Ohm’s Law

Ohm’s law states that the current (I) flowing through a conductor is directly proportional to the potential difference (V) across the ends of the conductor.

We know that,

but,

therefore,

also,

therefore, or = R a constant for a given conductor for a given value of n, l and at a given temperature. It is known as the electrical resistance of the conductor.

Thus, V = RI this is Ohm’s law.

Ohm’s law is not a universal law, the substances, which obey ohm’s law are known as ohmic substance.

Limitations of Ohm's Law

Although Ohm’s law has been found 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 broadly are one or more of the following types:

(a) V ceases to be proportional to I (below fig).

The dashed line represents the linear Ohm’s law. The solid line is the voltage V versus current I for a good conductor

(b) The relation between V and I depends on the sign of V. In other words, if I is the current for a certain V, then reversing the direction of V keeping its magnitude fixed, does not produce a current of the same magnitude as I in the opposite direction (below fig). This happens, for example, in a diode.

Characteristic curve of a diode.

(c) The relation between V and I is not unique, i.e., there is more than one value of V for the same current I (below fig). A material exhibiting such behaviour is GaAs.

Variation of current versus voltage for GaAs

Materials and devices not obeying Ohm’s law in the form of V = RI are actually widely used in electronic circuits.