  The electrostatic force on a spherical particle near a planar surface is
calculated for the cases of a uniform electric field applied in either normal
or tangential direction to the surface. The particle and suspending media are
assumed to be weakly conducting, so that that the leaky dielectric model
applies. The Laplace equation for the electric potential is solved in bipolar
coordinate system and the potential is obtained in terms of a series expansion
of Legendre polynomials. The force on the particle is calculated using the
Maxwell tensor. We find that in the case of normal electric field, which
corresponds to a particle near an electrode, the force is always attractive but
at a given separation it varies nontrivially with particle-suspending medium
conductivity ratio; the force on a particle that is more conducting than the
suspending medium is much larger compared to the force on a particle less
conducing than the suspending medium. In the case of tangential electric field,
which corresponds to a particle near an insulating boundary, the force is
always repulsive.
