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Ohm's Law | Equation Formula and Limitation of Ohm's Law

The most basic quantities of electricity are voltage, current and resistance. Ohms law shows a simple relationship between these three quantities. This law is one of the most basic laws of electricity. This law helps to calculate the power, efficiency and impedance of any element of electrical circuit.georg ohm
Ohms law first appeared in the book written by George Simon Ohm (German) in 1827.

Statement of Ohm's Law

Whenever we apply a potential difference i.e. voltage across a resistor of a closed circuit, current starts flowing through it.

The statement of Ohms law says that

This current (I) is directly proportional to the voltage (V) applied, provided temperature and all other factors remain constant. Mathematically, Where, R is constant of proportionality. This equation presents the statement of Ohm law. Here, we measure current in Ampere (or amps), voltage in unit of volt. The constant of proportionality R is the property of the conductor, we know it as resistance and measure it in ohm (Ω). Theoretically, the resistance has no dependence on the voltage applied, or on the flow of current. The value of R changes only if the conditions (like temperature, diameter and length etc.) of the resistor are changed by any means.

History of Ohm's Law

In the month of May 1827, George Simon Ohm published a book "Die Galvanische Kette, Mathematisch Bearbeitet". "Die Galvanische Kette, Mathematisch Bearbeitet" means "The Galvanic Circuit Investigated Mathematically". He presented the relationship between voltage (V), current (I), and resistance (Ω) based on his experimental data, in this book. George Simon Ohm had defined the fundamental interrelationship between current, voltage and resistance of a circuit which was later named Ohms law. Because of this law and his excellence in the field of science and academics, he got the Copley Medal award in 1841. In 1872 the unit of electrical resistance was named 'OHM" in his honor.

Ohm's Law Physics

We an understand the physics behind Ohms law well if we examine it from molecular level of a material. A conductor contains free electrons. These free electrons randomly move in the conductor. When, we apply a voltage, across the conductor, the free electrons keep being accelerated towards higher potential end due to electrostatic force of the applied voltage. This means they acquire some kinetic energy as they move towards the + Ve end of the conductor. However, before they get very far they collide with an atom or ion, lose some of their kinetic energy and may bounce back. Again due to presence of static electric field the free electrons again accelerate. This keeps happening. That means, even after application of external electric field, there will be still random motion in the free electrons of the conductor. But each free electron drift towards +Ve end with its inherent random motion. As a result the free electrons tend to "drift" towards the + Ve end, bouncing around from atom to atom on the way. This is how materials resist current. If we apply more voltage across the conductor, the more free electrons will move with more acceleration which causes more drift velocity of the electrons. The drift velocity of the electrons is proportional to the applied static electric field. Hence the current (I) we get is also proportional to the applied voltage (V). ohms law

Applications of Ohm's Law

The applications of ohms law are that it helps us in determining either voltage, current or resistance of a linear circuit when the other two quantities are known to us. Apart from that, it makes power calculation a lot simpler, like when we know the value of the resistance for a particular circuit, we need not know both the current and the voltage to calculate the power dissipation since P = VI. Rather we can use Ohms Law. To replace either the voltage or current in the above expression to produce the result  These are the applications of Ohms law as we can see from the results, that the rate of energy loss varies with the square of the voltage or current. When we double the voltage applied to a circuit, obeying Ohms law, the rate at which energy is supplied (or power) gets four times bigger. This phenomena occurs because increasing the voltage also makes the current rise by the same amount as it has been explained above.

Limitation of Ohm's Law

The limitations of Ohms law are explained as follows:
  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. Ohms law is also not applicable for non – linear elements.
Non-linear elements are those which do not give current through it, is not exactly proportional to the voltage applied, 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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