In the earlier chapters, we have discussed mechanics and waves. Mechanics deals with point particles and rigid bodies. However, these idealizations do not allow energy to be stored in them in any form - rotational or vibrational motions. As the constituents of rigid body remain fixed in length w.r.t. each other, energy cannot be exchanged with them. Physics of systems where energy may be stored, converted from one form to another, requires us to understand the laws of thermodynamics. One of the most important concepts is that of entropy. The mystery of this concept has been removed in a series of papers by Leff [84, 85, 87, 88, 103]. We take advantage of these recent essays and follow them rather closely.

There are four laws on which thermodynamics is based. Each of the laws, zeroth law to the second law have given a new concept - temperature, energy, and entropy. The third law has established a limitation on thermodynamic functions. We shall explain the ideas and concepts in some detail. The applications of these ideas to real life are everywhere around us. The changes occurring all around from one state to another are consistently described within these laws. To relate a few common instances [100], we have noticed that the valve on a bicycle pump gets hot when we are pumping up a tyre. Why? When we pump and compress the gas, there is no heat transfer from outside. The process is adiabatic. The internal energy of the air increases, leading to an increase in temperature. This hot air heats up the valve. As a second example, again drawn from our experience, is the warm (even hot in certain regions in Northern India) dry wind that blows down from mountains into plains. It is called differently in different parts of the world - Loo (India), Kachchan (Sri Lanka), Santa Ana (Southern California), Chinook (North America), Foehn (Switzerland), Berg Wind (South Africa). As winds come down from the mountains, they move into the regions of greater atmospheric pressure. Hence, the moving air is adiabatically compressed and heated. As the descent is fast, there is no time for any significant exchange with local air. So this wind is warm and dry.