# Homework Help: Capacitors and a battery

1. Jun 25, 2006

### renanpo

A 50 pF capacitor and a 200 pF capacitor are both charged to 2.6 kV. They are then disconnected from the voltage source and are connected together, positive plate to positive plate and negative plate to negative plate. Find the resulting potential difference across each capacitor.

I can find Q on each capacitor by using Q=VC. The Q total will be Q1+Q2, but I am not sure what to do from here. Could anyone give me some help? Thank you

2. Jun 26, 2006

### Staff: Mentor

If they have the same starting voltage and are connected together in parallel, there wil be no change in voltage, right? The harder question is what happens if they are reversed in polarity before being connected. Quiz question -- what is the final voltage if they are connected with reverse polarity. Harder quiz question -- discuss the energy stored before and after the connection -- where did the energy go?

3. Jun 26, 2006

### tim_lou

conservation of charge:
q1+q2=qtotal
if the polars are different, then... just make one negative.
add the initial charges and you'll get qtotal
then, at equilibrium,
V1=V2
q1/C1=q2/C2
q1+q2=qtotal
solve the system, find V1 using V1=q1/C1
now you have final charge and final voltage, you can find anything from there.

Last edited: Jun 26, 2006
4. Jun 27, 2006

### einstein_a_go_go

Another way of looking at the question as set:

If the voltage is the same across both, there will be no potential difference, there fore no charge will flow from one to t'other, therefore no overall change at all.

Is the question right or were they charged in seriesor as berkemann sadi is it a case of reversing the capacitors?

EaGG