# Homework Help: Cell Membrane and Hollow Spheres

1. Jan 25, 2005

### figs

there are 2 probs here.
1) The inner and outer surfaces of a cell membrane carry a negative and positive charge, respectively. Because of these charges, a potential difference of about 0.0680 V exists across the membrane. The thickness of the membrane is 7.95×10-9 m. What is the magnitude of the electric field in the membrane?

I thought i could just use E=-change in V/change in distance, but no.

2) Two hollow metal spheres are concentric with each other. The inner sphere has a radius of 0.1440 m and a potential of 86.0 V. The radius of the outer sphere is 0.146 m and its potential is 82.0 V. If the region between the spheres is filled with Teflon, find the electric energy contained in this space.

This one i played with in the equation V=(kq/rsquared)+(k/q'/rsquared).
I've just confused myself with working them so many times. so im seeking help here!

2. Jan 25, 2005

### gnome

(1) asks for magnitude, so I think it's just $E = \left | \frac{\Delta V}{d} \right |$

(2) determine the capacitance using the potential difference, the distance between the spheres, and the dielectric constant of the teflon. Then you can find the amount of energy stored using the potential difference and the capacitance.

3. Jan 25, 2005

### figs

i figured it out.
(1) I set V=kq/r equal to E=kq/r^2

(2) i solved Q=[(kEoA)/d]V
and then plugged into V=EPE/Q

its easier for me if the equations are
derived, and i worked it all out nicely.
That second problem was making me crazy!