Entropy Changes in Heat Flow and Energy Conversion

The first row of the image at right shows heat flowing from hot to cold, which is a decrease in order, as hot and cold become less separated, and thus an increase in disorder/entropy. It also shows the reverse, which doesn't happen! The second row shows the conversion of work, which is ordered macroscopic energy, into heat, also an increase in disorder/entropy. Its reverse similarly doesn't happen! As the green (heat engine) and blue (refrigerator) arrows indicate, the third row shows things that combine increasing and decreasing disorder and thus need not violate the second law of thermodynamics. They are as efficient as possible if the disorder/entropy is neither increased (which is a squandering of ordered energy) nor decreased (which is impossible), and this is the case if Q_H/T_H = Q_C/T_C, as it is for the Carnot cycle or any other reversible cycle operating between the two temperatures.