Physics and Astronomy Colloquium – Professor Paul Cassak; Department of Physics and Astronomy, West Virginia … – The University of Iowa

Professor Paul Cassak; Department of Physics and Astronomy, West Virginia University

One of the most important processes across subdisciplines of physics is the conversion of energy. The first law of thermodynamics has been in place for over 170 years and often is the go to solution to explore energy conversion. Applying the first law is much easier to do when the system is in local thermodynamic equilibrium (LTE), but many modern physics systems are not in LTE. Examples abound, including from astronomy and cosmology, nuclear physics, and quantum entanglement. An especially prominent example is in plasma and space sciences, where high temperatures, low densities, and large constituent mass differences often reduce the effect of collisions that normally would drive them towards LTE. Plasmas accessibility to direct measurement in space and the laboratory makes them excellent settings for studying non-LTE processes. Often, the evolution of systems out of LTE is described using only a few fluid variables called moments, namely the density, momentum, and energy. However, for a system not in LTE, an infinite number of moments can be important, and the evolution of the other moments is typically not considered. In this talk, we discuss a recent result that combines all the other moments in a single variable (Cassak et al., Phys. Rev. Lett., 130, 085201, 2023), namely the so-called relative entropy (Grad, J. Soc. Indust. Appl. Math., 13, 259, 1965). We derive an equation for its time evolution and argue its form complements or extends the first law of thermodynamics for systems not in LTE. We introduce a new quantity we call the higher order non-equilibrium terms (HORNET) which quantifies the rate system approaches or moves away from LTE (Barbhuiya et al., Phys. Rev. E, 109, 015205, 2024) with dimensions of power density, which is useful because it can be directly compared to standard power densities. We demonstrate the results in numerical simulations of various plasma processes. The results could have important applications in a wide array of systems that are out of LTE, both within and outside of physics.

Bio:Dr. Cassak received a Ph.D. in theoretical and computational plasma physics from the University of Maryland in 2006. He was a postdoc at the University of Delaware in 2007 and 2008. His research focuses on magnetic reconnection and its applications using analytical techniques, large scale numerical simulations, and observational data as appropriate.

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Physics and Astronomy Colloquium - Professor Paul Cassak; Department of Physics and Astronomy, West Virginia ... - The University of Iowa