Elementary thermodynamics of trapped particles
  I develop simple thermodynamic relations for a collection of noninteracting classical particles confined in a harmonic trap. The volume of such a trap is not a good thermodynamic variable, so conventional expressions of the first law of thermodynamics and the ideal gas law must be modified. I use the frequency of oscillations about the minimum of the trap as an external parameter characterizing the confinement, and derive elementary relations between particle number, temperature, energy, oscillation frequency, and a generalized pressure, that are analogous to conventional thermodynamic relations for an ideal gas in a rigid volume. I also discuss heat capacities for trapped particles. 
Martin Ligare