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The word "Thermodynamics" was first coined to denote the inter-relation between heat and mechanical energy i.e. thermodynamics is the study of the effects of work, heat and energy of a system. With the passage of time, however, the scope of thermodynamics increased into a vast domain, embracing a variety of phenomena in all branches of science.

So far, we have dealt with the mechanics of single particles, systems of particles, rigid bodies and fluids using Newton's laws. In thermodynamics, we have to broaden our views to deal with systems that are too complex to treat in terms of the motion of individual particles.

The dynamics of the gas molecules (systems) results in more disordered behavior since there is a large number of particles involved and in many different ways they can share the energy available to them.

Thermodynamics is only concerned with the large scale (macroscopic) observations. So we have to define new set of laws to govern the behaviour of gases, liquids or solids (macroscopic systems).

Thermodynamics is a macroscopic science. It deals with bulk systems and does not go into the molecular constitution of matter. Thermodynamic description involves relatively few macroscopic variables of the system, which are suggested by common sense and can be usually measured directly.

A microscopic description of a gas involves specifying coordinates and velocities of the huge number of molecules constituting the gas.

In mechanics, our concern is, the motion of particles or objects under the action of forces and torques. Thermodynamics is not concerned with the motion of the systems as a whole and it is concerned with the internal macroscopic state of the gases.
"The main theme of thermodynamics is temperature and its measurements".
Temperature is directly connected to the internal energy of the gas molecules.

The various scales of measurement of temperature are shown below. From this table we can easily know how different scales of measurement are differing.

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