Solving a Circuit: Find Ceq, Q, & U

[cristina045]

Homework Statement

For the cicuit shown, the voltage is 200V, C1= 4microF, C2= 15microF and C3= 12microF
(i drew it on paint)
Find
a) the equivalent capacitance between the terminals
b) the charge stored on each capacitor
c) the total stored energy

Homework Equations

Ceq= 1/c1 + 1/c2 +1/c3
or
Ceq = C1+C2+C3

C=Q/V

I think this is the right equation for the energy stored in a capacitor:
U= QV/2=CV^2/2

The Attempt at a Solution

Ok I'm really not sure because I'm having a lot of trouble with figuring circuits out. I think C1 and C2 are in series but in parallel with C3. For a) what i did is :

Ctot = 1/Ceq + C3
and
1/Ceq = 1/C1 + 1/C2
Therefore Ct = 3.15microF + 12microF
Ct= 15.15microF

b) I would use C= Q/V
so Q1 = 8*10^-4 C
Q2= 3*10^-3 C
Q3= 2.4*10^-3 C

c) U=CV^2/2
my equivalent capacitance was 15.15microF and my voltage was 200v, therefore my U should be 0.303J

It seems to me that b and c were too easy to get. Besides, I'm not even sure my a) part is right. I'm so confused...

the picture i drew is at the bottom, it's just pending approval.

 

[rl.bhat]

In series combination of capacitors, charges in each capacitors must be the same.

 

[tiny-tim]

Hi cristina045!

(have a mu: µ and try using the X² and X₂ tags just above the Reply box )

I think C1 and C2 are in series but in parallel with C3.
…
i'm not even sure my a) part is right.

Your a) is fine,

(except of course it should be C_eq + C₃ in the first line )

(and personally I'd have used something like C₁₂ instead of C_eq)

b) I would use C= Q/V
so Q1 = 8*10^-4 C
Q2= 3*10^-3 C
Q3= 2.4*10^-3 C

Your Q₃ is fine, because the voltage across C₃ is 200 V.

But the voltage across C₁ (or C₂) is not 200 V.

The total voltage across both of them is 200 V, so use that (and the fact that, as rl.bhat says, their charges must be the same).

 

[cristina045]

thank you guys soooo much
i'm really having trouble understand these things
it's so much easier to understand and solve with clear explanations :)