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==Thermodynamics and statistical mechanics==
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In [[thermodynamics]] and [[statistical mechanics]] contexts, the term phase space has two meanings: for one, it is used in the same sense as in classical mechanics. If a thermodynamic system consists of ''N'' particles, then a point in the 6''N''-dimensional phase space describes the dynamic state of every particle in that system, as each particle is associated with three-position variables and three momentum variables. In this sense, as long as the particles are [[Gibbs paradox|distinguishable]], a point in phase space is said to be a [[Microstate (statistical mechanics)|microstate]] of the system. (For [[identical particles|indistinguishable particles]] a microstate will consist of a set of ''N''<nowiki>!</nowiki> points, corresponding to all possible exchanges of the ''N'' particles.) ''N'' is typically on the order of the [[Avogadro
The phase space can also refer to the space that is parameterized by the ''macroscopic'' states of the system, such as pressure, temperature, etc. For instance, one may view the pressure-volume diagram or entropy-temperature diagrams as describing part of this phase space. A point in this phase space is correspondingly called a macrostate. There may easily be more than one microstate with the same macrostate. For example, for a fixed temperature, the system could have many dynamic configurations at the microscopic level. When used in this sense, a phase is a region of phase space where the system in question is in, for example, the [[liquid]] phase, or [[solid]] phase, etc.
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