# Confused on Capacitor problem.

1. Apr 2, 2008

### themrk

1. The problem statement, all variables and given/known data
An axon is the relatively long tail-like part of a neuron, or nerve cell. The outer surface of the axon membrane (dielectric constant = 5, thickness = 1 10-8 m) is charged positively, and the inner portion is charged negatively. Thus, the membrane is a kind of capacitor. Assuming that an axon can be treated like a parallel plate capacitor with a plate area of 4.20 10-6 m2, what is its capacitance?

2. Relevant equations
C = k * Eo * A / d

3. The attempt at a solution
I used C = 5 * 8.85*10^-12 * 4.20*10^-6 / 1*10^-8 = 1.9E-8 F

However this is not the correct answer and I am perplexed as to why. Is Eo not 8.85*10^-12 or is d not just the thickness?

2. Apr 2, 2008

### Mindscrape

So is this a parallel plate capacitor with with a dielectric in between? If so then you should be able to just use

$$C = \epsilon_r C_{vac}$$

where $\epsilon_r$ is the dielectric constant. I think you may have used the wrong electric permitivity.

3. Apr 2, 2008

4. Apr 2, 2008

### merryjman

I think the permittivity is correct, 8.85e-12 is the vacuum permittivity, and it was multipled by 5, the dielectric constant.

If you are doing this as a web-based problem, it's probably a matter of significant digits. Use more.