The “mean free path” is the average distance a gas molecule travels between collisions. If a molecule has a diameter “d”, then the effective cross section for a collision is “π d2“. This is used mostly with the Kinetic Theory of Gases, and is a good estimation of how particles move in a gas.
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Also useful when describing the motions of the molecules in a gas (again using the Kinetic Molecular Theory) is the average velocity of the molecules. Since the gas molecules are constantly moving about, it would be ridiculously hard to individually account for each molecule’s individual speed and direction at any given time. Instead, we make use of statistics to get an overall average estimate of the motion of the particles using the “root mean square” technique applied to gases.


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