Ohm’s Law | Statement, Applications and Limitation of Ohm’s Law

What is Ohm’s Law?

Ohm’s law deals with the relationship between current, voltage and ideal resistance. Ohm’s Law is a formula used to calculate the relationship between voltage, current and resistance in an electrical circuit. More formally, Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points.

Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship as “V=IR”, or “I=V/R”. The basic equation for Ohm’s law is shown below:

This relationship was introduced by German physicist George Simon Ohm. That is why the law is well known as Ohm’s law.

Georg Simon Ohm

Ohm’s law first appeared in the book written by Georg Simon Ohm (German) in 1827.

Ideal Resistance

Before going to actual topic let us know what is an ideal resistance. Ideal resistance is that resistance which possesses the property of pure resistance and also it doesn’t change its resistivity due to change in voltage or current imposed on it.

Statement of Ohm’s law

At constant temperature, the current through an ideal resistor is directly proportional to the voltage applied across the resistor.

The constant of proportionality is written as R and this is the resistance value of the resistor.

Ohm’s Law Formula

The relationship between current, voltage and resistance can be written in three different ways. When a known voltage is applied across a known resistance the current through the resistance can be determined by the relationship

When a known current flows through a known resistance the voltage appeared across the resistance can be determined by the relationship

When a known voltage is applied across a resistance and the current through the resistance is also known then the value of the reactance can be determined by the relationship:
Ohm's Law Triangle

Ohm’s law is a crucial part of a beginners learning of electrical engineering as it is used constantly throughout an electrical engineers career. For a more indepth discussion of Ohm’s law and other fundamental electrical concepts, we would recommend looking at some of the best books for electrical engineers.

Determining Power from Ohm’s Law

When current flows through a resistance there will be a dissipation of power from the resistance. This power can easily be determined by using Ohm’s law. As we know power is the product of current and voltage. If the current I ampere flows through a resistance and voltage V volts is the voltage across the resistance then power
Using Ohm’s law we can write
From that relationship we can determine power of a resistance if either voltage and resistance or current and resistance are known to us.

From the same relation for a given dissipated power, we can determine the unknown resistance value of either current or voltage is known.
If any two of power, current, voltage and resistance are known then with the help of Ohm’s law we can find out other two variables.

Why the temperature is kept constant in Ohm’s law

The main criteria for Ohm’s law is to keep the resistance constant because proportionality constant in the relationship is resistance R. But we know that the variation of temperature affects the value of resistance so to keep the resistance constant during experiments of Ohm’s law the temperature is considered constant.

Applications of Ohm’s Law

There are thousands of applications of this law in our daily life. We will show only a few of them in this article.

  • Conventional Domestic Fan Regulator is one very common device where the current through the fan gets regulated by controlling the resistance of the regulator circuit.
  • In voltage divider circuit this law is used to divide source voltage across the output resistance.
  • In electronic circuits, there are many purposes where intentional voltage drop is required to supply specific voltage across different electronic elements. This is done by applying Ohm’s law.
  • In mainly dc ammeter and other dc measuring instruments shunt is used to divert current. Here also Ohm’s law is used.

The list will continue as much as you think.

Limitation of Ohm’s Law

The limitations of Ohm’s law are outlined below:

  1. This law cannot be applied to unilateral networks.
    A unilateral network has unilateral elements like diode, transistors, etc., which do not have same voltage current relation for both directions of current.
  2. Ohm’s law is also not applicable for non – linear elements.
    Non-linear elements are those which do not have current exactly proportional to the applied voltage, that means the resistance value of those elements changes for different values of voltage and current. Examples of non – linear elements are thyristor, electric arc, etc.
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