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Thread: capacitor

  1. #1 capacitor 
    Forum Freshman
    Join Date
    Nov 2006
    assume an 80 mF capacitor is initially connected to a 9V battery. The battery is then removed and a wire is connected between the two terminals of the capacitor. What is the charge on the capacitor after the wire is connected?

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  3. #2  
    Forum Masters Degree bit4bit's Avatar
    Join Date
    Jul 2007
    Assuming the capacitor was fully charged before the battery was disconnected, then the voltage across it would be equal to that of the battery, 9V.

    Q = CV the charge across the capacitor would be,

    Q_initial = (80*10^-3)*9
    = 0.72 C

    The charge on the capacitor after the wire is connected is given by the expression:

    Q = Q_initial *e^(-t/CR)


    t = time
    C = capacitance = 0.08F
    R = resistance of wire

    You can see, the charge is a function of the load resistance, and the time as well as the initial charge.

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