  Quantum vacuum fluctuations of the electromagnetic field in empty space seem
not to produce observable effects over the motion of a charged test particle.
However, when a change in the background vacuum state is implemented, as for
instance when a conducting boundary is introduced, dispersions of the particle
velocity may occur. As a consequence, besides the existence of classical
effects due to the interaction between particle and boundary, there will be a
quantum contribution to the motion of the particle whose magnitude depends on
how fast the transition between the different vacuum states occurs. Here this
issue is revisited and a smooth transition with a controllable switching time
between the vacuum states of the system is implemented. Dispersions of the
particle velocity in both, zero and finite temperature regimes are examined.
More than just generalizing previous results for specific configurations, new
effects are unveiled. Particularly, it is shown that the well known vacuum
dominance reported to occur arbitrarily near the wall is a consequence of
assumed idealizations. The use of a controllable switching enable us to
conclude that thermal effects can be as important as or even stronger than
vacuum effects arbitrarily near the wall. Additionally, the residual effect
predicted to occur in the late time regime was here shown to be linked to the
duration of the transition. In this sense, such effect is understood to be a
sort of particle energy exchanging due to the vacuum state transition.
Furthermore, in certain arrangements a sort of cooling effect over the motion
of the particle can occur, i.e., the kinetic energy of the particle is lessen
by a certain amount due to subvacuum quantum fluctuations.
