RC Circuit defibrillator Question

[tmlfan_17]

Homework Statement

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?

Homework Equations

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=?

Q=Q_oe^-t/τ
(Q/Q_o)=e^-t/τ
ln(Q/Q_o)= -t/τ
τ=RC
ln(Q/Q_o)=-t/RC
Rearrange for R
R=-t/(C(ln(Q/Q_o))
=-(40x10^-3s)/((140 x10^-6F)(ln(0.95)))
=5.6kΩ

 

[ehild]

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

ehild

 

[jaxmoney]

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!