A cell membrane has a surface area of 1.0 × 10−7 m2, a dielectric constant of 5.2, and a thickness of 7.5 nm. The membrane acts like the dielectric in a parallel plate capacitor; a layer of positive ions on the outer surface and a layer of negative ions on the inner surface act as the capacitor plates. The potential difference between the "plates" is 90.0 mV.
(a) How much energy is stored in this capacitor?
(b) How many positive ions are there on the outside of the membrane? Assume that all the ions are singly charged (charge + e).