E 粉+ Figure 6l Under the action of an electric field E. which is the resultant of an external field and of the field of the dipoles within the dielectric, positive and negative charges in the molecules are separated by an average distance s In the process a net charge dQ= NOs da crosses the surface da, N being the number of molecules per unit volume and g the positive charge in a molecule. The vector da is perpendicular to the shaded surface. The circles indicate the centers of charge for the positive and for the negative charges in one molecule