Non-redundant and Natural Variables Definition of Heat Valid for Open Systems

Although an unambiguous definition of heat is available for closed systems, the question of how best to define heat in open systems is not yet settled. After introducing a set of physical requirements for the definition of heat, this article reviews the non-equivalent definitions of heat for open systems used by various authors, emphasizing which physical requirements are not met. A subsequent section deals with the main objective of this article and introduces a new definition of heat that avoids the difficulties of the existent definitions, providing (i) a complete distinction between open and closed systems, (ii) non-redundancy, (iii) natural variables for the thermodynamic potentials, and (iv) a sound, complete, and intuitive generalization of classical thermodynamic expressions.