电磁场与电磁波——电场.ppt

Under the action of extra electric field force, action centre of non-polar molecules positive electrical charge negetive electrical charge do not coincide together. And the equivalent electric dipole moments of polar molecules change their directions to make the vector sum of equivalent electric dipole moments be zero. This phenomenon is called polarization of dielectric. The result of polarization is to produce polarized charges inside the dielectric on the surface of the dielectric. These polarization charge produce electric field in the dielectric which is different in direction with the extra electric field. Therefore the electric field inside the dielectric is different with the electric field outside the dielectric. Polarization of dielectric (a) In normal status,action centre of positive electrical charge negetive electrical charge coincide together (b) The equivalent electric dipole moments of polar molecules Extra electric field Under the action of extra electric field force, in order to describe the polarized state of dielectric, intensity of polarization is introduced. In polarized dielctric: ΔV, Sum of electric moment inΔV: Polarization Intensity Vector: electric dipole moment in unit volume :average electric dipole moment inΔV N: the number of molecules in unit volume Polarization Intensity Vector: In linear, homogeneous and isotropic dielectric: Electric field of a point outside the polarized dielectric In the dielectric: dV′， Electic dipole moment in dV′: Electric potential produced by electic dipole moment at point P: Define: density of surface bound charge density of volume bound charge I.e. In free air, . Dielectric flux density is dielectric： Electric field in dielectric medium is produce by