Question

Carnot engine is a reversible engine. It can be proved from the:

• A. Zeroth Law of Thermodynamics
• B. First Law of Thermodynamics
• C. Second Law of Thermodynamics
• D. Third Law of Thermodynamics

Hint:

The Carnot engine’s reversibility (where changes can be reversed back to the starting point), combined with its operation between two constant-temperature heat reservoirs, leads to maximum possible efficiency, meaning it’s the ideal, theoretical scenario, and real-world engines always perform worse. The engine operates in four fully reversible steps: isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression.

Answer 1

The Correct Option is C

The Carnot engine, a theoretical thermodynamic cycle, achieves maximum possible efficiency through its reversibility. Its reversibility is proven by the Second Law of Thermodynamics, which governs entropy changes and the direction of thermodynamic processes. According to the Second Law, an isolated system's total entropy remains constant (∆S = 0) in reversible processes, but increases (∆S>0) in irreversible ones.
Comprising reversible steps, the Carnot cycle can operate in reverse, resulting in a zero net entropy change for the entire cycle. The proof demonstrates that no other engine operating between the same two temperatures can surpass the Carnot engine's efficiency without violating the Second Law. This relies on the principle that reversible processes leave no net trace and that a more efficient engine would contradict the Second Law.