remainig part of chapter 17

Conduction in Ionic Materials
In ionic materials, the band gap is too large for thermal electron promotion. Cation vacancies allow ionic motion in the direction of an applied electric field, this is referred to as ionic conduction. High temperatures produce more vacancies and higher ionic conductivity.
At low temperatures, electrical conduction in insulators is usually along the surface, due to the deposition of moisture that contains impurity ions.

Electrical Properties of Polymers
Polymers are usually good insulators but can be made to conduct by doping. Teflon is an exceptionally good insulator.
Dielectric Behavior
A dielectric is an electrical insulator that can be made to exhibit an electric dipole structure (displace the negative and positive charge so that their center of gravity is different).

Capacitance
When two parallel plates of area A, separated by a small distance l, are charged by +Q, –Q, an electric field develops between the plates
E = D/ee0
where D = Q/A. e0 is called the vacuum permittivity and e the relative permittivity, or dielectric constant (e = 1 for vacuum). In terms of the voltage between the plates, V = E l,
V = Dl/ee0 = Q l/Aee0 = Q / C
The constant C= Aee0/l is called the capacitance of the plates.

Field Vectors and Polarization
The dipole moment of a pair of positive and negative charges (+q and –q) separated at a distance d is p = qd. If an electric field is applied, the dipole tends to align so that the positive charge points in the field direction. Dipoles between the plates of a capacitor will produce an electric field that opposes the applied field. For a given applied voltage V, there will be an increase in the charge in the plates by an amount Q' so that the total charge becomes Q = Q' + Q0, where Q0 is the charge of a vacuum capacitor with the same V. With Q' = PA, the charge density becomes D = D0 E + P, where the polarization P = e0 (e–1) E .

Types of Polarization
Three types of polarization can be caused by an electric field:
Electronic polarization: the electrons in atoms are displaced relative to the nucleus.
Ionic polarization: cations and anions in an ionic crystal are displaced with respect to each other.
Orientation polarization: permanent dipoles (like H2O) are aligned.

Frequency Dependence of the Dielectric Constant
Electrons have much smaller mass than ions, so they respond more rapidly to a changing electric field. For electric field that oscillates at very high frequencies (such as light) only electronic polarization can occur. At smaller frequencies, the relative displacement of positive and negative ions can occur. Orientation of permanent dipoles, which require the rotation of a molecule can occur only if the oscillation is relatively slow (MHz range or slower). The time needed by the specific polarization to occur is called the relaxation time.

Dielectric Strength
Very high electric fields (>108 V/m) can free electrons from atoms, and accelerate them to such high energies that they can, in turn, free other electrons, in an avalanche process (or electrical discharge). This is called dielectric breakdown, and the field necessary to start the is called the dielectric strength or breakdown strength.