Question

The assertion that V = IR is a statement of Ohm’s law is not true. Why ?

Answer 1

The equation

\[ V=IR \]

is not itself the statement of Ohm's law. It is only a mathematical relation between voltage, current and resistance.

Ohm's law states that:

For a metallic conductor, provided the physical conditions such as temperature remain constant, the current through the conductor is directly proportional to the potential difference across it.

\[ V\propto I \]

or,

\[ \frac{V}{I}=R=\text{constant} \]

The equation \(V=IR\) can also be written for materials whose resistance changes with voltage or current (such as diodes, transistors, etc.), but such materials do not obey Ohm's law because \(R\) is not constant.

Hence, Ohm's law is the constancy of the ratio \(\frac{V}{I}\) under constant physical conditions, not merely the equation \(V=IR\).