A-Z Thermodynamics deals with the general principles and laws that govern the behaviour of matter and with the relationships between material properties. The origins of these laws and quantitative values for the properties are provided by statistical mechanics, which analyses the interaction of molecules and provides a detailed description of their behaviour. A physical explanation of entropy based upon the laws of probability is introduced. The equivalence of entropy and probability that results represents a return to the original viewpoint of Boltzmann, and it serves to demonstrate the fundamental unity of thermodynamics and statistical mechanics, a point that has become obscured over the years. The fact that entropy and probability are objective consequences of the mechanics of molecular motion provides a physical basis and a coherent conceptual framework for the two disciplines. The free energy and the other thermodynamic potentials of thermodynamics are shown simply to be the total entropy of a subsystem and reservoir; their minimisation at equilibrium is nothing but the maximum of the entropy mandated by the second law of thermodynamics and is manifest in the peaked probability distributions of statistical mechanics.About the AuthorProf. G.K. Bose has retired from a lifetime of teaching physics. A graduate of Harvard College, he began his teaching career at the secondary level, teaching chemistry and physics to students at Rohilla Academy, an independent school in Kanpur. In 1979, he returned to Harvard to study Particle physics with the late Professor A. David Antony. After receiving his Ph.D. there, he went on to teach introductory Physics (in both majors and nonmajors courses) and Physics at Tufts University where he became a tenured professor. In 1989 he returned again to Harvard where he taught Particle Physics and also participated in teaching the introductory course for majors. He has also published books on Atomic physics, Nuclear physics, Applied Physics, Molecular Physics, Wave and Optics, Heat and Thermodynamics and a widely-used text on Chemical kinetics and chemistry.