  I derive formulas for the electrostatic potential of a charge in or near a
membrane modeled as one or more dielectric slabs lying between two
semi-infinite dielectrics. One can use these formulas in Monte Carlo codes to
compute the distribution of ions near cell membranes more accurately than by
using Poisson-Boltzmann theory or its linearized version. Here I use them to
discuss the electric field of a uniformly charged membrane, the image charges
of an ion, the distribution of salt ions near a charged membrane, the energy of
a zwitterion near a lipid slab, and the effect of including the phosphate head
groups as thin layers of high electric permittivity.
