Electromotive force (EMF) is defined as the maximum potential difference between the two terminals of an electrical source when no current is flowing. Conceptually, it represents the amount of energy gained per unit charge delivered to an external circuit, typically originating from a chemical reaction or physical process within the source itself. EMF measures the driving voltage of the source and is a fundamental characteristic in the analysis of electrochemical cells and batteries. While it is distinct from the terminal voltage when current is actively drawn, it establishes the theoretical capacity of the system to maintain an electrical potential difference under open-circuit conditions.