According to the Second Law, you cannot have a cycle with 100% efficiency.

According to the Second Law, you cannot have a cycle with 100% efficiency.

• A. 100% efficiency
• B. 50% efficiency
• C. 0% efficiency
• D. 200% efficiency

Answer: (A)

The key idea is that you cannot convert all of the heat you take in into useful work in a cyclic process. The Second Law sets a fundamental limit: some heat must be rejected to the cold reservoir, so the efficiency can never reach 100%. In fact, the Carnot bound shows the maximum possible efficiency is η = 1 − Tc/Th, which is always less than 1 for any finite temperatures Th and Tc. So 100% efficiency is impossible. Choosing the option that states 100% efficiency reflects that impossibility. The other numbers don’t capture the core limit: 200% would violate energy conservation, and 50% or 0% can be realized in various cycles under the right conditions (0% is a degenerate case of no work produced).

The key idea is that you cannot convert all of the heat you take in into useful work in a cyclic process. The Second Law sets a fundamental limit: some heat must be rejected to the cold reservoir, so the efficiency can never reach 100%. In fact, the Carnot bound shows the maximum possible efficiency is η = 1 − Tc/Th, which is always less than 1 for any finite temperatures Th and Tc. So 100% efficiency is impossible.

Choosing the option that states 100% efficiency reflects that impossibility. The other numbers don’t capture the core limit: 200% would violate energy conservation, and 50% or 0% can be realized in various cycles under the right conditions (0% is a degenerate case of no work produced).