Thermodynamics and Energy


Further Insight Relative to Cavity Radiation III: Gedanken Experiments, Irreversibility, and Kirchhoff's Law

Authors: Pierre-Marie Robitaille

Recently, gedanken experiments have been proposed in order to examine the validity of Kirchhoff’s Law of Thermal Emission (P.-M. Robitaille, Further Insight Relative to Cavity Radiation: A Thought Experiment Refuting Kirchhoff’s Law, Prog. Phys., 2014, v. 10, no. 1, 38–40; P.-M. Robitaille, Further Insight Relative to Cavity Radiation II: Gedanken Experiments and Kirchhoff’s Law, Prog. Phys., 2014, v. 10, no. 2, 116–120). In the second of these works, real materials (i.e. graphite a nd silver) were utilized in order to construct two separate cavities at the same temperature which are then placed in thermal contact with one another. It was hypothesized that the graphite cavity initially contained blackbody radiation and that the silver cavity was devoid of radiation. In the case of the silver cavity, all of the energy of the system was assigned to the phonons in its walls. When the cavities were brought together and a small hole introduced between the cavities, it was hypothesized that thermal contact between the cavity walls would enable the transformation of phonon energy into photon energy, eventually resulting in filling the silver cavity with black radiation. Energy contained within the wall of the silver cavity was believed to be reversibly trapped. However, in allowing energy to flow reversibly out of the walls of the silver cavity in this context, it has been assumed that the silver conduction bands could be neglected and that only phonon energy need be considered. However, the reflectivity attributed to the silver cavity should be considered uniquely as a result of energy associated with the formation of its conduction bands. Such formation must be considered irreversible. It will be demonstrated that under these conditions Kirchhoff’s law, once again, does not hold. The lack of thermal radiation within the silver cavity does not lead to a violation of the second law of thermodynamics.

Comments: 4 pages. First published in: Progress in Physics, 2016, v. 12, no. 1, 85-88.

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[v1] 2016-02-01 08:30:24

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