Homework Help: A bit of help with a capacitor problem

1. Mar 17, 2012

bob92

1. The problem:
In the capacitor circuit shown below C1 = 4␣F, C2 = 12␣F, C3 = 12␣F, and C4 = 3␣F. The switch (S) between A and B is closed and the energy stored in C3 is 1.35x10-3 J. If the switch is now opened calculate the change in the charge on C2. Assume the potential difference between C and D is constant.

Here is the work that I have: (sorry the picture is too big to directly link)

http://i.imgur.com/qnTvK.jpg

The question asks what is the change in charge in C2 (the one on the lower left). The correct answer is -99uC but I can't even get my answer close.

Last edited: Mar 17, 2012
2. Mar 17, 2012

PeterO

You either read the question incorrectly, or typed the question incorrectly - see red above.

3. Mar 17, 2012

bob92

Sorry, I apologize, I mean't C2. C3 is what is given at the start.

4. Mar 17, 2012

PeterO

When did you mean C2 - in the question or in the answer?

You have still not removed the ambiguity.

5. Mar 17, 2012

bob92

C2 in the answer. The questions asks for the change in the charge of C2.

6. Mar 17, 2012

PeterO

Back to square 1 then.

If 1.35 x 10-3 J of energy is stored on C3, which has capacitance 12μF, what is the PD across it, and what charge is stored on it.

7. Mar 17, 2012

bob92

1.35 x 10-3 J would imply a PD of .015 using formula E=(CV^2)/2, so V = .015. Using the formula C = Q/V, gives the charge on c3 to be .18C

8. Mar 17, 2012

PeterO

Very small Voltage there. When you stated in the original 12␣F did you mean 12 μF or what?

With 12μF I get 15V [and a charge of 180 μC]

EDIT: DOn't just accept my figures, I could easily be out by a factor of 10

9. Mar 18, 2012

PeterO

Just checking that you realise that with the switch closed, we have C1 and C3 connected in parallel, and C2 and C4 connected in parallel, and those two pairs connected in series,

But when the switch is open, we have C1 and C2 in series, and C3 and C4 in series, and each of those pairs connected in parallel.