1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar

    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


    User Avatar

    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


    User Avatar
    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook