Okey guys, here's another difficult capacitor circuit... :yuck:(adsbygoogle = window.adsbygoogle || []).push({});

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

Find the proportions (the quotient) between the energies in the two capacitors when the switch is closed in the circuit below.

2. Relevant equations

(1) C = Q / V

(2) W = CV^2

(3) Current division (maybe?): I2 = I*R1/(R1+R2)

Ohm's and Kirchhoff's.

And maybe some others...?

3. The attempt at a solution

I tried to find the potential over each capacitor, because then I could use equation (1) and (2) above and find the solution. But I don't know how to get grip of the potentials. It's hard to do any potential walk here, cause there's so many branches all over the place...

The key said this:

W1 = 1/2 * C1 * U1^2 = 1/2 * C1 * (R1*I)^2

W2 = 1/2 * C2 * U2^2 = 1/2 * C2 * (R2*I)^2

=> W1/W2 = C1*R1^2 / (C2*R2^2)

4. Questions

a) According to the key, U1 and U2 (which I suppose are over the capacitors, the key doesn't tell) equals R1*I and R2*I respectively. How could that be? Why is not R3 included?

b) What's happening when the switch is closed? Is the battery driving current through the capacitors, or is the capacitors driving current, i.e., they are being discharged? How can you see that in that case? Isn't the battery still trying to hold the charges on the capacitors at place?

c) How to approach a problem like this in general... (if it's not evident from a) and b))

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# Homework Help: Energy in capacitors in complicated circuit

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