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First Order Transient: RC Circuit

  1. Nov 10, 2008 #1
    1. The problem statement, all variables and given/known data


    i have to find Vc(t)

    2. Relevant equations

    I know that Vc(0+) = Vc(0) = Vc(0-) and Vc = Vs*e^(-t/RC)

    3. The attempt at a solution

    What i'm unsure on is what the equivalent resistance of the circuit would be so I could find the time constant. I know since the Capacitor is fully charged it now behaves as a voltage source, but for any nodal / mesh equation I come up with I'm always one equation short of having equal unknowns & equations.
    So ... any help on how to find the Req would be nice.
  2. jcsd
  3. Nov 11, 2008 #2


    User Avatar
    Science Advisor

    Find the Thévenin equivalent around the capacitor.

    EDIT: Before, or after t=0? Which will affect the time constant?
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