\sect{Pressure via the virial theorem}

%Baruch's  A07, phrasing modified by DC

A gas of $N$ particles is confined in a box 
of volume $V$ at temprature of $T$.
The two-body interaction between the particles is ${u(r) \propto r^{-\gamma}}$.
%
Write the virial theoprem and deduce that the mean kinetic energy is

\[\ K \ \ = \ \ \frac{1} {\gamma +2}\left (3PV + \gamma E\right)\]

where ${E=K+U}$ is the total energy. 
What happens for ${\gamma =-2}$ ? 

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