# Multiple electrodes - capacitance.

1. Oct 16, 2008

### Dafe

Hey,
I have a length of pipe. Inside the pipe walls I have placed four electrodes. If one is giving out
x volts, and I know the dielectric constant of the material between them, is there an easy way of finding the capacitance on the other three electrodes?

I'm looking for hints as well as good resources on the subject.

Thank you.

2. Oct 16, 2008

### Phrak

It really depends on the geometric placement.

What's an electrode?

3. Oct 16, 2008

### Dafe

By electrodes I mean plates of some metal.

Let's say the plates are facing each other if you know what I mean..

4. Oct 16, 2008

### Phrak

You're language is hard to follow, but you should elaborate more. Are they stacked like coins with some space in between? What's the spacing.

The "capacitance on them" doesn't mean anything. You might validly ask about the capacitance between two plates (or electrodes). Each plate can have a voltage relative to some other plate. The pipe itself is a "plate".

5. Oct 16, 2008

### Dafe

Let's say that the pipe is made of glass.
I make four rectangular shapes of some metal.
I glue them inside the pipe walls.
I have a battery connected to one of the metal shapes.
Can I then find out the capacitance on the other metal shapes?

Is this a better explanation?

Thank you for your patience!

6. Oct 16, 2008

### Phrak

C = e(A/d)

For d is small compared to the size of the plate:

C is the capacitance between[i/] two plates.
e is the dialectric constant of the material
A is the area of a plate
d is the distance between them

7. Oct 16, 2008

### Dafe

What if d is not small compared to the size of the plates?

8. Oct 16, 2008

### Redbelly98

Staff Emeritus
Then the simple formula Phrak gave is not accurate. You would have to measure the capacitance.

It can be measured with a capacitance meter (some DVM's have that feature). Or measure the time to discharge the cap through a resistor by measuring the capacitor voltage during discharge -- either with a DVM if the discharge time is longer than several seconds, or on an oscilloscope if the discharge time is shorter.

From your description, it sounds like you would a capacitance meter or an oscilloscope -- discharge time should much faster than 1 sec if d is the thickness of a glass pipe.