1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: RC Circuit defibrillator Question

  1. Nov 11, 2013 #1
    1. The problem statement, all variables and given/known data

    A 140μF defibrillator capacitor is charged to 1500 V. When fired through a patient's chest, it loses 95% of its charge in 40 ms. What is the resistance of the patient's chest?

    2. Relevant equations

    3. The attempt at a solution
    According to "Mastering Physics," the solution that I have provided is incorrect. I don't know what I've done wrong.

    Given: C=140μF V=1500 V
    -loses 95% of its charge in 40 ms

    Needed: R=?

    ln(Q/Qo)= -t/τ
    Rearrange for R
    =-(40x10-3s)/((140 x10-6F)(ln(0.95)))

    Attached Files:

    Last edited: Nov 11, 2013
  2. jcsd
  3. Nov 12, 2013 #2


    User Avatar
    Homework Helper

    The solution looks correct, try to use more significant digits.

  4. May 21, 2017 #3
    Sorry that I can't help you, but this is for any future students coming here for a similar problem. The OP did almost everything right. His only problem was that since the capacitor loses 95% of its charge, the final equation should include ln(0.05) for the 5% of the charge remaining, not ln(0.95). Hope this helps!
    Last edited: May 21, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted