Question

In one cycle of the engine, the net change in the entropy of both reservoirs (hot and cold together) is:

• A. zero
• B. greater than or equal to zero
• C. less than zero
• D. depends on the type of engine

Answer 1

The correct option is B greater than or equal to zero

Greater than or equal to zero. By the $2^{nd}$ Law, the entropy of the universe (engine + hot bath + cold bath) can never decrease. The entropy of the engine remains constant (see previous question) so the entropy of the reservoirs must increase. In the case of an ideal Carnot cycle, the process is reversible and the net entropy change is zero.