Solving a Circuit: Find Ceq, Q, & U

  • Thread starter cristina045
  • Start date
  • Tags
    Circuit
In summary, when capacitors are in series, the charges in each capacitor must be the same. When capacitors are in parallel, the charges in each capacitor are not necessarily the same, but the total voltage across them is still the same.
  • #1
cristina045
4
0

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.
 

Attachments

  • capacitance sketch.jpg
    capacitance sketch.jpg
    4.6 KB · Views: 1,801
Last edited:
Physics news on Phys.org
  • #2


In series combination of capacitors, charges in each capacitors must be the same.
 
  • #3
Hi cristina045! :smile:

(have a mu: µ and try using the X2 and X2 tags just above the Reply box :wink:)
cristina045 said:
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 Ceq + C3 in the first line :wink:)

(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 :smile: says, their charges must be the same).
 
  • #4


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 :)
 

1. How do I find the equivalent capacitance (Ceq) of a circuit?

To find the equivalent capacitance of a circuit, you need to first determine the type of capacitors present in the circuit (series or parallel). Then, use the respective formula to calculate the equivalent capacitance. For series capacitors, use the formula 1/Ceq = 1/C1 + 1/C2 + ... + 1/Cn. For parallel capacitors, use the formula Ceq = C1 + C2 + ... + Cn.

2. What is the formula for finding the total charge (Q) in a circuit?

The formula for finding the total charge in a circuit is Q = Ceq * V, where Ceq is the equivalent capacitance and V is the voltage across the circuit.

3. How do I calculate the total energy (U) stored in a circuit?

The formula for calculating the total energy stored in a circuit is U = (1/2) * Ceq * V^2, where Ceq is the equivalent capacitance and V is the voltage across the circuit.

4. Can I use the same formula to find Ceq, Q, and U for any type of circuit?

Yes, the formulas for finding Ceq, Q, and U are applicable to any type of circuit as long as the capacitors are connected in series or parallel.

5. How do I know if I have calculated the correct values for Ceq, Q, and U?

To ensure you have calculated the correct values, you can check your calculations using the laws of conservation of energy and charge. The total energy stored in the circuit should be equal to the energy supplied by the power source, and the total charge in the circuit should be equal to the sum of the charges on individual capacitors.

Similar threads

  • Introductory Physics Homework Help
Replies
27
Views
3K
  • Introductory Physics Homework Help
Replies
11
Views
1K
  • Introductory Physics Homework Help
Replies
7
Views
2K
  • Introductory Physics Homework Help
Replies
11
Views
3K
  • Introductory Physics Homework Help
Replies
2
Views
12K
  • Introductory Physics Homework Help
Replies
4
Views
2K
  • Introductory Physics Homework Help
Replies
9
Views
1K
  • Introductory Physics Homework Help
Replies
3
Views
3K
  • Introductory Physics Homework Help
Replies
19
Views
2K
  • Introductory Physics Homework Help
Replies
4
Views
2K
Back
Top