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 QH/TH
= QC/TC, as it is for the Carnot cycle or
any other reversible cycle operating between the two temperatures.