G m 3 eg with the molecular weight of air around 29

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Unformatted text preview: les is n (e.g., m 3): (e.g., • With the molecular weight of air around 29 g/mole (~75% N2 @ 28; ~25% O2 @ 32), 293 °K: 28; 32), – the number of collisions in time t is notZ = 4 nr2vt – m = 29 1.67 10-27 kg – <v> = 461 m/s – note same ballpark as speed of sound (345 m/s) Winter 2008 • Correcting for relative molecular speeds, and expressing as collisions per unit time, we have: 5 Winter 2008 6 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Mean Free Path, cont. Relevance of Mean Free Path • Mean free path is related to thermal conduction of air • Now that we have the collision frequency, Z, we can get the average distance between collisions as: – if the mean free path is shorter than distance from hot to cold surface, there is a collisional (conductive) heat path between the two = v /Z • So that • Once the mean free path is comparable to the size of the vessel, the paths are ballistic – collisions cease to be important • Though not related in a 1:1 way, one also cares about transition from bulk behavior to molecular be...
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