ined as: D = εξ. (19) where ξ electric field strength, ξ = V / l.. (20) In the presence of the dielectric, surface charge density can also be presented as D = εξ = ε 0 ξ + P (21) where polarization is defined as, P ε ( ε 1) ξ... (22) = 0 r the increase in charge density above that for a vacuum because of the presence of the dielectric. Figure 14-5: Schematic representation of capacitor and polarization phenomenon.
11 The polarization may also be thought of as the total dipole moment per unit volume of the dielectric material or as a polarization electric field within the dielectric that results from the mutual alignment of the many a atomic or molecular dipoles. Polarization of a dielectric material may be caused by at least one of four major types of polarization: electronic, ionic (atomic), orientation, and space charge (interfacial). Electronic polarization arises because the center of the electron cloud around a nucleus is displaced under an applied electric field. Ionic polarization occurs in ionic materials because an applied field acts to displace cations in the direction of the applied field while displacing anions in a direction opposite to the applied field. Orientation polarization can occur in materials that possess permanent electric dipole moments. These permanent dipoles tend to become aligned with the applied electric field, but entropy and thermal effects tend to counter this alignment, thus it is highly temperature dependent. Space charge polarization results from the accumulation of charge at structural interfaces in heterogeneous materials. Such polarization occurs when one of the phases has a much higher resistivity than the other. The total polarization subjected to an alternating electric field depends upon the ease with which the permanent or induced dipoles can reverse their alignment. The time required for dipole reversal is called the relaxation time, and its reciprocal is called the relexation frequency. As the frequency of the applied electric field approached the relaxation frequency, the polarization response increasingly lags behind the applied field. The reorientation of each type of dipole is opposed by internal friction, which leads to heating in the sample and power loss. The power loss depends on the degree to which the polarization lags behind the electric field. This is also called dielectric loss, and a low dielectric loss is desired at the frequency of utilization. At high enough frequencies, the dielectric will experience electrical breakdown initiated by the field-induced excitation of a number of electrons into the conduction band, and the insulator become a conductor. The magnitude of the electric field required to cause dielectric breakdown is called the dielectric strength or breakdown strength. Many ceramics and polymers are utilized as insulators and in capacitors. For example: glass, porcelain, stealite, mica. These have dielectric constants within the range of Typical applications: electrical insulation, switch bases, light receptacles. Dielectric constant for most polymers lies in the range of 2-5, less than that for ceramics, since the later exhibit greater dipole moments. Typical applications: insulation for wires, cables, motors, generators, some capacitors Ferro-electricity and Piezoelectricity Two peculiar phenomena related to electric dipoles are ferro-electricity and piezoelectricity. Ferro-electricity is defined as the spontaneous alignment of electric dipoles by their mutual interaction in the absence of an applied electric field. This arises from the fact that the local field increases in proportion to the polarization. Thus, ferro-electric materials must possess permanent dipoles. Ex.: BaTiO 3, Rochelle salt
12 (NaKC 4 H 4 O 6.4H 2 O), potassium dihydrogen phosphate (KH 2 PO 4 ), potassium niobate (KNbO 3 ). These materials have extremely high dielectric constants at relatively low applied field frequencies. Thus, capacitors made from ferro-electric materials are smaller than capacitors made of other dielectric materials. Piezo-electricity, or pressure electr