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Homework Help: Charging/Discharging Capacitor & Magnetic Field

  1. Apr 6, 2012 #1
    1. The problem statement, all variables and given/known data
    The magnetic field in the region shown below is increasing by 5 T/s. The circuit shown (with R=25 , C=95 µF, dimensions 1 m by 2 m) is inserted into the region as shown. The capacitor is initially uncharged.

    See Diagram: http://www.webassign.net/userimages/23-31.gif?db=v4net&id=50305

    a.) What is the current in the circuit at the moment the circuit is put in the B field?
    .4 A
    direction: counterclockwise

    b) How long will it take the capacitor to charge to 25% of its final charge?

    c) How much charge will the capacitor hold after a long time (assuming this magnetic field rate increase is maintained)?

    2. Relevant equations



    3. The attempt at a solution

    I'm using Q(t)=Qf(1-e^t/rc) to find time.

    First, I found Qf by using Q=VC. I know that V=IR so, Q=IRC, =(.4)(25)(9.5e-6)

    Then, I used Q(t)=Qf(1-e^t/rc)


    I'm not getting a real or correct answer for this.

    Your assistance, any help you could offer is much appreciated. THANK YOU!
  2. jcsd
  3. Apr 6, 2012 #2
    I think you numbers are correct but when the charge is 25% that means 25% x final charge.
    I think you have just forgotten to multiply by the final charge??
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