Ohm's law is not valid when

CONCEPT:

​Ohm's law:

• A conductor through which a current I is flowing and let V be the potential difference between the ends of the conductor. Then Ohm’s law states that

⇒ V ∝ I

⇒ V = IR

where R is called the resistance of the conductor. Resistance depends on the shape, size, and material of the conductor.

Limitations of Ohm's law:

• Ohm’s law is valid over a large class of materials.
• There do exist materials and devices used in electric circuits where the proportionality of V and I does not hold. 

The deviations in occurring are one or more of the following types:

• V is not to be proportional to I for some range of V or I

• The relation between V and I depends on the sign of V. 
  • Reversing the direction of V keeping its magnitude fixed does not produce a current of the same magnitude as I in the opposite direction.

• The relation between V and I is not unique
  • There is more than one value of V for the same current I 

EXPLANATION:

Ohm's law:

• A conductor through which a current I is flowing and let V be the potential difference between the ends of the conductor. Then Ohm’s law states that

⇒ V ∝ I

⇒ V = IR

where R is called the resistance of the conductor. Resistance depends on the shape, size, and material of the conductor.

• The above equation is that of a straight line, where the slope is constant and is equal to R.
• Ohm's law will not be valid if the slope of the equation is variable.
• Meaning Ohm's law will not be valid if the rate of change of potential with respect to current is variable. Therefore option 2 is correct.