Finding Effective Capacitance in a Circuit | Voltage Across Capacitor

[delsoo]

Homework Statement

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 can't 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

Homework Equations

The Attempt at a Solution

 

[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.

 

[mfb]

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.

 

[rude man]

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.

Ho ho, good point! Lousy choices!