It is a device which converts chemical energy into electrical energy.

EMF of a cell is defined as the terminal p.d. when no current is being drawn from the cell.

Terminal Potential difference (V) is defined as the terminal potential difference when current is being delivered to external load resistance.

Internal Resistance (r) of a cell is the hindrance offered by the electrolyte of cell to the flow of current

When a current I is passed in cell in opposite direction by external battery, then terminal potential difference