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Homework Help: Compute the charge on the capacitor

  1. Mar 8, 2008 #1
    1. The problem statement, all variables and given/known data
    http://img259.imageshack.us/img259/1535/question2az5.th.jpg [Broken]

    Compute the charge on the capacitor 10.0 ms after the switch is thrown from position 2 back to position 1.

    I know from previous parts of this problem that the Voltage across the resistor before the switch is thrown is 8.28 V. Afterwards it is 9.72V.

    I figured that the charge q before switch is thrown is 133 micro coulombs.

    2. Relevant equations

    3. The attempt at a solution

    I tried using the new voltage after the switch is thrown to find q but it didn't work. Can someone please help me out?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Mar 9, 2008 #2
    C = Q/V;

    and I found the voltages using the equations relating to RC charging and discharging.

    but I am still not getting the right answer.
  4. Mar 9, 2008 #3
    I can't tell you what you did wrong, because you didn't show your work. Didn't the problem
    give R, C and E?
  5. Mar 10, 2008 #4
    What kind of equation governs a capacitor?
  6. Mar 10, 2008 #5
    You're looking for an equation with time t in it. You're going to need that to use your given value of 10.0 ms.
  7. Mar 10, 2008 #6
    A capacitor with C = 1.30×10−5 F is connected as shown in the figure View Figure with a resistor with R = 990 Omega and an emf source with E = 18.0 V and negligible internal resistance. Initially the capacitor is uncharged and the switch S is in position 1. The switch is then moved to position 2, so that the capacitor begins to charge. After the switch has been in position 2 for 10.0 ms, the switch is moved back to position 1 so that the capacitor begins to discharge.

    I used the equations for V and I for discharging and charging capacitors.

    V = E(1-e^(-t/RC)) charging
    V = V_oe^(-t/RC) discharging

    I = (E/R)e^(-t/RC) charging
    I = (V_o/R)e^(-t/RC) discharging

    I then used C = Q/V to find Q for the first part where before switch is thrown...

    hope this helped clarify some things, sorry for the lack of info
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