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Homework Help: Time constant and parallel RC

  1. Mar 21, 2013 #1
    the attached file is the question

    I know the time constant after opening the switch

    but how can i know the time constant when the switch is on?

    This is NOT the classic initial and AFTER A LONG LONG LONG Time question

    How can i know the potential of C in t=2s ?

    What is the equation for charging ?

    Plz help, i googled for a long time but still can't get the ans
    Last edited by a moderator: Mar 21, 2013
  2. jcsd
  3. Mar 21, 2013 #2


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    Staff: Mentor

    With Kirchhoff's laws, you can get a differential equation for the charge of the capacitor. The time constant can be seen in this equation.
  4. Mar 21, 2013 #3


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    Staff: Mentor

    One approach would be to find the Thevenin equivalent sources that the capacitor "sees" when the switch is open and when the switch is closed. That'll give you two simple RC circuits to work with...
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