1. This law is valid to conductors only at constant temprature. The resistance of a conductor increases with the increase in its temperature. Hence, the V-I graph for a conductor would not be a straight line.
2. Insulators do not conduct electricity at all, so the Ohm's law is not applicable to the insulators in some cases.
3. Ohm's law is also not applicable for the semiconductors, as the material begins to conduct at certain voltage which will give a steep rise to V-I graph and make it non-linear.
4. This law is not applicable to the unilateral networks(has elements such as diode or transistors etc; which do not have same voltage current).
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There is a range of voltages over which the resistance is fairly constant. Due to dissipation of heat, the resistance is heated and its resistance changes.
The resistance does not obey near the breakdown voltage of the materials. The materials become super conductors at very high voltages.
Some materials do not obey ohm's law below a certain temperature. the material behaves differently.
Resistance is also altered, there is a presence of magnetic field or electric field, if that induces an emf in the conductor. That is, if there is an electromagnetic effect.
Every real conductor or resistor has an inherent capacitance and inductance. That could depend on the geometrical shapes too. It is not a pure resistance.