Blundell Thermal Physics · Problem 7.1
Molecular effusion
Physics — Statistical Mechanics · molecular-flux-surface-coverage
Problem
In a vacuum chamber designed for surface–science experiments, the pressure of residual gas is kept as low as possible so that surfaces can be kept clean. The coverage of a surface by a single monolayer requires about atoms per m. What pressure would be needed to deposit less than one monolayer per hour from residual gas? You may assume that if a molecule hits the surface, it sticks.Intuitive explanation
The key is connecting molecular collision rate to pressure using kinetic theory. Every gas molecule that hits the surface sticks, so you need to find what pressure produces fewer than collisions per m² per hour. The molecular flux hitting a surface depends on both the number density (from ideal gas law) and the average molecular speed—together they tell you how many molecules arrive per unit area per unit time.
Intuitive explanation
The key is connecting molecular collision rate to pressure using kinetic theory. Every gas molecule that hits the surface sticks, so you need to find what pressure produces fewer than collisions per m² per hour. The molecular flux hitting a surface depends on both the number density (from ideal gas law) and the average molecular speed—together they tell you how many molecules arrive per unit area per unit time.