1. E.M.F. of a cell : the voltage of a cell in an open circuit condition, measured by a very high resistance voltmeter is called e.m.f. of a cell. This is denoted as E, measured in volts. This is shown in the Fig. 1.
2. Internal resistance of a cell : the cell completes path of current from its positive terminal to negative terminal through external circuit. But to complete the closed path, the current flows from negative to positive terminal of cell, internally. The opposition by a cell to a current, when it flows internal to the cell is called the internal resistance of the cell. It is denoted as r and measured in ohms.
In an equivalent circuit of a cell, its internal resistance is shown in series with that cell. The resistance is shown in series with that cell. The Fig. 2 shows a cell and its internal resistance.
3. Terminal voltage : when an external resistance is connected across the terminals of the cell, the current I flows through the circuit. There is voltage drop 'Ir' across the internal resistance of the cell. The cell e.m.f. E has to supply this drop. Hence practically the voltage available at the terminals of the cell is less than E by the amount equal to 'Ir'. This voltage is called the terminal voltage V. This is shown in the Fig. 3.
Mathematically, the terminal voltage is given by,
From external resistance side we can write,
Note: Practically internal resistance of the cell must be as small as possible.
It can also be observed that on no load i.e. external resistance not connected, the open circuit terminal voltage is same as e.m.f. of the cell, as current I = 0.
