Which thermodynamics law introduces the concept of entropy as a measure of thermal energy per unit temperature that is unavailable for doing useful work?

💡 Solution & Explanation

The second law of thermodynamics establishes that heat cannot spontaneously flow from a cooler body to a warmer body and introduces entropy as a measure of unavailable thermal energy per unit temperature. Unlike the first law which focuses on energy conservation or the zeroth law covering thermal equilibrium, this principle dictates the natural direction of thermodynamic processes. A related concept for competitive exams is the Carnot theorem, which sets the maximum theoretical efficiency for any heat engine operating between two temperatures.