emf of the cell

In order to maintain continuous flow of current through a conductor AB of resistance R, we should always keep A at a positive(higher) potential and B at a negative(lower) potential. We connect A to positive terminal P of the cell and B to negative terminal Q of the cell. Through chemical reaction, the cell always maintains P at a constant positive potential and Q at a constant negative potential.

In external circuit, current(+ve charge) flows from P to Q via the conductor AB. But, inside the cell the same positive charge moves from lower potential to higher potential. To do this, the cell must be able to do work on the charge. The energy to do this work is derived from the chemical process inside the cell.

The influence that makes charge move from lower potential to higher potential is called the electromotive force and is denoted by E.

The emf of a cell is defined as the work done in carrying a unit positive charge through the complete circuit including the charge flow inside the cell.

The emf is measured in the units of Joule/Coloumb or Volt. Thus emf has the same units as the units of potential difference.

The resistance to flow of current inside the electrolyte solution of the cell is called internal resistance of the cell. The emf and internal resistance of the cell will be fairly constant only when small current is drawn from the cell.