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Homework Help: Capacitance Question

  1. Jan 21, 2010 #1
    A working circuit contains a 3V power supply, a 5000uF Capacitor, and a 200ohm resistor.

    a) What is the time constant for the circuit?
    b) What is the charge in coloumbs on the capacitor at a potential difference of 3V?
    c) How much energy is stored on the Capacitor at a potential difference of 3V?
    d) Calculate the initial value of the current when the fully charged capacitor starts to discharge through the resistor.

    a) T=RC
    = 200 X (5000X10-3)
    =1000 seconds

    b) Q=CV
    Q = 5000X10-3 X3
    = 15C

    c) E=1/2CV2
    E=1/2 X 5000X10-3 X 32
    E = 22.5J

    d) Q=Q0e-t/RC
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jan 21, 2010 #2


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    Gold Member

    I think you did everything fine until the d) part. Currents are measured in amperes. What you wrote is the charge of the capacitor. How is i(t) related to q(t)? What you wrote is q(t).
    How much is worth t when the capacitor starts to discharge?
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