# Homework Help: Finding the Resistance in an RC Circuit

1. Apr 8, 2013

### hamhamt

1. The problem statement, all variables and given/known data
A 10-µF capacitor is charged by a 10-V battery through a resistance R. The capacitor reaches a potential difference of 4 V in a period of 3 s after the charging began. Find the value of R.

2. Relevant equations

c=q/v
q=CE(1-e^-t/RC)
i=(E/R)e^(-t/RC)

3. The attempt at a solution

I found the charge on the capacitor when its at 4 volts and got 4.0 * 10^-5 C. I then tried to plug this into the equation for a capacitor charging, but I dont know the time at which the capactior is at 4 volts.

At this point, I attempted finding another equation, so I could attempt a system of 2 equations with 2 unknowns. I tried to use the current in a capactor charging equation, but I ended up getting infinitely many solutions.

I am not entirely sure how to approach this problem.

2. Apr 8, 2013

I thought...

3. Apr 8, 2013

### XianForce

Use c = q/v to find the charge. Then plug that into the second equation at t = 3 s, because the capacitor begins charging at t = 0. So since it reaches 4 V after 3 s of charging, t = 3 s when the capacitor is charged to 4 V.

4. Apr 8, 2013

### hamhamt

Jesus. I had half of the question cut off at the most important part. Thank you both lol