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Finding voltage from a fully charged capacitor?

  1. Apr 13, 2013 #1
    1. The problem statement, all variables and given/known data

    I am given a circuit in which my homework is asking my to find the time constants of this circuit:
    Capture.PNG

    The problem wants the time constants and then wants to find the voltage response when the circuit is in the position connecting the 2k ohm resistor.

    the capacitors are assumed to be fully charged at the t=0

    2. Relevant equations

    time constants in a RC circuit are simply R*C
    v(t)=v(f)-(v(i)-v(f))e^(-t/RC)


    3. The attempt at a solution

    I have finished this problem. However, I was wondering, because I do not see an obvious way, is there a way to figure the voltage at the very beginning? I am not given a voltage to which the capacitor was charged, I simply only know that there is an equivalent capacitance of 150uF.

    I wrote my voltage response as:
    V(t)=V(initial)*e^(-t/0.1125)

    Time constant = (2000*3000)/(5000)*150uF
     
    Last edited: Apr 13, 2013
  2. jcsd
  3. Apr 13, 2013 #2

    gneill

    User Avatar

    Staff: Mentor

    The circuit as given shows no sources for either current or voltage, so no, there's no way to tell the initial charge or potential on the capacitors.

    It's hard to see how the capacitors could have any charge at all since there's always a resistive load across them no matter what the switch position. Perhaps we are to assume that the circuit is assembled instantaneously with charges already on the capacitors and the switch in the "R2" position? If so, what's the point of the switch and R1 :confused: Well, it's a mystery...
     
  4. Apr 13, 2013 #3
    That's kind of what I thought too. I left my voltage as V(c), or the voltage of the capacitor. I wasn't sure if I was just missing some equation out there. This is one of those 1 known-2unknown situations, so I'm leaving it as a vague variable :)
     
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