Conceptual Question on Parallel Plate Capacitors

[waters]

Homework Statement

Two capacitors, C1 and C2, are separately charged to 166 C and 348 C, respectively. They are then attached in parallel so that the + plate of one goes to the - plate of the other, and vice versa, as shown on the diagram below (notice how C2 was rotated 180 degrees before the capacitors were connected). C1 has a capacitance of 39.8 F, and C2 has a capacitance of 174 F. What is the final voltage across C1?

http://lon-capa.mines.edu/res/csm/csmphyslib/type56_capacitors/AttachedInParallel.jpg

Homework Equations

V(C1+C2) = Q1 - Q2
V = (Q1-Q2)/(C1+C2)
V = .851 V

The Attempt at a Solution

My question is, why are the charges subtracted? What goes on in this circuit to make that happen? Why does this happen when capacitors are aligned in this manner? Also, why is the final voltage reported as positive, when it should be negative?

 

[ehild]

"Authorization Required" to your link.

ehild

 

[waters]

Sorry about that. It should be fine now.

 

[Muphrid]

When the capacitors are connected, the "circuit" is really two disconnected halves--a top half and a bottom half. It follows from conservation of charge that on the top half, with AD, you have +Q1 from C1 and -Q2 from C2, and the opposite charge on the BC half. This charge will redistribute itself over the plates so that the new voltages for the capacitors are the same (as is required for a parallel circuit), but the net charge Q1-Q2 cannot change.