Write the limitations of Ohm's law.

(N/A) Ohm's law states that the current flowing through a conductor is directly proportional to the potential difference applied across its ends,provided physical conditions remain constant. However,it has certain limitations:
$(a)$ $V$ and $I$ are not proportional to each other for all materials.
In the provided graph,the dotted line represents the region where Ohm's law is obeyed (linear behavior). The continuous curve indicates where Ohm's law is not obeyed (non-ohmic behavior).
When current flows through a conductor,Joule heating occurs. As the temperature of the conductor increases,its resistance changes. Consequently,for such materials,the $V-I$ graph is a non-linear curve.
Examples: Diodes,Transistors.
$(b)$ The relationship between $I$ and $V$ depends on the polarity of the applied voltage $V$. Thus,for the same magnitude of positive or negative voltage,different current values are obtained.
Example: $PN$ junction diode.