# Homework Help: Capicatance & magnitude of capacitor

1. Apr 13, 2012

### Alouette

1. The problem statement, all variables and given/known data
There's a spherical capacitor, inner radius a and outer radius b, with a filling in between of epoxy, and also connected to battery of voltage V. What is the capacitance?

Also, what is magnitude of surface charge density outer shell?

2. Relevant equations
For Q1
1)C=4∏ε0(ab/b-a)

2)C = Q/ΔV

3)C = κC0

For Q2
s=Q/4∏r2

3. The attempt at a solution
Used above equation, but it doesn't include any voltage V nor dialelectric constant κ. Are these the right equations?

Haven't attempted Q2 since it seems to require C/m2 as units for answer...

2. Apr 13, 2012

### darkxponent

First you show the attempt. Then only we will know how to help

3. Apr 13, 2012

### Alouette

Sure. Except I already did. I said I used C=4∏ε0(ab/b-a) but this gave me the wrong answer. I noticed that it didn't even used V or dialelectric constant κ, but I'm unable to find how these fit it. The only other equations I found involving them were equations 2) and 3).

C = Q/ΔV , this has V but i'm unsure how there's a change in V when they gave me just a constant voltage. Also there wasn't any charge in the question either...

C = κC0, I would have to use the equation 1) C and then multiply by epoxy constant, which I found to be either 3.9, 4.1, or 4.4, not sure which one but I was going to plug them in to find out.

4. Apr 13, 2012

### Alouette

4*π*k*ε/((1/a)-(1/b)), where k is 3.6 and ε is 8.85e-12

I don't understand where these equations come from...

the 4πε is from the charge of the sphere?

why the subtraction of inverses?

Not looking for a detailed lecture, just a simple explanation would do to better help me understand the process of arriving at this answer.

Last edited: Apr 13, 2012
5. Apr 13, 2012

### Staff: Mentor

Do a web search on "spherical capacitor". The Hyperphysics site has some appropriate info.

6. Apr 14, 2012

### darkxponent

seems to me that you are attempting the question without studying the chapter. You dont have a clear idea of what capacitor is. The question you have asked is quite very simple. I dont know how to help you with this question as it is only formula based. I would advice you to first go through the chapter once then only attempt the question