# Homework Help: Capacitance of four metal plates

1. May 7, 2015

### Patrikp

1. The problem statement, all variables and given/known data
Four identical parallel metal plates with area $$S$$ and distance between each two $$d$$ are located in air. The external plates are connected by wire. What is the equivalent capacitance between A and B?
This is problem from Irodov.

2. Relevant equations
$$q=UC$$

3. The attempt at a solution
Plates 1 and 4 are always on same potential which im not sure how to use.
I tried to simplify as on the picture but that turns out to be wrong(why?).

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2. May 7, 2015

### CWatters

As I see it there are three nodes, A, B and C (the wire connecting the outer plates 1 and 4).

Draw the three nodes on paper. There is capacitance between...

A and B
A and C
B and C

3. May 7, 2015

### Patrikp

Can you explain how did you get that? Thanks

4. May 7, 2015

### Suraj M

You have to recognize the 3 capacitors, its easier if you number them, say capacitor plate 1,2,3 and 4 , so the 3 capacitors are (12),(23) and (34), now try figuring out the the arrangement of these capacitors, by considering which are connected and which are not.
If you want to know how to recognize the 3 capacitors try giving the plates charges, imagine A is the +ve terminal and see which all plates should have +ve and which -ve, once you figure that out, i think you'll be able to get the answer.

5. May 7, 2015

### CWatters

Try..

PS the colours don't mean anything they are just to highlight the three nodes.