The Electromotive Force (EMF, $E$) of a cell is the potential difference between its terminals when no current is drawn (open circuit).

• Physically, $E=\frac{W}{Q}$, where $W$ is the work done by chemical forces to move a charge $Q$ within the cell from one terminal to the other against the electrostatic force.
• When a current $I$ flows, the terminal voltage $V=E-Ir$, where $r$ is internal resistance.