The electrochemical cell contains two half cells. The electrodes in these cells have different electrode potentials. When the circuit is completed the loss of electrons occurs at the electrode having lower reduction potential whereas the gain of electrons occurs at the electrode with higher reduction potential. The difference in the electrode potentials of the two electrodes of the cell is termed as electromotive force (abbreviated as EMF) or cell voltage (Ecell). Mathematically, it can be expressed as

EMF = E_red (cathode) - E_red (anode) or simply as
                                                 
E_cell = E_cathode - E_anode 

Since in the representation of a cell, the cathode is written on right hand side and the anode on left hand side, therefore, EMF of a cell is also sometimes written as:
                                             
EMF = E_right - E_left = E_R - E_L

EMF of the cell may be defined as the potential difference between the two terminals of the cell when either no current is drawn from it. EMF is measured with the potentiometer or vacuum tube voltmeter.

The EMF of the cell depends on nature of the reactants, concentration of the solutions in the two half cells and the temperature. The EMF of the cell at the standard state conditions is called standard EMF and can be calculated from the standard electrode potentials of the two half cells.