Finding Total Capacitance with Electrical Breakdown

[catie1981]

Homework Statement

In figure 25-29, a potential difference of V = 100 V is applied across a capacitor arrangement with capacitances C1 = 13.0 µF, C2 = 6.76 µF, and C3 = 4.14 µF. If capacitor 3 undergoes electrical breakdown so that it becomes equivalent to conducting wire, what is the increase in (a) the charge in microcoulombs on capacitor 1 and (b) the potential difference across capacitor 1?
http://img45.imageshack.us/img45/9895/fig2528iq2.gif

Homework Equations

series capacitance 1/Ctotal= 1/C1 + 1/C2 + 1/C3
parallel capacitance Ctotal = C1 + C2 + C3
q= CV

The Attempt at a Solution

well, I've tried many different ways- but I guess my biggest problem comes from not knowing whihc of the capacitors to add up first- meaning do I add together C1 and C2 first, or C2 and C3, or C1 and C3? If you add them up starting with those different pairs, the total capacitance comes to different values each time, so that could be a problem. After, that I might be abel to work it backwards to figure out the rest of the problem...I just can't get it off the ground...thanks

 

[Integral]

C1 and C2 form a parallel pair. Compute their total capacitance.

The parallel combination of c1 and c2 are in series with C3.

 

[catie1981]

uh-oh...ok so I have that C123=3.42microFarad, which will also give q123=3.42e-4C...since C3 will be essentially taken out of the equation, I would need to figure out what the new total capctitance is without c3, right? Or did I miss something?

 

[catie1981]

oooo, ok so I've got part (b)...which I got by simply dividing q123 by C12...however, how to get part (a)...that's the tricky part...I'm not really sure how to manipulate these numbers to determine q1 in the first place...

 

[catie1981]

OK, so I now know where I'm having trouble...I don't know what the ORIGINAL value for q1 is, so if anyone can point me in the right direction to determine q1 before, maybe I can do q1 after...thanks

 

[catie1981]

wooops, I got it solved...thanks anyway :)

 

[Fletcherdavis]

how did you solve this first one?