Initially three variables pressure, volume and temperature represent the thermodynamic state of a system. In 1854, Rudolf Clausius while studying thermodynamic systems realized that to represent the thermodynamic state of a system, in addition to these three variables there was one quantity named “Entropy”. Like pressure, volume and temperature the this is also a function of the state of the system.
There are many reasons for which we need to introduce this concept . As we can represent the changes in the state of a system in different ways. For example the isothermal change (in which the temperature remains unchanged), the isobaric change (in which the pressure remains constant) and the isochoric change (in which the volume of the system remains constant).
It was concluded that the entropy remains constant in an adiabatic change in the system. The systems have a tendency to change from a more ordered state to a more disordered state. The perception of entropy expresses this in a better physical and mathematical form. The entropy of a substance is a real quantity, just like pressure, volume and temperature.
Despite being an very important quantity, we can’t represent this in some physical form. It, therefore, becomes very difficult to visualize it and to understand its exact nature. It is conveniently to understand the concept of Entropy by studying its effect, properties and other aspects.
We conclude that dimensions of entropy are the same as the ratio of heat (or energy) and temperature. Its unit is joule/Kelvin (J/K).
The change in the entropy (S) of a substance defined by the relation dS=dq/dt shows that the heat energy has the same dimensions as the product of S and absolute temperature T. In earth’s gravitational field the potential energy of a body is proportional to the product of its mass and the height above some zero level. A comparison indicates that if we regard height as corresponding to temperature, and then mass corresponds to entropy. Thus, entropy of a system is a quantity which bears to heat motion a similar relation as mass bears to linear motion.
Principle of Increase of Entropy
In a reversible process, the entropy remains unchanged while in an irreversible process, it increases. Since, in general, most of the processes are not perfectly reversible, therefore, there is always an increase in it. If the processes occur in succession the it goes on increasing and tends to a maximum value. This is the principle of increase of entropy. It states that the entropy of an isolated or self-contained system either increases or remains constant accordingly as the processes it undergoes are irreversible or reversible.
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