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RC Circuits: Voltage as a function of time

  1. Jan 18, 2008 #1
    1. The problem statement, all variables and given/known data

    There is an RC circuit with a 6 V battery, a 100 Ohm resistor, and 2.0 microFarad capacitor. At time t = 0, the switch is closed. While this happens, an ammeter measures the current through the resistor. At t = 0.000139 s the ammeter finds that 0.030 A is flowing through the resistor.

    a) At this instant in time (t = 0.139 milliseconds) what is the charge on the capacitor?

    2. Relevant equations

    V(T) = E (1-e^(-t/RC))
    Q(t) = CE(1-e^-t/RC)

    3. The attempt at a solution

    I solved the equation for t = 0.139, but I was wondering whether or not the stuff about the ammeter measuring the current matters? I mean, if so, then I = 0.030 A and I = R/V and Q = CV, so I could also find the charge that way, but when I do that, I get two different answers. What do you suggest? Thanks in advance!
  2. jcsd
  3. Jan 19, 2008 #2
    I don't think the part about the current through the resistor is important - it's different from the current and charge on the capacitor. I think. :)
  4. Jan 19, 2008 #3
    u find the voltage across the resistor
    then u subtract it from the emf of the battery to find the voltage across the capacitor
    then u multiply that voltage with the capacitance to get charge
  5. Jan 20, 2008 #4
    But the formula is for V_C, the voltage across the capacitor.

    And there is a separate formula for charge - the Q(t) one. I tried solving both and got completely different answers (as in I solved the voltage one then did Q = CV).

    I don't think that works, esalihm.
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