. cl as s - ph ] 1 3 Ju l 2 01 0 Is the electrostatic force between a point charge and a neutral metallic object always attractive ?

We give an example of a geometry in which the electrostatic force between a point charge and a neutral metallic object is repulsive. The example consists of a point charge centered above a thin metallic hemisphere, positioned concave up. We show that this geometry has a repulsive regime using both a simple analytical argument and an exact calculation for an analogous twodimensional geometry. Analogues of this geometry-induced repulsion can appear in many other contexts, including Casimir systems.