Electric potential and fields-capacitors

[smoics]

electric potential and fields--capacitors

Homework Statement

What is the potential difference across each capacitor in the figure?
What is the potential difference across each capacitor in the figure?

Homework Equations

Q=C/V
Parallel capacitors: add up
series capacitors: (1/c + 1/c...)^-1

The Attempt at a Solution

http://teacher.pas.rochester.edu/phy122/Lecture_Notes/Chapter27/chapter27.html

I tried using Q=C/V and got 45,36,54 microC. And I tried what they have on the site above (although it didn't really make a whole lot of sense as to why they were doing what they did). I understand the concept of how to add capacitors, but how to get their individual charges...? I also tried dividing the voltage by 3 because there are three capacitors...Thanks!

 

[cupid.callin]

First find net capacitane and thus net charge given by battery
then find charge going in each parallel branch by using Q₁/C₁ = Q₂/C₂ (as both branches are parallel so V₁=V₂)

find Q₁ and Q₂ and thus use them and the formula Q=CV to find C for all the capacitors.

 

[smoics]

Q1 = C1V and Q2 = C2V
total charge: Q = Q1 + Q2 = C1V + C2V = ( C1 + C2 )V
Ceq = Q/V = C1 + C2

I know C1 and C2, but if I use Q1/C1 = Q2/C2, I have two unknowns, both Q's. Don't I? Or can I solve for them using one of the above equations? I know Qnet, does that help??

 

[gneill]

The potential difference across the 5μF capacitor is clear -- it's fixed by the 9V battery.

For the series capacitors, consider that they must both have the same amount of charge on them (any current that pushes a charge onto one of them must push the same charge onto the other because they are connected in series) and this will also be the same amount of charge that's stored on the equivalent capacitance of the pair.

 

[cupid.callin]

Find the net capacitance and thus the net charge ...

then proceed by Q1 = C1V and Q2 = C2V

and a hint, for questions like these don't find answer in terms of variables but find the value of each variable at each step ... that'll make your problem less confusing ...