# Capacitance question

1. Jul 9, 2006

### StudentofPhysics

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 4.7 10-6 m2, what is its capacitance?

I used the formula C = (KE0 A )/ d

and assigned the following:
KE0 = 5
A = 4.7 x 10^-6 m2
d = 1 x 10^-8 m

and came up with:
C= 2350.
this isn't correct though.

What am I doing wrong?

2. Jul 9, 2006

### StudentofPhysics

ok, on further work, i believe it should just be K=5. i dont know what formual to use to find E though...

3. Jul 9, 2006

### Mute

You mean "E0", in your capacitance equation? That's just the permittivity of free space (usually denoted ε with a subscript 0), a constant equal to 8.85x10^(-12) Coulomb^2/(Nm^2).

Last edited: Jul 9, 2006
4. Jul 9, 2006

### StudentofPhysics

i got it know
thank you