Limilation of the Ohm's Law

(1)Non-ohmic condutor do not obey ohm's law

(2)in non-ohmic resistors there is no linear relationship 

(3)

Potential difference 

(V)versus current 

(I)current is not a straight line

(4)Expression for resistance

R=lim delta V/delta I

E.g liquid electrolytes ,vacuum tubes,