The emergence and evolution of the Second Law of Thermodynamics

Abstract

The results of measurements on thermal properties of common substances provided a backdrop to the successful development of heat engines in the eighteenth and nineteenth centuries. Early in the nineteenth century, Carnot’s theory of an idealised heat engine provided the stimulus to the ongoing studies, both practical and theoretical, of the properties of materials and of mechanical and thermal processes. By mid-century, the First and Second Laws of Thermodynamics had been established, in various equivalent forms. These clarified the role of energy and its transformations and led to the introduction of a new thermodynamic function, entropy, to complement energy and, from its thermodynamic properties, to clarify the limitations imposed by the Second Law. The understanding of thermal properties of matter from the laws and techniques of classical thermodynamics was enhanced by the introduction of and alignment with statistical thermodynamics. This provided an understanding of properties of assemblies of large numbers of molecules by incorporating the rules of probability theory, leading to a statistical interpretation of entropy. A merger with the well-known laws and processes of chemistry led to substantial advances in chemical thermodynamics, permitting an insightful subsequent reassessment of thermodynamics as a whole.