WHAT IS ENTROPY- Basic Of Thermodynamics

WHAT IS ENTROPY- Basic Of Thermodynamics

Introduction to Entropy : 

Entropy is useful property and serves as valuable tool in second law analysis of engineering devices. But entropy is such a complex concept, we cannot give an adequate answer to the question—“What is Entropy”.

Not being able to describe entropy fully, however does not mean it is less useful. We could not define energy, but it did not interfere with our understanding of energy transformation and energy conservation.

Entropy can be viewed as a measure of molecular disorder or molecular randomness. As system becomes more disordered, the position of molecules becomes less predictable and the entropy increases. Thus entropy is lowest in solid phase and highest in gaseous phase.


From microscopic point of view, the entropy of system increase whenever the thermal randomness of system increases. Thus entropy can be defined as measure of thermal randomness or molecular disorder, which increases anytime when system goes under process.

As we know molecules of substance in solid phase continuously oscillate about their position. These oscillations decreases as temperature of substance decreases, and motion of molecules stops at absolute zero. This state represents ultimate molecular order(or minimum energy). Therefore entropy of a pure crystalline substance at absolute zero temperature is zero since there is no disorder about the state of molecules at that instant. This statement is known as third law of thermodynamics.

The third law of thermodynamics provides reference point for the determination of entropy. The entropy determined relative to this point is called absolute entropy.  Engineers usually concerned with the changes in entropy than absolute entropy.

Entropy is non-conserved property, and there is no such thing as the conservation of entropy principle. Entropy is conserved only during idealized reversible process and increases during all actual processes.

Causes of the Increase in Entropy of the System
In actual practice the reversible isentropic process never really occurs, it is only an ideal process. In actual practice whenever there is change in the state of the system the entropy of the system increases. Here are the various causes of the increase in entropy of the closed system are:

1) Due to external interaction: In closed system the mass of the system remains constant but it can exchange the heat with surroundings. Any change in the heat content of the system leads to disturbance in the system, which tends to increase the entropy of the system.

2) Internal changes in the system: Due to internal changes in the movements of the molecules of the system there is further disturbance inside the system. This causes irreversiblities inside the system and an increase in its entropy.

Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

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