Second Law Of Thermodynamics

Grounded in NCERT Class 11, Chapter 12 (Thermodynamics), the Second Law of Thermodynamics places a fundamental restriction that goes beyond mere energy conservation expressed by the first law. The Kelvin-Planck statement asserts that no process is possible whose sole result is the absorption of heat from a single reservoir and its complete conversion into work. A cyclic engine must always reject some heat to a colder reservoir to return to its starting state, so a single-reservoir engine achieving 100 percent conversion is forbidden in principle. The option that the first law permits it is misleading: the first law only requires an energy balance and does not by itself forbid full conversion, but the second law explicitly does. The option about a very large reservoir is wrong because reservoir size does not lift the second-law prohibition; the obstacle is the impossibility of zero heat rejection in a cycle, regardless of how big the reservoir is. The option about perfect insulation is wrong because a truly adiabatic engine cannot draw heat from a reservoir to run a continuous cycle at all. A consistency check seals the conclusion: such a device would have efficiency exactly 1, yet any real cyclic engine has $\eta <1$, confirming that the student's claim violates well-established physics.