What is the potential difference Vad-Vac?

[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 V_ab = + 210 V.

What is the potential difference V_cd?

Homework Equations

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

The Attempt at a Solution

Trying to find V_cd:
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 V_ad and V_ac by using V=Q/C. I don't think this is the right way to do this problem though.

 

[LowlyPion]

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

 

[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, V_ad=140V and V_ac=70V, so V_ad-V_ac=70V.