Replace particle description by average. 1023 molecules in a molar volume is too many to handle numerically. So divide full system into a large number of "small" volumes dV such that:
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Macroscopic Properties: Temperature, velocity, pressure are essentially constant in volume
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In principle, use statistical physics (or Particle Dynamics averaged as "Transport Equations") to describe volume dV in terms of macroscopic (ensemble) properties for volume
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Volume size = dV must be small enough so macroscopic properties are indeed constant; dV must be large enough so can average over molecular motion to define properties
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As typical molecule is 10-8 cm in linear dimension, these constraints are not hard
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Breaks down sometimes e.g. leading edges at shuttle reentry etc. Then you augment continuum approach (computational fluid dynamics) with explicit particle method
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