I.D The Second Law

The historical development of thermodynamics follows the industrial revolution in the 19 century, and the advent of heat engines. It is interesting to see how such practical considerations as the efficiency of engines can lead to abstract ideas like entropy. An idealized heat engine works by taking in a certain amount of heat QH , from a heat source (for example a coal fire), converting a portion of it to work W , and dumping the remaining heat QC into a heat sink (e.g. atmosphere). The efficiency of the engine is calculated from W QH −QC η = = ≤ 1. (I.16) QH QH An idealized refrigerator is like an engine running backward, i.e. using work W to extract heat QC from a cold system, and dumping heat QH at a higher temperature. We can similarly define a figure of merit for the performance of a refrigerator as