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When two terminals of a battery are connected by a conductor, an electric current flows through the conductor. One terminal continuously sends electrons into the conductor, while the other continuously receives electrons from it. In this way, the battery behaves like an electric pump. The property of a cell (or generator) that makes the charge to move in a particular direction is called electromotive force or emf. Just like a force which moves a mass, an emf moves a mass, an emf moves a charge in a circuit. The emf of a source is not a force but work done in moving a unit charge round the circuit. The emf of a cell is the energy is supplied by the cell to move unit charge round a circuit join to it. It is the potential difference between the terminals of the cell in an open circuit. It does not depend on the size of a cell but depends on the chemical used in it. It is denoted by E. its unit is volt in SI-units.

Terminal resistance: the resistance offered by a source for a current to pass through it is called its internal resistance. The internal resistance of a cell is the resistance offered by the electrolyte between the electrode of the cell when electric current passes through it. Internal resistance of a fresh new cell is small but the resistance increases as we use it more. It is shown externally in series to the cell. It is denoted by r.

Internal resistance of a cell depends on many factors:                                                                                                   

Terminal potential difference: terminal potential difference of a cell is defined as the potential difference between the positive and negative electrodes of a cell sends a current in an external circuit. It is measured in volt and denoted by v. the terminal potential difference depends on the external resistance connected to a closed circuit. It is smaller than the emf of the cell when the cell is discharging.