# Solve Capacitor Circuit: Is My Method Correct?

• 1st1
In summary, the capacitors are in series, so the equivalent capacitance equation would be 1/(1/C3+1/C4).
1st1
Capacitor Circuit: Is My Method Correct?

## Homework Statement

Four capacitors, a battery and a switch are assembled in the circuit below. Initially, the switch is set to position A and C4 is uncharged.
At t = 0, the switch is moved to B.
Find Q4, the charge on C4 when the switch is on B.
http://online.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?courses/phys212/oldexams/exam1/fa09/fig20.gif

Q= CV

## The Attempt at a Solution

The initial charge on the C3 capacitor = C3 * V1

Now as the switch to moves to B.

The total charges remains the same, but the voltage changes.

Then

Q = ( C3 + C4 ) V2

V2 = Q / ( C3 + C4 )

next with V2 found

Q4 = C4 V2

*****************

Ok, so that is the only way I've been able to figure this out, however, arent the capacitors in series? In that case I would have to change the equivalent capacitance equation to 1/(1/C3+1/C4) which doesn't work out. Can someone please explain what's going on?
Help would be greatly appreciated. Thanks in advance!

Last edited by a moderator:
The question doesn't quite make sense at the moment.
Is the switch first connected to A, and then to B?

Yes, I'm sorry its a three part question and I forgot the main part of the question statement. Fixed now.

Last edited:
Anyone able to help? I've got the answer, I just want to know if I am doing it correctly and whether the capacitors are considered to be in parallel or in series and why.

In the first part, C1 and C2 are combined in series, and the result of that combination is combined with C3 in parallel. C4 has no effect.
When the switch is on B, some of the charge on C3 moves on to C4 until the pd is the same across both (=V). The total charge that was on C3 (=Q) is now distributed between C3 and C4. So C3xV + C4xV= Q
This gives you V
Knowing V will give you Q for C4.

Stonebridge said:
In the first part, C1 and C2 are combined in series, and the result of that combination is combined with C3 in parallel. C4 has no effect.
When the switch is on B, some of the charge on C3 moves on to C4 until the pd is the same across both (=V). The total charge that was on C3 (=Q) is now distributed between C3 and C4. So C3xV + C4xV= Q
This gives you V
Knowing V will give you Q for C4.

Thank you, the bold part is where I was confused. But now I see I was looking at it the wrong way. Thanks!

## 1. How do I know if my method is correct when solving a capacitor circuit?

There are a few ways to check if your method is correct. One method is to use Kirchhoff's laws to verify that the voltage and current values in your circuit are consistent. Another method is to use a simulator or software to simulate the circuit and compare the results to your calculations. Additionally, you can consult with a colleague or mentor to get a second opinion on your method.

## 2. What are the common mistakes when solving a capacitor circuit?

Some common mistakes when solving a capacitor circuit include incorrect application of Kirchhoff's laws, forgetting to account for the initial charge on the capacitor, using incorrect formulas or values, and not considering the non-ideal characteristics of real capacitors.

## 3. How can I simplify a complex capacitor circuit?

One way to simplify a complex capacitor circuit is to break it down into smaller, simpler sub-circuits. You can also use equivalent circuit techniques, such as Thevenin's theorem or Norton's theorem, to reduce the complexity of the circuit. Another option is to use a simulator or software to analyze the circuit and provide a simplified representation.

## 4. Is there a standard method for solving capacitor circuits?

Yes, there are standard methods for solving capacitor circuits, such as using Kirchhoff's laws and equivalent circuit techniques. However, the specific method used may vary depending on the complexity of the circuit and the desired level of accuracy. It is important to understand the principles and concepts behind these methods in order to apply them correctly.

## 5. What should I do if my calculations do not match the simulation results?

If your calculations do not match the simulation results, you should double-check your calculations and make sure you are using the correct formulas and values. You may also want to compare your method to other methods or seek advice from a colleague or mentor. If the discrepancy is significant, it could indicate a mistake in your method or a problem with the simulation. In this case, it may be helpful to troubleshoot the circuit and make any necessary adjustments.

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