• id: JTHERM-02-N09
• title: Average Distance Between Molecular Collisions
• tags: mean-free-path, collision, molecular-diameter

The mean free path $\lambda$ is the average distance a molecule travels between successive collisions: $\lambda = 1/(\sqrt{2}\pi d^2 n) = k_BT/(\sqrt{2}\pi d^2 P)$, where $d$ is the molecular diameter and $n = N/V$ is the number density. The $\sqrt{2}$ factor accounts for the relative motion of molecules. At STP, $\lambda \approx 60$ nm for nitrogen. $\lambda$ increases with temperature (molecules spread out) and decreases with pressure (more crowding). At very low pressures (high vacuum), $\lambda$ can exceed the container dimensions — this is the Knudsen regime where molecules collide with walls more often than with each other.