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The particle number, N, is the number of constituent particles in a thermodynamical system. The particle number is a fundamental parameter in thermodynamics and it is conjugate to the chemical potential.

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. Our definition of elementary particle is thus temperature dependent.