# Parallel Capacitors

1. May 22, 2012

### ZxcvbnM2000

1. The problem statement, all variables and given/known data

A capacitor of capacitance 9 μF is fully charged from a 200 V dc supply. The
capacitor is now disconnected from the supply and connected in parallel with an
uncharged 3 μF capacitor. Once the 3 μF capacitor is fully charged, it is removed
and discharged. a) What charge remains on the 9 μF capacitor? b) How many
times would this process have to be repeated in order to reduce the charge on the 9
μF capacitor to below 50% of its initial charge? c) What would be the pd between
the plates of the 9 μF capacitor now be?

2. Relevant equations

Q=CV

3. The attempt at a solution

a) I said that the 3μF capacitor will be fully charged when the voltage across both capacitors is the same,let's say Vx.

Since energy has to be conserved then Ei=Ef therefore

C1V^2 = C1Vx^2 + C2Vx^2 <=> Q1' = 1.5588*10^-3 C . Am i doing this correct ?

2. May 22, 2012

### Staff: Mentor

Unfortunately, much as in elastic collisions of masses, connection of capacitors in parallel is not an operation that conserves energy; some energy is lost when the charges get shuffled into their new configuration.

What is conserved through the connection operation, though, is charge.

3. May 22, 2012

### ZxcvbnM2000

Thank you very much !!!