This article describes the use of an emissive probe to map the time-invariant plasma potential in a rf generated plasma in a GEC Reference Cell. The mapping has been done throughout a region expected to trap contamination particles for four different geometric configurations of materials on the powered electrode of the GEC Cell. For one case, there is a potential maximum which is a trap for negatively charged particles. For the other three cases, trapping can occur if a second force besides the electrostatic force is present. We postulate that this is the ion drag force. When particles are present in the system, the resulting particle clouds are observed using laser light scattering, and are superimposed of the maps. These clouds occur just where one would expect them to, given the electromagnetic force configuration exhibited by the maps plus the supposition of the existence of an ion drag force. It is shown that, at the location of a cloud, the ion drag force is of the right magnitude relative to the electrostatic force to permit trapping of particles.