ENGG 2230 Lecture : Ch 1 Solutions.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: p. P1. 2 table a. 6 lists the density of the standard atmosphere as a function of altitude. Use these values to estimate, crudely, say, within a factor of 2, the number of molecules of air in the entire atmosphere of the earth. Solution: make a plot of density versus altitude z in the atmosphere, from table a. 6: This writer"s approximation: the curve is approximately an exponential, o exp(-b z), with b approximately equal to 0. 00011 per meter. Integrate this over the entire atmosphere, with the radius of the earth equal to 6377 km:
