What Is Heat? Can Heat Capacities Be Negative?

• Published in: Entropy (Basel, Switzerland) Vol. 25; no. 3; p. 530
• Main Author: Roduner, Emil
• Format: Journal Article
• Language: English
• Published: Switzerland MDPI AG 19.03.2023; MDPI
• Subjects: Analysis; Atomic properties; Atoms & subatomic particles; deficiencies of bulk thermodynamics; Electrons; Emission; Energy; Entropy; entropy of self-gravitating systems; Gases; Heat; Heat capacity; Heat exchange; Internal energy; Internal energy (Physics); Kinetic energy; Nanoclusters; negative heat capacities; Opinion; Phase transitions; Potential energy; Specific heat; Stars; Statistical thermodynamics; Thermodynamics; Translational motion; Virial theorem; virial theorem and heat; work; Germany; heat; deficiencies of bulk thermodynamics; negative heat capacities; entropy of self-gravitating systems; work; virial theorem and heat
• ISSN: 1099-4300; 1099-4300
• DOI: 10.3390/e25030530

In the absence of work, the exchange of heat of a sample of matter corresponds to the change of its internal energy, given by the kinetic energy of random translational motion of all its constituent atoms or molecules relative to the center of mass of the sample, plus the excitation of quantum states, such as vibration and rotation, and the energy of electrons in excess to their ground state. If the sample of matter is equilibrated it is described by Boltzmann's statistical thermodynamics and characterized by a temperature . Monotonic motion such as that of the stars of an expanding universe is work against gravity and represents the exchange of kinetic and potential energy, as described by the virial theorem, but not an exchange of heat. Heat and work are two distinct properties of thermodynamic systems. Temperature is defined for the radiative cosmic background and for individual stars, but for the ensemble of moving stars neither temperature, nor pressure, nor heat capacities are properly defined, and the application of thermodynamics is, therefore, not advised. For equilibrated atomic nanoclusters, in contrast, one may talk about negative heat capacities when kinetic energy is transformed into potential energy of expanding bonds.