OHM’S LAW:

So far you have been introduced to the concepts of electric current (as a movement of free electrons through a conducting material), voltage (or potential) and potential difference and to the resistance to current flow by any conducting material.  The relationship which exists between these quantities was discovered by a physicist called Ohm and is now referred to as Ohm's Law.  This is the most fundamental law in all electric's and electronics.

Ohm's law states: For a fixed metal conductor, with temperature and other conditions remaining constant, the current through it is proportional to the potential difference between its ends.

Mathematically this is expressed as:

                                      I ∝ V

Thus the ratio  V/I

and this ratio is called the resistance of the conductor.

Hence we may write  V/I  =  R       where V is in volts

                                                   I is in amperes

                                                    R is in ohms