# Capacitor discharge?

1. Oct 5, 2009

### fball558

capacitor discharge???

1. The problem statement, all variables and given/known data

A 1.4 uF capacitor, initially uncharged, is connected in series with a 10 k ohms resistor and a 6 V battery of negligible internal resistance.

How long does it take the capacitor to reach 99% of its final charge?

this is part b. i already found out part a which asked What is the charge on the capacitor after a very long time?
answer to this part was 8.4 uC

2. Relevant equations

for part b i think i will have to use the equation
Q=Qf(1-e^(-t/T)) where t is time and T is the time constant defined by T= RC

3. The attempt at a solution

i plugged in arbatrary values just to define my 99% i picked 5 but that should not matter

5=4.95(1-e^(-t/(10000*1.4e-6)))
solve for t
this will be in seconds i assume and my answer wants it displayed in ms
i got an answer of around 32 ms but it says im wrong.
any help would be great!!!

2. Oct 5, 2009

### Redbelly98

Staff Emeritus
Re: capacitor discharge???

First of all, the 5 and 4.95 should be switched around. If 5 is the final charge Qf, then you want to know when Q=4.95. Other than that, your equation is correct.

I get 1 - e^(-0.032/0.014) = 0.898.., not 0.99 as desired. So you probably made an algebra mistake somewhere in getting the 0.032 sec answer.