Chapter 26 Entropy
Microscopically, all the laws of physics and the movements of particles are reversible in time, and yet, we find that everyday processes at the macroscopic scale occur in one direction of time only. For instance, we do not find that a cup that had fallen to the ground from a table rises by itself and lands back on the table. There appears to be a natural direction to time. The reason has to do with the irreversibility of thermodynamic processes, which we now understand in terms of a new property of thermodynamic system called entropy.
Rudolf Clausius first introduced the concept of entropy into physics in 1865, and recast the second law of thermodynamics in terms of entropy. Clausius found that the entropy of the entire universe, i.e., an isolated system, must be increasing with time. This says that the forward direction of time is the direction in which entropy of an isolated system would increase with time.
In 1880s Ludwig Boltzmann hypothesized a relation between the entropy of a system and the number of microstates of the system. Since, there are more arrangements of molecules in a disordered state of a system, a disordered state will have a higher entropy than an ordered state. Thus, increasing entropy of the universe corresponds to the increasing disorder in the universe.
In addition to the understanding the forward direction of time and the natural tendency towards disorder, entropy has also been helpful in analyzing the information content in computer science and other places. In this chapter, we will study the foundations of entropy and its uses in thermodynamics.
