Voltaic cells for physicists: Two surface pumps and an internal resistance

A proper discussion of voltaic cells—the basic elements of any battery—must invoke the fact that chemical reactions occur at each of the two electrode–electrolyte interfaces. The corresponding physical picture is that voltaic cells have two chemical-reaction-driven “ion pumps” and an internal resistance within the electrolyte. (These ion pumps can move electric charge; hence the historical terminology of “electromotive force,” or emf.) A correct physical picture for the “seat of emf” of a voltaic cell gives, not a “volume pump” (represented by a linear rise in voltage, as in many introductory physics textbooks), but two “surface pumps” (represented by two steplike jumps in voltage). Within this framework, some basic properties of voltaic cells are considered: their emf, maximum current, internal resistance, and energy storage. We also discuss some diffusion effects within voltaic cells, whose manifestations—but not origins—are well known when batteries are charged and discharged. Because of these diffusion effects, treating the electrolyte as an internal resistance is an oversimplification.