ABSTRACT
In deriving the second law of thermodynamics, Sadi Carnot imagined a heat-engine in which a cylinder of perfect gas absorbed heat slowly and reversibly from a source, and gave it out at a sink at lower temperature. Since none was wasted, and each stage in the cycle was reversible, Carnot could show that such a machine was the most efficient imaginable; actual machines have to run fast, some waste is inevitable, and reversibility is only sometimes important. Carnot’s engine run in reverse would function as a refrigerator, taking heat from cool bodies and giving it out at a higher temperature: and his work showed that energy is required for this process. If one Carnot cycle were driving another in reverse, then no work would be done and there would be no net result; but the real world is not like that. Many processes, like those of the sausage factory, are irreversible; and in closed systems the steady increase in entropy is an indication that not everything can be reversed, and that time’s arrow has a direction.
