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Homework Help: Time Constant, Resistance, Energy

  1. Feb 18, 2010 #1
    1. The problem statement, all variables and given/known data

    A 11.0 uF capacitor in a heart defibrillator unit is charged fully by a 12000 V power supply. Each capacitor plate is connected to the chest of a patient by wires and flat "paddles," one on either side of the heart. The energy stored in the capacitor is delivered through an RC circuit, where R is the resistance of the body between the two paddles. Data indicate that it takes 72.1 ms for the voltage to drop to 20.0 V.

    a) time constant?
    b) Determine resistance "R"
    c) How much time does it take for the capacitor to lose 89% of its stored energy?
    d) If the paddles are left in place for many time constants, how much energy is delivered to the chest/heart area of the patient?

    2. Relevant equations

    T = RC (im not sure how to find capacitance)
    q = qoe^-t/RC ???

    3. The attempt at a solution
    i attempted a nd b and got them wrong. for c and d i really have no clue how to start. Im pretty sure Im using the wrong equations. Any help would be appreciated
  2. jcsd
  3. Feb 18, 2010 #2
    can someone help me figure out capacitance?
    Last edited: Feb 18, 2010
  4. Feb 18, 2010 #3
  5. Feb 18, 2010 #4
    The capacitance is 11.0 uF (micro-farads). It's given right in the beginning of the problem.
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