  We investigate an ideal gas in a time--dependent external trapping potential.
We use the Boltzmann equation with the relaxation time ansatz to explore the
time--dependent energy of an adiabatically isolated system. In particular we
are interested on the dissipation during a potential change along a given
protocol with finite velocity. The role of the relaxation less evolution as a
limiting case is studied: starting from an equilibrium distribution and for
small times the energy of the gas with relaxation is always smaller than that
of the relaxation--less. This means that relaxation processes show an
ambivalent behavior: on the one hand entropy production, but on the other hand
reduction of dissipation by driving back the system into equilibrium.
