This concept establishes entropy as a legitimate thermodynamic state variable by contrasting it with an illegitimate candidate ("heat content"), showing that a quantity is path-independent only if its net change around any closed cycle is zero. Using the Carnot cycle, it defines entropy change as heat transferred divided by the absolute temperature at which the transfer occurs (dS = Q/T), and proves via the ideal gas law and isothermal work integrals that the sum of Q/T terms around the cycle vanishes. This belongs to classical thermodynamics, extending the state-variable concept (already established for internal energy U) to a new quantity governing heat exchange.
This concept establishes entropy as a legitimate thermodynamic state variable by contrasting it with an illegitimate candidate ("heat content"), showing that a quantity is path-independent only if it…