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The particle number (or number of particles) of a thermodynamic system, popconventionally indicated with the letter ''N'', is the number of constituent particles in that system. The particle number is a fundamental parameter in thermodynamics which is conjugate to the chemical potential. Unlike most physical quantities, particle number is a dimensionless quantity. It is an extensive parameter, as it is directly proportional to the size of the system under consideration, and thus meaningful only for closed systems. A constituent particle is one that cannot be broken into smaller pieces at the scale of energy ''k·T'' involved in the process (where ''k'' is the Boltzmann constant and ''T'' is the temperature). For example, for a thermodynamic system consisting of a piston containing water vapour, the particle number is the number of water molecules in the system. The meaning of constituent particle, and thereby of particle number, is thus temperature-dependent. ==Determining the particle number== The concept of particle number has a main role in theoretical considerations. In situations where the actual particle number of a given thermodynamical system needs to be determined, mainly in chemistry, it is not practically possible to measure it directly by counting the particles. If the material is homogeneous and has a known amount of substance ''n'' expressed in moles, the particle number ''N'' can be found by the relation :''N'' = ''nNA'', where ''NA'' is the Avogadro constant.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「particle number」の詳細全文を読む スポンサード リンク
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