Statement and Limitations of ohm's law

Unit - I

BASIC CIRCUITS ANALYSIS

OHM'S LAW AND ITS LIMITATIONS

Statement:

The ratio between the potential difference across two terminals of a conductor and the current through it remains constant, when the physical conditions of the conductor remain unchanged. Here the physical condition is the temperature.

Mathematically,

V/I = constant

Or V / I = R

V = I × R

This is the ohm's law. By knowing any two quantities, the third can be calculated.

P = VI = I²R= V²/R

Note 1: Take example of the resistor in the Fig. (1.3). Terminal A is connected to +ve and B connected to -ve. If you interchange the terminals, the same current will flow, but in the opposite direction. Such element which will have same property for either direction of current is called bidirectional or (bilateral) element i.e., resistor is example of bilateral element.

Note 2: Take example of a diode. When it is forward-biased, the current is higher, and when it is reversed-biased, the current is very low. So when the polarities of the applied voltage are changed, the direction of current changes and also its strength. That means the resistance offered by the diode is different depending upon the direction of flow of current. Such element whose properties change, when the direction of current changes is called unilateral or unidirectional element.

Note 3: Taking current along X axis and voltage along Y axis, a graph can be plotted to represent ohm's law. This will be a straight line passing through origin. The slope of this line represents the resistance.

Limitations of ohm's law: 1. Ohm's law cannot be applied to non-metallic conductors 2. It cannot be applied to non-linear devices such as Zener diode, voltage regulator (VR) tubes etc. 3. Ohm's law holds good only for constant temperature. If the temperature changes, this law cannot be applied.