Limitations of Ohm’ Law-12 CBSE
Ohm’s law is not a fundamental law of nature but in actual practice, there are certain cases where the slope of the V-I graph is not constant and the relation
V = R I
does not hold always. The following are the cases where Ohm’s Law ceases to be valid.
The potential difference across a conductor should increase proportionally with current as per Ohm’s law as depicted in fig. 1 in the name of the Ideal Curve.
But the observed curve is practically found to have deviated from the theoretical(Ideal Curve) at a higher value of current. This deviation occurs due to the reason of the heating effect of the current. Due to an increase in current to sufficiently higher values, the temperature of the conductor increases, and consequently the resistance of the conductor also increases which causes the potential difference across the conductor to increase sharply disagreeing with ohm’s law. Since the temperature of the conductor could not be kept constant at higher temperatures, causing the deviation of proportional change in potential difference developed across the conductors.
Graphical representation of Limitations of ohm’s law
![](https://physicsmadesimple.in/wp-content/uploads/2022/12/Screenshot-70.png)
In non-linear devices, like semiconductor diodes, where V-I characteristics strongly depend on the sign of V. It is observed that on reversing the direction of voltage keeping its magnitude the same, the value of current is different. This is a clear violation of ohms law…
graphical observations of ohm’s Law
![](https://physicsmadesimple.in/wp-content/uploads/2022/12/Screenshot-71.png)
Similar is the case of other devices like transistors, and thermistors, the variation of the potential difference with the electric current is not proportional. Hence Ohm’s Law does not hold. For example, the V-I characteristics of GaAs are not uniform but there exists more the one value of currents corresponding to a single value of V applied.
However, It is observed that there are some materials and devices widely used in electronic circuits not obeying ohms law.
![](https://physicsmadesimple.in/wp-content/uploads/2022/12/Screenshot-72.png)
![](https://physicsmadesimple.in/wp-content/uploads/2022/12/Screenshot-64.png)