Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-12 GTBL042-Callister-v2 August 13, 2007 18:22

500 • Chapter 12 / Electrical Properties

Table 12.5 Dielectric Constants and Strengths for Some Dielectric Materials

Dielectric Constant

Material 60 Hz 1 MHz Dielectric Strength(V/mil)a

Ceramics

Titanate ceramics — 15–10,000 50–300

Mica — 5.4–8.7 1000–2000

Steatite (MgO–SiO 2 ) — 5.5–7.5 200–350

Soda–lime glass 6.9 6.9 250

Porcelain 6.0 6.0 40–400

Fused silica 4.0 3.8 250

Polymers

Phenol-formaldehyde 5.3 4.8 300–400

Nylon 6,6 4.0 3.6 400

Polystyrene 2.6 2.6 500–700

Polyethylene 2.3 2.3 450–500

Polytetrafluoroethylene 2.1 2.1 400–500

aOne mil=0.001 in. These values of dielectric strength are average ones, the magnitude

being dependent on specimen thickness and geometry, as well as the rate of application and

duration of the applied electric field.

If a dielectric material is inserted into the region within the plates (Figure 12.28b),

then

C=

A

l

(12.26)

Capacitance (for

parallel-plate

capacitor, with

dielectric material)—

dependence on

permittivity of the

material, and plate

area and separation

distance

whereis the permittivity of this dielectric medium, which will be greater in magni-

tude than 0. The relative permittivityr, often called thedielectric constant,is equal

dielectric constant

to the ratio

r=



 0

(12.27)

Definition of

dielectric constant

which is greater than unity and represents the increase in charge-storing capacity by

insertion of the dielectric medium between the plates. The dielectric constant is one

material property that is of prime consideration for capacitor design. Thervalues

of a number of dielectric materials are contained in Table 12.5.

12.19 FIELD VECTORS AND POLARIZATION

Perhaps the best approach to an explanation of the phenomenon of capacitance is

with the aid of field vectors. To begin, for every electric dipole there is a separation

between a positive and a negative electric charge as demonstrated in Figure 12.29.

An electric dipole momentpis associated with each dipole as follows:

Electric dipole p=qd (12.28)

moment

whereqis the magnitude of each dipole charge anddis the distance of separation

between them. In reality, a dipole moment is a vector that is directed from the negative

to the positive charge, as indicated in Figure 12.29. In the presence of an electric field

e, which is also a vector quantity, a force (or torque) will come to bear on an electric