# Homework Help: Basic Electric Potential Theory

1. Sep 28, 2010

### mcpoopants

Alright, so there is a very basic theory involving capacitors and electric potential that is throwing me off. I have a very basic problem here: http://img444.imageshack.us/img444/2251/73619554.png [Broken]

Assume the switch is closed and the capacitor is fully charged. From here I'm prompted to find the final voltage across the capacitor. Pretty obvious, you use V=IR, but I'm missing out on the value of "R". In this problem it is just R2, which is given to you. My problem is that I do not understand how the voltage across that resistor is equivalent to the voltage across that fully charged capacitor. It'd really help to explain as slowly as possible, because it is a basic idea that is kicking my butt in more complicated problems. Thanks to all.

Last edited by a moderator: May 4, 2017
2. Oct 1, 2010

### zhermes

Use kirchoff's loop rule around R2 and the capacitor. When you move from a point in a circuit, back to the same point, the net voltage change must be zero. This is the same thing as saying that the voltage difference between a point and itself is zero.

So, if a point on the top wire---between the resistor and capacitor---has a given voltage difference from a point on the bottom wire---between the resistor and cap---across the resistor, it has to be the same as across the capacitor.... because they're the same two points.

Does that make any sense?