  The friction in plasmas consisting of two species with different temperatures
is discussed together with the consequent energy transfer. It is shown that the
friction between the two species has no effect on the ion acoustic mode in a
quasi-neutral plasma. Using the Poisson equation instead of the
quasi-neutrality reveals the possibility for an instability driven by the
collisional energy transfer. However, the different starting temperatures of
the two species imply an evolving equilibrium. It is shown that the relaxation
time of the equilibrium electron-ion plasma is, in fact, always shorter than
the growth rate time, and the instability can thus never effectively take
place. The results obtained here should contribute to the definite
clarification of some contradictory results obtained in the past.
