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