Ohm's Law

| Labels: Physics

Ohm’s law deals with the relationship between current, voltage and ideal resistance. It was given by Georg Simon Ohm in 1827.

Statement of Ohm’s Law

At constant temperature, the current through an ideal resistor is directly proportional to the voltage applied across the resistor. Ideal resistance is defined as the resistance which doesn’t change its resistivity due to change in voltage or current imposed on it and hence is a pure resistance.

Limitation of Ohm’s Law

Ohm’s law is followed by many substances under specific conditions, however, it is not a fundamental law.

The conductors that follow Ohm’s law are called Ohmic conductors. For these conductors, the linear relationship between voltage and current holds true, i.e. V is proportional to I. The resistance is independent of the current I through the conductor. In these conductors, the current I gets reversed in direction when a potential difference, V, is reversed, but the magnitude of current changes linearly with voltage. Therefore, the V-I graph will be a straight line passing through the origin.

Non-Ohmic Conductors

The conductors that don’t obey Ohm’s law are called non-ohmic conductors. The resistance of such conductors is not constant even at a constant temperature. Some examples can be:

Metallic conductors, which have a straight graph up to a certain current (acts as ohmic) but become non-ohmic at larger currents.
P-n junction diode, which consists of a p-type and n-type semiconductors. A voltage is applied across the junction, resulting in a current I. I is not proportional to V. Very little currents flow for fairly high negative voltage and a current begins to flow for much smaller positive bias. Thus, the junction diode allows current to flow only in one direction, i.e, it acts as a rectifier.

Some other examples of non-ohmic conductors are incandescent bulbs, semiconductors- like Germanium and Silicon.

The bulb’s tungsten filament heats up when the voltage increases, changing the resistance of the wire. So the bulb filament does not obey ohm’s law. (Bulb is not to be confused with LED, a light-emitting diode)

Summarised Notes for Limitations of Ohm’s Law

There are two types of conductors→ ohmic and non-ohmic
Ohmic conductors follow Ohm’s law and form a straight line on the graph
Non-ohmic conductors do not follow Ohm’s law in every situation
P-n junction diode, Metallic conductors, tungsten filament, semiconductors like germanium, silicon
Resistors tend to be Ohmic at their designed operating temperature