Question

Consider a 30-cm sphere traveling through the atmosphere and take the diameter as the characteristic length. Find the altitude (in km) above which free molecular flow prevails. Assume a collision diameter of 3.7 Angstroms and a temperature of 280 K. Use the following relation to relate atmospheric number density to altitude.

Answer 1

Explanation:

Knudsen Number is the ratio of the molecules' mean free path to the system's longitude scale.  

When a gas is stored in a cubic container then the length of the box can be chosen as the characteristic length and mean free path as the average distance traveled by a molecule prior to collision. The mean free path depends on molecular diameter and number of molecules present in the system. Larger diameter and greater number of molecules would the the molecules' mean free path

.