Inside a flat layer of thickness D=10 cm there is a uniform charge distribution with density p=1C/m. In the layer there is also a spherical cavity, free of charge, of radius R= D/2, centered on the point in the center of the layer, so that the sphere is tangent to the external surfaces of the layer (A is a point of tangency). a) Determine the expression of electric field along the ray connecting O with A. b) Determine the difference of potential between point O and point A. Suggestion: analyze the field produced by the layer and the sphere separately then apply the principle superposition. of