The polarization properties of dielectrics are described by the polarization vector:
, C/m2
where is the electric susceptibility, and is the permittivity of vacuum. The relation between the three electric field vectors, E, D and P, is
, C/m2.
It shows that the total flux in a dielectric medium is due to the flux of the free charges (flux density D), which is partially cancelled by the flux due to the bound charge (flux density P).
In a linear isotropic medium, it is common to use the relative permittivity , such that . Obviously, , and
.
The P vector is also useful when we are interested in the amount of bound surface charge deposited at the interface between a polarized dielectric and air. Its normal (to the interface) component is directly equal to the bound surface charge:
, C/m2
where is the unit normal pointing outward from the dielectric. The above relation is very similar to the relation between the normal component of the flux density D and the free surface charge deposited on the surface of a conductor.
You can observe the change of the magnitude of the polarization in a dielectric slab due to the change of its relative permittivity . The change in the bound surface charge density at the upper dielectric-to-air interface is also shown. Note that the slab is in the field of two flat electrodes: the bottom (black) one is grounded, while the top (red) one is at V. The red color of the top electrode is not related to the P vector, it simply emphasizes the nonzero potential.
Note also that the polarization is zero in the air surrounding the slab (blue background). |