Conductors and dielectrics a general view

2 DIELECTRIC PROPERTIES OF SUBSTANCES 4.2.1 Conductors and dielectrics a general view 

From the point of view of electric properties, all substances can be divided into two main classes—conducting and nonconducting an electric current. Metals, their alloys and a small number of chemical compounds with metal character of interatomic interactions relate to the class of conductors. The second class includes other substances and represents the overwhelming majority. Conductivity is defined by the presence of free charge carriers in a substance their absence determines dielectric properties. So, dielectrics are substances in which there are no free charges capable of covering long distances in the substance (in comparison with molecular sizes). 

Depending on their molecular structure, all dielectrics can, in turn, be divided into two large groups—polar and nonpolar. In polar dielectrics, molecules themselves represent the electric dipoles with the electric moment p it appears due to displacements of electric charges from positions of their equilibrium in free atoms as a result of chemical bonding (e.g., HjO, HCl and NHj). The molecular dipoles of polar dielectrics participate in thermal motion this can be translational motion (in gases and liquids), oscillation 

Molecules of nonpolar dielectrics do not possess a dielectric moment. This means that the positions of the positive and negative charge centers in the molecules coincide. Examples of nonpolar molecules are H2, CCI4, CgHg, CH4, etc. In this case, the macroscopic dielectric sample does not possess a dipole macroscopic moment either. 

For the quantitative description of dielectrics, the notion of the polarization (degree) or the polarization vector Sd is introduced numerically, it is equal to the electric dipole moment of a unit volume. Accordingly 

---