1. Not finding help here? Sign up for a free 30min 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!

Compute the charge on the capacitor

  1. Mar 8, 2008 #1
    1. The problem statement, all variables and given/known data

    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: Mar 9, 2008
  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
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Compute the charge on the capacitor
  1. Charge on capacitor (Replies: 10)

  2. Charge in a capacitor (Replies: 11)

  3. Charging of Capacitors (Replies: 0)

  4. Charge on a Capacitor (Replies: 3)

  5. Charged capacitor (Replies: 1)