《Electromagnetism, principles and application》教学课件（PPT讲稿）Chapter 6 Dielectrics：I

6.1 Electric Polarization p The electric polarization P is the dipole moment per unit volume at a given point where p- the average electric dipole moment per molecule N-the number of molecules per unit volume

6.2 The Bound Charge Densities Pb and Ob The displacement of charges within the dielectric gives rise to net volume and surface charge densities Pb=-V. P, where n is the normal to the surface, pointing outward ------------- Figure 6-1 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 o 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

The Surface Bound Charge Densities ob(see Fig 6-1) The positive charges move along e, and the negative charges move in opposite direction The separation btwn them is s, forming a parallelepiped with cross section da The volume of this parallelepiped is d T=s·da the net charge accumulated on da is dQ= NQs. da= Np da=P da where Q is the point charge of a molecule

Thus the bound surface charge density do d where n is the unit vector normal to the surface Conclusion: the bound surface charge density is the normal component of the polarization p at the surface

The bound volume charge density pb As just shown, the net charge flowing out of a volume t across an element da is p. da Thus the total net charge flowing out of the surface s bounding t is P. da Thus the net charge that remains within T isQ Q=-/s P da By the divergence theorem -/ P da=-(v p)dr Th Us pb V·P