麻省理工的电磁场教材-6.pdf

6 POLARIZATION 6.0 INTRODUCTION The previous chapters postulated surface charge densities that appear and disap- pear as required by the boundary conditions obeyed by surfaces of conductors. Thus, the idea that the distribution of the charge density may be linked to the field it induces is not new. Thus far, however, no consideration has been given in any detail to the physical laws which determine the occurrence and behavior of charge densities in matter. To set the stage for this and the next chapter, consider two possible pictures that could be used to explain why an object distorts an initially uniform electric field. In Fig. 6.0.1a, the sphere is composed of a metallic conductor, and therefore composed of atoms having electrons that are free to move from one atomic site to another. Suppose, to begin with, that there are equal numbers of positive sites and negative electrons. In the absence of an applied field and on a scale that is large compared to the distance between atoms (that is, on a macroscopic scale), there is therefore no charge density at any point within the material. When this object is placed in an initially uniform electric field, the electrons are subject to forces that tend to make them concentrate on the south pole of the sphere. This requires only that the electrons migrate downward slightly (on the average, less than an interatomic distance). Because the interior of the sphere must be field free in the final equilibrium (steady) state, the charge density remains zero at each point within the volume of the material. However, to preserve a zero net charge, the positive atomic sites on the north