# Classical fluid

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{{Short description|Concept in physics}}
'''Classical fluids'''<ref>R. Balescu, ''Equilibrium and Nonequilibrium Statistical Mechanics'', (John Wiley, 1975)</ref> are systems of particles which retain a definite volume, and are at sufficiently high temperatures (compared to their [Fermi energy](/source/Fermi_Energy)) that [quantum](/source/quantum) effects can be neglected. A system of [hard spheres](/source/hard_spheres), interacting only by hard collisions (e.g., billiards, marbles), is a model classical fluid. Such a system is well described by the [Percus–Yevik equation](/source/Percus%E2%80%93Yevick_approximation). Common liquids, e.g., liquid air, gasoline etc., are essentially mixtures of classical fluids. Electrolytes, molten salts, salts dissolved in water, are classical charged fluids. A classical fluid when cooled undergoes a freezing transition. On heating it undergoes an evaporation transition and becomes a classical gas that obeys [Boltzmann statistics](/source/Maxwell%E2%80%93Boltzmann_statistics).
 
A system of charged classical particles moving in a uniform positive neutralizing background is known as a one-component plasma (OCP). This is well described by the hyper-netted chain equation (see [classical-map hypernetted-chain method](/source/classical-map_hypernetted-chain_method) or CHNC). An essentially very accurate way of determining the properties of classical fluids is provided by the method of [molecular dynamics](/source/molecular_dynamics). An [electron gas confined in a metal](/source/Free_electron_model) is ''not'' a classical fluid, whereas a very high-temperature plasma of electrons could behave as a classical fluid. Such non-classical [Fermi systems](/source/Fermi_gas), i.e., quantum fluids, can be studied using [quantum Monte Carlo](/source/quantum_Monte_Carlo) methods, Feynman [path integral formulation](/source/path_integral_formulation), and approximately via [CHNC](/source/Classical-map_hypernetted-chain_method) integral-equation methods.

==See also==
*[Bose–Einstein condensate](/source/Bose%E2%80%93Einstein_condensate)
*[Fermi liquid](/source/Fermi_liquid)
*[Many-body theory](/source/Many-body_theory)
*[Quantum fluid](/source/Quantum_fluid)
==References==
{{reflist}}

Category:Concepts in physics

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