Posted by **Sandra** on Thursday, March 4, 2010 at 4:53am.

A simplified model for an erythrocyte is a spherical capacitor with a positive inner surface of area A and a membrane of thickness b. Use potential difference of 94 mV across the membrane and a dielectric constant K = 5.0. If the capacitance of the membrane is C = 0.15 pF, what is the radius of the erythrocyte? The thickness of the membrane is b = 90 nm.

The way I thought to solve for this question was that I found the area using the Capacitance of a dielectric eqn, then from there used A = pi*r^2 to solve for r...is this the correct approach? I'm just concerned because I'm not using the potential difference anywhere...

- Physics -
**drwls**, Thursday, March 4, 2010 at 5:43am
The membrane surface area should be

A = 4 pi r^2

for a spherical cell

- Physics -
**Sandra**, Thursday, March 4, 2010 at 8:32am
Thanks, so there wouldn't be any need to use the potential difference anywhere right?

- Physics -
**bobpursley**, Thursday, March 4, 2010 at 8:45am
No, no V. Capacitance is not dependent on potential.

- Physics -
**Anonymous**, Saturday, March 6, 2010 at 2:09am
Isnt capacitance defined as how much charge per unit potential?

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