The same argument that we have used to discuss the efficiency of a heat engine can be
used to discuss the efficiency of a refrigerator, a device for transferring energy as heat
from a cold object (the contents of the refrigerator) to a warm sink (typically, the
room in which the refrigerator stands). The less work we have to do to bring this
transfer about, the more efficient is the refrigerator.
When an energy |qc | migrates from a cool source at a temperature Tc into a warmer
sink at a temperature Th, the change in entropy is