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Electro Motive Force

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 move in a particular direction is called electromotive force or emf. Just like a force that 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 around the circuit


The emf of a cell is the energy supplied by the cell to move unit charge around a circuit join to it. It the potential difference between the terminals of the cell in an open circuit. It does not depend on the size of a cell but it depends on the chemical used in it. It is denoted by E. Its unit is volt in SI-unit


Internal 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 an electric current passes through it Internal resistance of a fresh new cell is small but the resistance increases as we use it more and more. It is shown externally in series to the cell as shown in

Fig  It is denoted by r



Figure  Representation of internal resistance


The internal resistance of a cell depends on many factors


The distance between the electrodes: r increases with an increase in distance between the



 The nature of the electrolyte, greater the conductivity of the electrolyte, lesser is the internal resistance


 Nature of the electrodes


The area of the plates or electrodes immersed in the electrolyte: internal resistance decreases with an increase in the area of the plates immersed


 The temperature of the electrolyte, internal resistance decreases with the rising of temperature.