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Homework Help: Defibrillator discharges. need to find time constant

  1. Apr 10, 2010 #1
    A defibrillator uses a capacitor to deliver a large current to a person needing heart stimulation. Assume that the capacitance used is 60microfarads and the resistor it discharges through is 220 ohms. the defibrillator is charged up to 5 kiloVolts and it is discharged through a patient for 1.0 millisecond.

    a) What fraction of the time constant is 1.0 millisecond?
    b) How much charge is left on the capacitor after the 1.0 millisecond?
    c) Find the total charge delivered to the patient.
    d) Determine the average current delivered to the patient.'

    I used time constant=R*C to find the time constant but it comes out to be smaller than 1.0ms, which does not make sense. I just need to know which equations to use to get the answers right.

    Please help. And Thanks to whoever helps!
  2. jcsd
  3. Apr 10, 2010 #2


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    Welcome to PF!

    Hi gujrichick! Welcome to PF! :smile:

    What did you get for the time constant? :confused:
  4. Apr 10, 2010 #3

    I got 1.32*10^-5 seconds as my time constant.. and I dont understand how I would get a fraction from that or how that would help me answer the other questions.
  5. Apr 10, 2010 #4


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    Hi gujrichick! :smile:

    (try using the X2 tag just above the Reply box :wink:)
    I get 220 Ω times 60 10-6 F = 1.32 10-2 s
  6. Apr 10, 2010 #5
    oh! I know what I did wrong now, I thought that micro was -9 instead of -6.

    but which equations should I use for the other parts? I think for part c I can use C=Q/V and find the total charge. and for d I can do V=IR. But what about part b? Do I use Q=CVe^(-t/RC)? And plug in 1ms for t?
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