Limitations of Ohm's Law: Although Ohm’s law has been found valid over a large class of
materials, there are a number of commonly used circuit elements which do not
obey this law called Non – Ohmic conductors.
Non – Ohmic conductors have one or more of the following properties:
1. Potential difference (V) depends on current (I) non-linearly.
2. The relation between V and I is non-unique, that is, for the same current I,
there is more than one value of voltage V.
3. The relationship between V and I depends on the sign of V for the same
absolute value of V.
Variation of Resistivity with Temperature: The resistivity of a material is temperature dependent. All
materials do not exhibit the same dependence on temperatures. Over a limited
range of temperatures, that is not too large, the resistivity of a metallic
conductor is approximately given by,
ρT = ρo [1 + α (T–To)]
Where ρT is the resistivity at a temperature T and
ρo is the same at a reference temperature To. α is called
the temperature co-efficient of resistivity, and SI unit of α is K–1.
