Write limitations of Ohm's law. - Vidyadip

Question

Write limitations of Ohm's law.

✓

Answer

For certain materials and devices used in day to day life, voltage (V) and current (I) are not proportional to each other. So in such cases Ohm's law is not obeyed.

(a) V and I are not proportional to each other.
In graph, dotted line part shows that Ohm's law is obeyed, where as continuous curve of the graph shows that Ohm's law is not obeyed.
$\rightarrow $ When current flows through conductor, Joule heat is produced. Hence, when conductor is heated, its resistance increases. Hence, for such materials V and I graph will be continuous curve.
Example : Diode, Transistor.

(b) Relation between I and V depend on polarity of V . Hence, for same positive or negative values of voltage, different current values are obtained.
Example : PN junction diode.
$\rightarrow$ In graph shown for positive and negative value of voltage and current, scales are different.

(c) I and V relation is not one-one function. For one value of current there are more than one values of voltage. For example, GaAs exhibit such behaviour.
$\rightarrow$ AV Figure shows $I \rightarrow V$ graph for GaAs. In such graph, $\frac{\Delta V }{\Delta I }$ is used to obtain resistance.

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