# Homework Help: Series and Parallel

1. Feb 10, 2012

### Instinctlol

Hi,
I was wondering how you can tell if 2 capacitor is in series or parallel with each other? I have seen some configuration where it looks like they are in parallel but they are not. Is there any specific way to tell?

Last edited: Feb 10, 2012
2. Feb 10, 2012

### PeterO

Re: Serires and Parallel

I would be interested to see a picture of that circuit.

I hope you are not getting confused by calculating overall capacitance - it is the opposite formula of resistors.

ie resistors in Series R = R1 + R2 + R3.

Capacitors in Parallel C = C1 + C2 + C3

WHEREAS

Resistors in parallel 1/R = 1/R1 + 1/R2 + 1/R3

Capacitors in series 1/C = 1/C1 + 1/C2 + 1/C3
I

3. Feb 10, 2012

### Instinctlol

Re: Serires and Parallel

I thought C3 and C4 are in parallel but they are in series. What makes it in series?

In this picture, I think that C1 and C3 are in parallel because the ends are connected to the same potential Va (points a and b is Vab). Is this correct?

If you can explain it in a way that relates to my reasoning, that would be great. Thank you!

4. Feb 10, 2012

### PeterO

Re: Serires and Parallel

example 1: C3 and C4 are in series on their little branch - the path goes through C3 and then through C4.

If we begin at the common connection of C3 and C4 ; the Right hand side of your picture ; then if you pass from there, through either capacitor, you do NOT end up at the came point electrically speaking - C2 gets in the way.

If you re-draw the first circuit with points a and b on each side [like example 2] rather than both on the left with the circuit looping round, it will be much clearer I feel.

That re-drawing of complicated circuits in a straight line can make it muck clearer.

Example 2.
Assuming your connection from d to c represents just a wire, then C1 and C3 are in parallel with each other, and C2 and C4 are in parallel with each other

5. Feb 10, 2012

### Instinctlol

Re: Serires and Parallel

What do you mean when you say end up at the same point?

6. Feb 10, 2012

### PeterO

Re: Serires and Parallel

Try this

If you imaging the two junctions in the circuit are labelled d at the top and c at the bottom, then if you leave d, passing through C3, the only way to c is to pass through C4 also, showing they are in series.
Note that C2 is in parallel with that pair, as passing through C2 is the alternate way to get to that point c

7. Feb 10, 2012

### PeterO

Re: Serires and Parallel

I forgot; I didn't say the same point, I said the same point electrically.

In your example 2, points c and d are electrically the same - they are connected by a piece of wire.

8. Feb 10, 2012

### Instinctlol

Re: Serires and Parallel

So let me try to summarize, if anything is connected with the same wire (no other potential is involved), it is series. In example 2, C1 and C2 are NOT in series because of the wirecd.

And something is connected parallel if the same potential enters such as C2 and C4 in example 2.

9. Feb 10, 2012

### PeterO

Re: Serires and Parallel

Reasonable description - certainly in example 2, C1 and C2 are NOT in series.

You could re-draw Example 2 as a circuit with two hexagons with each having a flat top and bottom, and sloping sides.

a is the side vertex of the left hexagon, the two hexagons touch at the centre vertex, and b is the right hand vertex of the right hexagon.

C1 is in the top side of the left hexagon
C3 is in the bottom side of the left hexagon
C2 is in the top side of the right hexagon
C4 is in the bottom side of the right hexagon

drawn that way, the parallel/series relation between the components is obvious; the left hand parallel connection is in Series with the right hand parallel connection.

10. Feb 10, 2012

### Instinctlol

Re: Serires and Parallel

I think I understand now, thanks a lot!

11. Feb 10, 2012

### PeterO

Re: Serires and Parallel

Describing this circuit starting at a.

First you pass through C1 [so C1 is in series with the rest of the circuit.

You then have a choice of passing through C2 or C3, so those two are potentially in parallel.
If you choose the C3 path, then you must continue through C4, showing that C3 and C4 are in series with each other.
You then get to a junction wher the parallel branches finish, so C2 was in parallel with the C3 / C4 combination.
The only place to go now is through C5, so like C1, it is in series with the rest of the circuit.

12. Feb 10, 2012

### Instinctlol

Re: Serires and Parallel

Here is a more complicated problem. Did I do this correctly?

13. Feb 10, 2012

### Instinctlol

Re: Serires and Parallel

I think instead of doing the parallel first on the bottom, I think it would be correct to do it in series then parallel

14. Feb 10, 2012

### PeterO

Re: Serires and Parallel

Bottom left,

The 16 & 24 are in series with each other (net 9.8 )
The 24 & 6 are in series with each other (net 4.8 )

Those two combinations are in parallel with each other (net 14.6)

Top example:
The 9 & 15 are certainly in parallel
The 4,8&12 are certainly in parallel
each of those combinations are part of a greater circuit, which alters depending on the switch position [open or closed]

Bottom right - certainly those 2 are in series.

15. Feb 10, 2012

### PeterO

Re: Serires and Parallel