# Homework Help: Capacitors with switch

1. Feb 8, 2009

### dwstrait

1. The problem statement
The capacitors in the figure are initially uncharged and are connected, as in the diagram, with switch S open. The applied potential difference is Vab = + 210 V.

What is the potential difference Vcd?

2. Relevant equations
(series) 1/C = 1/c1 + 1/c2 + ...
(parallel) C = c1 + c2 + ...
C=Q/V

3. The attempt at a solution
Trying to find Vcd:
I first found the equivalent capacitance of the series, which is 4*10-6 F. I calculated the charge on the capacitors using Q = CV, which was 8.4*10-4. Then, I tried to find Vad and Vac by using V=Q/C. I don't think this is the right way to do this problem though.

Last edited: Feb 8, 2009
2. Feb 8, 2009

### LowlyPion

That looks like the way to do it as far as you went.

3. Feb 8, 2009

### dwstrait

Ah, I figured it out. The upper and lower series each have half the value of the charge that I calculated. Then using V=QC, Vad=140V and Vac=70V, so Vad-Vac=70V.