Finding the combined capacitance between A and B

[Eitan Levy]

Homework Statement

Find the combined capacitance between A and B

Relevant Equations

Parallel and series connection of capactiors

First of all I believe that capacitors 1, 2 and 3 are connected in series, because the charge on them must be equal so I combined them.

After that, I suspect that the result of their combined capacitor (C123) is connected in parallel to C4 because the voltages on them are the same.

After that I struggle to find capacitors that will have the same charge or voltage on them due to the many wires that don't connect to anything.

I can't even understand how the capacitors on the edges may be charged, how can charge move from the plate closer to the edge? It should remain constant.

How do I proceed?

 

[haruspex]

believe that capacitors 1, 2 and 3 are connected in series

From the perspective of A? 1 is down the left path, while 2 and 3 are down the right path.

because the charge on them must be equal

I don’t see why that would be, or what the relevance of it would be to whether they are in series.

 

[Delta2]

For start, I believe capacitors 5, 6 and 8 are irrelevant and can be removed from the schematic without affecting the total capacitance between A and B.
After that I believe you can identify more easily which capacitors are in series and which in parallel.

 

[Eitan Levy]

From the perspective of A? 1 is down the left path, while 2 and 3 are down the right path.

I don’t see why that would be, or what the relevance of it would be to whether they are in series.

Then how do J determine when are capacitors connected by series or by parallel? That pretty much was what I was taught in class.

I I understand my mistake with 1 2 and 3. 2 and 3 are still connected by series I believe, but how can I proceed from there?

 

[Eitan Levy]

I

For start, I believe capacitors 5, 6 and 8 are irrelevant and can be removed from the schematic without affecting the total capacitance between A and B.
After that I believe you can identify more easily which capacitors are in series and which in parallel.

Is it okay to ignore them? Is it because they can't be charged due to the way the diagram was constructed?

 

[Delta2]

I

Is it okay to ignore them? Is it because they can't be charged due to the way the diagram was constructed?

yes I believe it is okay to ignore them. They can't be charged as you say, because if we imagine current flowing from A to B via all possible paths, these possible paths do not include capacitors 5 , 6 and 8.

 

[gneill]

Yes, as @Delta2 said, capacitors (or components in general) that are just "hanging" from circuit nodes without connections to other components at their other ends can be ignored. They will play no part in the operation of the circuit. Note that nodes A and B are considered to be important to the circuit operation since you would "connect" a hypothetical capacitance meter between them. So capacitor 9 is not just "dangling" without connection. It must remain.

Removing the extraneous capacitors you should be able to identify the remaining capacitors that are in either series or parallel, and hence reduce the circuit accordingly. Try redrawing the circuit without the unnecessary components and see what you can do to reduce the circuit.

 

[Eitan Levy]

yes I believe it is okay to ignore them. They can't be charged as you say, because if we imagine current flowing from A to B via all possible paths, these possible paths do not include capacitors 5 , 6 and 8.

So I get C2 and C3 connected in series, C23 is connected in parallel to C14 where C1 and C4 are also connected in series. After that C1234,C7,C9 are all connected in series. Is that right?

 

[gneill]

So I get C2 and C3 connected in series, C23 is connected in parallel to C14 where C1 and C4 are also connected in series. After that C1234,C7,C9 are all connected in series. Is that right?

Looks good! So,... now do the math...