A model has been developed for interpreting the observed dependence of the activation energy for surface electromigration damage on an applied potential in polycrystalline Cu films with clean surfaces. This potential gives rise to an electric field approximately perpendicular to the surface of the film. The model is based an the presence of ridges at grain boundary-surface intersections. It is shown that high fields can exist at ridge peaks and that such high fields can affect adatom mobility in ways similar to those observed in field ion microscopy. Several implications of this model are presented.