When a membrane is permeable to a single ion species, that ion's transmembrane concentration gradient drives net efflux, but because the accompanying impermeant counter-ions cannot follow, charge separation builds an opposing electrical force across the membrane — the membrane potential — that drives the ion back. The equilibrium potential of an ion is defined as the membrane voltage at which these two opposing forces are equal in magnitude, producing zero net flux; for potassium in a typical cell it is approximately −92 mV, a value determined by the concentration gradient established by active transport (the Na⁺/K⁺ pump exchanging three sodium ions out for two potassium ions in). This concept belongs to cell physiology and membrane biophysics, providing the electrochemical foundation for resting membrane potential and excitability, and it depends on the boundary condition that the ionic flux required to establish the potential is negligible relative to bulk ion numbers, so the concentration gradient is effectively unchanged.
When a membrane is permeable to a single ion species, that ion's transmembrane concentration gradient drives net efflux, but because the accompanying impermeant counter-ions cannot follow, charge sep…