# Circuit with capacitors , need to find Ceq, charge on each and total stored energy

#### cristina045

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

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

2. Relevant 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

3. 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.

#### Attachments

• 4.6 KB Views: 1,103
Last edited:
Related Introductory Physics Homework News on Phys.org

#### rl.bhat

Homework Helper
Re: Circuit with capacitors , need to find Ceq, charge on each and total stored energ

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

#### tiny-tim

Homework Helper
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.

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

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

Re: Circuit with capacitors , need to find Ceq, charge on each and total stored energ

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 :)

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving