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\sect{The Van der Waals equation}

${N}$ spheres with radius ${R}$ are contained in box volume $V$.
The temperature is $T$. Find the pressure using a mean-field  
one particle approximation. Extend the result if there is an extra 
potential $u(r)$ between the particles. Show that you get 
the Van der Waals equation. Define the term "excluded volume" 
in this context, and identify the~$a$ and~$b$ coefficients. 

