Chapter-3|Current Electricity| NCERT 12th Physics:
Cells and Electromotive Force(EMF) of a Cell:
An electric cell is a source of energy that maintains a continuous flow of charge in a circuit. Electric cell changes chemical energy into electrical energy.
Electromotive Force(EMF) of a Cell:
Electric cell has to do some work in maintaining the current through a circuit. The work done by the cell in moving unit positive charge through the whole circuit(including the cell) is called EMF of the cell.
If during the flow of q coulomb of charge in an electric circuit, the work done by the cell is W, then
Internal Resistance of a Cell(r):
Internal resistance of a cell is defined as the resistance offered by the electrolyte of the cell to the flow of current through it.
It is denoted by r. Its SI unit is ohm.
It is depends upon separation between two plates of cell, area of plates and nature of electrolyte.
Terminal Potential Difference(V):
The potential drop across the terminals of a cell when a current is being drawn from it is called its terminal potential difference(V).