Electrochemistry is a branch of physical chemistry dealing with the interconversion of electrical and chemical energy. Many chemical species are electrically-charged ions, and a large class of reactions – oxidation and reduction – consist of electron-transfer reactions between ions and other species. If the two half-reactions (oxidation, reduction) are made to occur at different electrodes, the electron transfer occurs by the passing of a current through an external circuit between them (see battery). The electromotive force driving the current is the sum of the half-reactions, which represent the free energy produced by them. Conversely if an emf is applied across the electrodes of a cell, it causes a chemical reaction if it is greater than the sum of the potentials of the half-reaction (see electrolysis). Such potentials depend both on the nature of the reaction and on the concentrations of the reactants. Cells arising through concentration differences are one cause of corrosion. See also electrophoresis.