ENGG 2230 Lecture : FluidMechWhite5eCh01.pdf
9 Apr 2013
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1. 1 a gas at 20 c may be rarefied if it contains less than 1012 molecules per mm3. Solution: the mass of one molecule of air may be computed as m. Then the density of air containing 1012 molecules per mm3 is, in si units, Finally, from the perfect gas law, eq. (1. 13), at 20 c = 293 k, we obtain the pressure: 1. 2 the earth"s atmosphere can be modeled as a uniform layer of air of thickness 20 km and average density 0. 6 kg/m3 (see table a-6). Use these values to estimate the total mass and total number of molecules of air in the entire atmosphere of the earth. Solution: let re be the earth"s radius 6377 km. Then the total mass of air in the atmosphere is m t dvol. 2 e (0. 6 kg/m )4 (6. 377e6 m) (20e3 m) (air vol) avg avg. Dividing by the mass of one molecule 4. 8e 23 g (see prob.
