Question

A molecule in a gas moves equal distances l between collisions with equal probability in any direction. After a total of n such displacements, what is the mean square displacement r2 of the molecule from its starting point

Answer 1

Consider N spherical molecules with radius r in a volumeV.Suppose only one molecule is moving.When it collides with anothermolecule,the distance between centers is 2r.The volume of the cylnder is 4πr2vdt.There are N/V molecules.