ABSTRACT
This chapter explores some consequences of combining the first law with the concept of entropy. The discussion of the entropy axiom so far seems rather mathematically oriented, without any obvious physical significance. Actually, the entropy axiom has very deep physical significance, perhaps one of the deepest in physics! To bring this out, first demonstrate an equivalence between the entropy axiom, and universality of Carnot cycles. What the second law can determine is the directionality of heat flow, something that the first law is incapable of doing. As a result, the second law discriminates between the initial and the final states of a general transformation of a system. Closely woven into this fabric is the notion of irreversibility. Furthermore, entropy as a state function provides a quantitative description of this directionality. As the name indicates, a perpetuum mobile is a machine that keeps running forever.
