If V is the potential difference across the ends of any conductor, then current I will flow through it. The value of the current changes with changes in potential difference. German scientist simen ohm found out that:

The value of current I passing through a conductor is directly proportional to the potential difference V appalled across its ends, provided the temperature and the physical state of the conductor does not change.

If a graph is drawn between V and I, it will be a straight line (fig. 16.5 which represents the direct proportional relation between V and I i.e.,


Where R is the constant of proportionality the value of which can be determined from V-I graph. The value of R is different for different conductors. 

Limitations of ohms law 

Ohms law is applicable only in case of metallic conductors when their temperature and physical state do not change.


Fig. 16.5