# Homework Help: Surface in capacitor equation

1. Apr 10, 2014

### baby_1

Hello
i want to calculate the capacitance of a two-surface parallel capacitor with famous equation

as you see the attachment (question 28) writer divided the depth size by 3. but if we know the surfaces of the capacitor are rectangular such as this picture

and each surface of capacitor has depth=15cm and width=20cm
so S=.15*.20=.03
but why the writer divided the depth size by 3(15/3)?because their depth are same and width should be changed via the figure.
Thanks

#### Attached Files:

• ###### ch6.pdf
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2. Apr 10, 2014

### haruspex

I can't interpret the diagram, either in the post or in the attachment. Please post the original question.

3. Apr 11, 2014

### baby_1

Hello
question is:
find the capacitance of parallel surface that different dielectrics are between two surface.
distance between surface:2mm
depth of surface:15cm
width:20cm

my question is : when we want to define the S in (area) why the writer divide depth by 3?
because depth for each is same and width is different so S=.15*.2 is correct.doesn't it?

4. Apr 11, 2014

### ehild

The three parts are filled with different dielectrics. See the pdf file.

ehild

5. Apr 11, 2014

### haruspex

Still not sure I understand the original question - terms like depth and width are not well-defined. But certainly the working in attachment does not look like a reasonable interpretation.
The question as posted just above I interpret as: There are three capacitors in parallel. Each has a thickness 2mm and a surface of 20x15 cm.
The diagram in the attachment fits with that.
The working in the attachment treats the capacitors as being 20cm thick and an area of 5cmx2mm (which is a bizarre shape for a capacitor). With that interpretation of its own diagram, that would put them in series, not in parallel.
You can safely ignore the working in the attachment.