks a lot :D
    now I do understand it
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