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Homework Help: Charge on a capacitator

  1. May 17, 2009 #1
    given the following:
    http://lh3.ggpht.com/_H4Iz7SmBrbk/ShATOIWSXEI/AAAAAAAAA_4/M57_Bf03Z2M/s720/Untitled.jpg [Broken]

    and knowing:

    the switch is closed to position A for a time of t=RC1/2 and then moved to position B
    what is the charge of C1 the moment the switch is moved from A to B??

    as far as i can see none of my equations have anything to do with time, other than P=VI (since W=J/s) but i dont think that helps me very much,, how do i incorporate the element of time into this, i tried kirchhoff but that didnt really get me anywhere

    since i dont know q1 I or R i think this is not the right way to go,
    i would think that the R needs to cancel out since the t is dependant on R, so i am looking for something that will give me t/R

    i thought that since W=J/s, P=E/t = q2/2C * 2/RC1 but that doesnt help either
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 17, 2009 #2
    There is an equation for exponential charging of a capacitor in a RC circuit.
  4. May 17, 2009 #3
    what is that equation
  5. May 17, 2009 #4
    You never solved for the differential equation of an RC circuit in your book or class?

    [tex]Q = Q_0 \left( 1 - e^{-t/RC}\right)[/tex]

    That is the equation for a battery charging a capacitor in an RC circuit. Where [tex]Q_0 = C\epsilon[/tex].
  6. May 17, 2009 #5
    nope, never seen that before, probably in the next few lessons, what is the root of this equations?
  7. May 17, 2009 #6
    Are you doing homework ahead of time?

    Too much to explain but I found this on google:

    http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/DC-Current/RCSeries.html [Broken]

    It explains how a capacitor charges and discharges in an RC circuit.
    Last edited by a moderator: May 4, 2017
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