电磁学ch6.ppt

Chapter 6 Dielectrics: I Electric Polarization P Bound Charges Gauss’ Law, Electric Displacemant D In contrst to conductors,dielectrics have no free charges that can move through the material under the influence of an electric field. All electrons are bounded. Positive and negative charges canmove in opposite direction with a small displacement of atomic dimensions. A dielectric in which this charge displacement has taken place is said to be polarized, and its molecules are said to possess induced dipole moments.(See Fig6-1)These dipoles produce their own field, which adds to that of the external charge.The dipole field and the external applied field can be comparable in magnitude. The applied electric field also tends to orient molecular dipole moments, so that the dipoles are aligned with the external field.However, thermal motions of molecules tend to destroy this alignmant.An equilibrium polarization is thus established, in which there is a net alignment, on the average. The electric field E and D * *