Friday, 24 June 2016



We were discussing thermodynamic state, path,process and cycles in our previous post. We have also discussed various basic concepts of thermodynamics such as quasi static process, path function and point function in thermodynamics and also thermodynamic equilibrium, intensive and extensive properties in our recent post.

Today we will see here the difference between macroscopic and microscopic view point in thermodynamics. 
There are two approaches or points of view for studying the behaviour of mater or for describing the thermodynamic systems and these approaches are as mentioned below
  1. Macroscopic approach
  2. Microscopic approach

Macroscopic approach in thermodynamics

In macroscopic approach a certain quantity of matter will be considered without considering the events occurring at molecular level. We can say in simple words that macroscopic approach will be based on the study of overall behaviour or gross behaviour of a number of molecules.
Study of behaviour of matter based on macroscopic approach will be considered as classical thermodynamics in the field of thermal engineering. Macroscopic effects i.e. average effects could be easily analysed by human sense or by measuring instruments such as pressure and temperature. Pressure and temperature could be measured easily with the help of measuring instruments or by human sense.

Let us consider one example for understanding the study of a thermodynamic system based on macroscopic point of view

Let we have one closed container filled with gas. There will be millions of molecules of gas in closed container. Now we want to know the pressure of the gas in closed container. Pressure of gas will be the average value of pressure exerted by the millions of individual molecules of gas in closed container. Here we have not shown the interest to determine the action of each individual molecule.

Microscopic approach in thermodynamics

Microscopic approach in thermodynamic system considers that matter will be composed with large numbers of molecules. Each molecule will have certain position, velocity and energy for a given instant of time and as a result of collision there will be frequent changing in the values of the above variables with respect to time. 

Behaviour of matter will be determined by considering the behaviour of each molecule and such an approach for studying the matter will be considered as microscopic approach in thermodynamics. Study of behaviour of matter based on microscopic approach will be considered as statistical thermodynamics in the field of thermal engineering.

Properties such as velocity, kinetic energy, impulse and momentum will be studied and determined on the basis of microscopic approach.

Even both approaches are seem to be different but when both approaches will be applied over a particular system then both approaches will provide the similar result.

We will discuss “Pure substance in thermodynamics” in our next post.

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Engineering thermodynamics by P.K. Nag
Engineering thermodynamics by R. K. Rajput

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