  The force on a small sphere with a plasma model dielectric function and in
the presence of a perfectly reflecting plane is considered. The contribution of
both the vacuum modes of the quantized electromagnetic field and of plasmon
modes in the sphere are discussed. In the case that the plasmon modes are in
their ground state, quasi-oscillatory terms from the vacuum and plasmon parts
cancel one another, leading a monotonic attractive force. If the plasmon modes
are not in the ground state, the net force is quasi-oscillatory. In both cases,
the sphere behaves in the same way as does an atom in either its ground state
or an excited state.
