Editing Thermodynamic system (section)

==Isolated system==
{{main|Isolated system}}
An isolated system is more restrictive than a closed system as it does not interact with its surroundings in any way. Mass and energy remains constant within the system, and no energy or mass transfer takes place across the boundary. As time passes in an isolated system, internal differences in the system tend to even out and pressures and temperatures tend to equalize, as do density differences. A system in which all equalizing processes have gone practically to completion is in a state of [[thermodynamic equilibrium]].

Truly isolated physical systems do not exist in reality (except perhaps for the universe as a whole), because, for example, there is always gravity between a system with mass and masses elsewhere.<ref>{{cite book |title=Thermodynamics of Spontaneous and Non-Spontaneous Processes |author1=I.M.Kolesnikov |author2=V.A.Vinokurov |author3=S.I.Kolesnikov |page=136 |url=https://books.google.com/books?id=2RzE2pCfijYC&pg=PA136 |isbn=978-1-56072-904-4 |year=2001 |publisher=Nova science Publishers}}</ref><ref>{{cite web |title=A System and Its Surroundings |website=ChemWiki |publisher=University of California - Davis |url=http://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/A_System_And_Its_Surroundings#Isolated_System |access-date=9 May 2012}}</ref><ref>{{cite web |title=Hyperphysics | publisher=The Department of Physics and Astronomy of Georgia State University |url=http://hyperphysics.phy-astr.gsu.edu/hbase/conser.html#isosys |access-date=9 May 2012}}</ref><ref>{{cite web |title=Open, Closed and Isolated Systems in Physical Chemistry |website=Foundations of Quantum Mechanics and Physical Chemistry |author=Bryan Sanctuary |publisher=McGill University (Montreal) |url=http://quantummechanics.mchmultimedia.com/2011/physical-chemistry/open-closed-and-isolated-systems-in-chemistry/ |access-date=9 May 2012 |archive-date=30 May 2012 |archive-url=https://web.archive.org/web/20120530190200/http://quantummechanics.mchmultimedia.com/2011/physical-chemistry/open-closed-and-isolated-systems-in-chemistry/ |url-status=dead }}</ref><ref>{{cite book |title=Material and Energy Balances for Engineers and Environmentalists |page=7 |publisher=Imperial College Press |url=http://www.icpress.co.uk/etextbook/p631/p631_chap01.pdf |access-date=9 May 2012 |url-status=dead |archive-url=https://web.archive.org/web/20090815150041/http://www.icpress.co.uk/etextbook/p631/p631_chap01.pdf |archive-date=15 August 2009 }}</ref> However, real systems may behave nearly as an isolated system for finite (possibly very long) times. The concept of an isolated system can serve as a useful [[scientific modeling|model]] approximating many real-world situations. It is an acceptable [[Idealization (science philosophy)|idealization]] used in constructing [[mathematical model]]s of certain natural [[phenomenon|phenomena]].

In the attempt to justify the postulate of [[entropy]] increase in the [[second law of thermodynamics]], Boltzmann's [[H-theorem]] used [[Ludwig Boltzmann#Boltzmann equation|equations]], which assumed that a system (for example, a [[gas]]) was isolated. That is all the mechanical [[Degrees of freedom (physics and chemistry)#Thermodynamic Degrees of Freedom for Gases|degrees of freedom]] could be specified, treating the walls simply as [[mirror]] [[boundary condition]]s. This inevitably led to [[Loschmidt's paradox]]. However, if the [[stochastic]] behavior of the [[molecule]]s in actual walls is considered, along with the [[random]]izing effect of the ambient, background [[thermal radiation]], Boltzmann's assumption of [[molecular chaos]] can be justified.

The second law of thermodynamics for isolated systems states that the entropy of an isolated system not in equilibrium tends to increase over time, approaching maximum value at equilibrium. Overall, in an isolated system, the internal energy is constant and the entropy can never decrease. A ''closed'' system's entropy can decrease e.g. when heat is extracted from the system.

Isolated systems are not equivalent to closed systems. Closed systems cannot exchange matter with the surroundings, but can exchange energy. Isolated systems can exchange neither matter nor energy with their surroundings, and as such are only theoretical and do not exist in reality (except, possibly, the entire universe).

'Closed system' is often used in thermodynamics discussions when 'isolated system' would be correct – i.e. there is an assumption that energy does not enter or leave the system.