  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.
