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Entropy

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Salt particles that have dissolved in water never spontaneously come back together in solution to reform solid particles. Moreover, a gas that has expanded in a vacuum remains dispersed and never spontaneously reassembles. The unidirectional nature of these phenomena is the result of a thermodynamic state function called entropy (S). Entropy is the measure of the extent to which the energy is dispersed throughout a system, or in other words, it is proportional to the degree of disorder of a...

Entropy

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Related Experiment Video

Updated: Dec 24, 2025

Non-equilibrium Microwave Plasma for Efficient High Temperature Chemistry

Non-equilibrium Microwave Plasma for Efficient High Temperature Chemistry

Published on: August 1, 2017

Energy dissipation and entropy in collisionless plasma.

Senbei Du¹, Gary P Zank², Xiaocan Li³

¹Department of Space Science, University of Alabama in Huntsville, Huntsville, Alabama 35899, USA.

Physical Review. E

|April 16, 2020

Summary

Collisionless plasma heating occurs through magnetic reconnection and turbulence, processes that convert magnetic energy into heat. This study reveals how fluid entropy changes drive this dissipation, highlighting the role of heat flux.

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Area of Science:

Plasma Physics
Astrophysics
Space Physics

Background:

Collisionless systems are typically considered dissipation-free, conserving entropy.
However, phenomena like magnetic reconnection and turbulence convert magnetic energy into heat.
Understanding this energy conversion is crucial for astrophysical and space plasma phenomena.

Purpose of the Study:

To investigate the energization and heating mechanisms in collisionless plasma.
To analyze the dissipation process using fluid entropy in isotropic and gyrotropic forms.
To derive and verify evolution equations for fluid entropy.

Main Methods:

Derivation of fluid entropy evolution equations.
Analysis of dissipation mechanisms in collisionless plasma.
Verification using collisionless particle-in-cell simulations of reconnecting current sheets.

Main Results:

Entropy evolution equations reveal mechanisms driving fluid entropy changes.
The study confirms previous findings on the pressure tensor's role.
Heat flux is identified as a significant factor in the dissipation process.

Conclusions:

Fluid entropy changes are key to understanding dissipation in collisionless plasmas.
Heat flux plays a critical role in converting magnetic energy into plasma heat.
Particle-in-cell simulations validate the derived entropy evolution equations.