THEORY OF EMF AND INTERNAL RESISTANCE
INTERNAL RESISTANCE of a cell is the resistance offered by the electrolyte of the cell when electric current flows through it.
ELECTROMOTIVE FORCE is the potential difference between the two poles of a cell in an open circuit (when no current is drawn from the cell). It is also defined as the energy supplied by the cell to drive a unit charge once around the circuit. It is denoted by E.
TERMINAL POTENTIAL DIFFERENCE IS the potential difference between the two poles of a cell in a closed circuit (when current is drawn from the cell).
When current flows in the circuit, there is a fall of potential across its internal resistance. Hence the PD is less than e.m.f. by an amount equal to potential drop across its internal resistance.
Definition: EMF of a cell is said to be one volt, if one joule of work is done by the cell to drive one coulomb of charge once around the circuit.
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