ABSTRACT
The second law of thermodynamics and the concepts of reversibility and irreversibility indicate that certain processes are possible in one direction but not in the reverse. For example, a cyclic machine that consumes work from the environment and delivers heat to a single reservoir can be built. On the other hand a machine cannot be built to do the opposite, namely, to receive heat from a single reservoir while producing positive work. The directionality in nature is obvious, yet we still have to develop a quantitative measure to evaluate this effect.
