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Homework Help: Finding the function of the cap.

  1. Dec 16, 2008 #1

    This is one of the questions I got in my finals, and I couldn't do it back then. I want to find Vc(t) , given the cap is initially charged up to 10V, the switch is then opened. My approach is to find the thevein eq of the circuit, which I would came up with a resistor in parallel with the cap, and then find the function using the expression Vc(t) = final + (initial - final) e^(t/RC).
    sounds good?
    Last edited: Dec 16, 2008
  2. jcsd
  3. Dec 16, 2008 #2


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    Staff: Mentor

    After the switch is opened, the cap disappears from the circuit, no?
  4. Dec 17, 2008 #3
    ops the switch should be opened initially and then closed(drawing error)
  5. Dec 18, 2008 #4


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    Staff: Mentor

    What is the source on the right? It can't be a current source of value 5ix, since there is only one ix flowing through the resistor before the switch is closed....

    I think you need to re-draw the circuit correctly, and then show us your work on the solution. Then we may be able to offer some help.
  6. Dec 18, 2008 #5
    Finding the function of the 0.1uF cap Vc(t) when the switch is closed at t=0 given that the cap is initial charged up to 10V.


    ok my attempt at this I find the thevenin eq of the right hand side circuit and come up with :

    Vc(infinite) = 0, Vc(0)=10 , time constant = RC = 1.67x10^-7

    Vc(t) = 0 + ( 10-0) e^(t/(1.67x10^-7 ))

    Appreciate if u could have a look at it.
  7. Dec 19, 2008 #6


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    Homework Helper

    How did you get 1.67 ohms for your thevenin equivalent circuit? I got an indeterminate answer for the thevenin equivalent circuit. You could do it without that. Furthermore, your final answer doesn't make much physical sense; Vc(t) would increase exponentially without limit as time passes.
  8. Dec 19, 2008 #7
    This is what I did :

    I hook up a 1V source and solve for I1 and thus the Rth.

    And I left out a - sign in my final answer
    Vc(t) = 10e^-t/RC
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