limitations of Ohm's law | Distinguish Ohmic and non ohmic circuit elements

limitations of Ohm's law | Distinguish Ohmic and non ohmic circuit elements 

Ohm's law is not considered to be a fundamental law. It explains the common behaviour of many substances/materials under given conditions like constant temperature and pressure. It is, therefore, possible that some materials may not strictly follow the Ohm's law (i.e. $V \propto I$).

Ohm's law is not obeyed in the following cases :

1. When temperature of a conductor increases considerably.

Ohm's law ($V \propto I$ or $R = \frac{V}{I}$) for a conductor at constant temperature is shown by a dotted line in Figure 13. However, when temperature of the conductor increase, its resistance increases and hence $V$ is not directly proportional to $I$. The behaviour of a conductor at high temperature is shown by a solid curve 

2. Ohm's law is not obeyed by semiconductor diode.

A semiconductor diode conducts, when forward biased and does not conduct, when reverse biased. (We shall discuss semiconductor diode later on).

The V-I characteristic of semiconductor diode is shown in Figure 14. When semiconductor is forward biased or reverse biased, it does not obey ohm law (i.e., $V \propto I$).

3. Ohm's law is not obeyed by a material gallium arsenide (Ga As), a compound semiconductor.

The variation of current with the voltage applied across Ga As is shown in figure

In this graph, there is more than one value of voltage $V$ for the given current $I$. Hence, Ga As does not obey ohm's law.

Distinguish between Ohmic and non ohmic circuit elements.

1.Ohmic materials or circuit elements :

The materials or circuit elements which strictly obey Ohm's law are known as Ohmic materials or Ohmic circuit element. For example, metals are Ohmic materials.

The graph between $V$ and $I$ for Ohmic materials is a straight line passing through the origin O such that $I$ varies linearly with $V$.

2.Non-ohmic materials or circuit elements :

The materials or circuit elements which do not strictly obey Ohm's law are known as non-ohmic materials or circuit elements. For example, vacuum tubes, semiconductor diodes, transistors, thermistors etc.

The graph between $V$ and $I$ for non-ohmic materials or circuit elements is not a straight line as shown in Figure. In such cases, ratio of change in $V$ to the change in $I$ for a given voltage gives the dynamic resistance of the circuit element.

That is,

$r = \frac{\Delta V}{\Delta I}$