Which statement about entropy is compatible with why perpetual-motion machines are impossible?

• A. Entropy can be fully reversed by adding energy.
• B. Entropy causes energy to be unavailable for work, preventing perpetual motion.
• C. Entropy has no effect on the feasibility of perpetual motion.
• D. Entropy is always conserved in all processes.

Answer: (B)

Entropy measures how energy becomes less available to do work as systems become more disordered. In real processes, energy lost as heat spreads into the surroundings, increasing the overall disorder. A machine that runs forever would need to return all that dispersed energy and restore the original order without leaving any net increase in entropy. The second law says this cannot happen: the total entropy of the universe cannot decrease in real processes. So entropy is what makes most of the energy in a cycle unusable for continued work, preventing perpetual motion.

Thus, the statement that energy becomes unavailable for work because entropy increases best explains why perpetual-motion machines aren’t possible. The ideas that entropy can be fully reversed by adding energy, that entropy has no effect on feasibility, or that entropy is always conserved misrepresent how entropy works in real systems.