The force attracting a charged, insulative particle to a nearby surface is often calculated by assuming that the total charge is concentrated at the center of the particle. While this is true when the particle is far away, there is evidence that a different model should be used when the particle is very close, or in contact. This attractive force is calculated in terms of the bispherical coordinate system, which provides an analytical solution valid when the particle approaches and contacts the surface. Comparison of this model with other experimental and analytical work is carried out to determine the validity of the various models, and to provide approximate formulas useful in engineering work. This work indicates that electrostatic adhesive force may be larger than normally expected when the surface charging is larger than traditional assumption, as reported in several experiments.