# Homework Help: Voltage across capacitor

1. May 5, 2014

### delsoo

1. The problem statement, all variables and given/known data
here's my question and the working in the same photo.. my question is to find to find the effective capacitance in the circuit i use (1/c1 +1/c2)^-1 , why should cant just take effective c =
(1/c1 +1/c2)^-1 but must times 2 ? since (1/c1 +1/c2)^-1 is already effective capacitance. please ignore the irrelevant part . sorry for the messy work

2. Relevant equations

3. The attempt at a solution

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2. May 5, 2014

### rude man

Why should effective capacitance be the criterion for obtaining the resultant voltage across the capacitors?

One way:
You can write 3 equations in 3 unknowns: Qb, Q1 and Q2 where
Qb is the charge on each plate before reconnection,
Q1 is the charge on plate 1 after reconnection
Q2 is the charge on plate 2 after reconnection

Then V after reconnection is obvious.

Last edited: May 5, 2014
3. May 5, 2014

### Staff: Mentor

Can you explain what you calculated? It is hard to understand that with those unexplained formulas.

By the way, as this is a multiple choice question, there is a shorter way to find the right answer: three out of four answers do not have the correct units.

4. May 5, 2014

### rude man

Ho ho, good point! Lousy choices!