Past exam of the physics course of the year 4 of the natural sciences course of the University of Cambridge / 2016 / Paper 1 / Section B / B2 / a

The Van der Waals equation replaces the freely available volume by $V-Nb$ to represent short-range repulsion and excluded volume. The attractive mean field reduces the pressure by $a{N}^2/V^2$: particles near the boundary are pulled back into the gas, lowering their momentum transfer to the walls. Both are effective corrections, rather than an exact microscopic interaction law.