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Homework Help: Capacitors/Opening Circuit/Charge

  1. Oct 27, 2008 #1
    1. The problem statement, all variables and given/known data
    All capacitors of the open circuit (figure below) are discharged when, at t=0, the switch S1 is closed.
    At some point later, t= t1, the switch S2 is then closed.
    What is the charge Q1( t2) on the capacitor C1 at time t= t2 > t1?


    2. Relevant equations
    C = Q / V ... Q = C * V
    Ceq = (1/C1) + (1/C2)

    3. The attempt at a solution
    I need some help getting started. Having no numbers is throwing me off and i don't quite understand.
    Last edited: Oct 27, 2008
  2. jcsd
  3. Oct 27, 2008 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Having no numbers is immaterial. It simply means that your answers will be given in terms of known parameters (e.g. E, R4, C1, etc.). In time, you'll actually prefer questions like this. It makes the question conceptually cleaner, so that you can focus on the goal (understanding and using circuit theory to solve the circuit) and not worry about things as mundane as arithmetic. ;-) Your purely algebraic answers will be true regardless of the specific component values.

    The first thing I would do is combine all of the resistors into a single effective resistance, and all of the capacitors into a single effective capacitance for this circuit (hint, how do capacitors in parallel combine)? Once you've done that, you'll be left with a simple RC circuit, and it'll be easy to answer the question. I guess you would do this for the circuit as it appears during the time interval t = 0 to t = t1.
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