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

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

502 • Chapter 12 / Electrical Properties

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Net negative

charge, Q'

at surface

Region of

no net

charge

Net positive

charge,

+Q' = PA

at surface

Area of plate, A

Vacuum

(a)

(b)

(c)

Q 0

+Q 0

V

V P

Q 0 + Q'

Q 0  Q'

 l^



















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Figure 12.31 Schematic

representations of (a) the

charge stored on capacitor

plates for a vacuum, (b) the

dipole arrangement in an

unpolarized dielectric, and

(c) the increased charge storing

capacity resulting from the

polarization of a dielectric

material. (Adapted from A. G.

Guy,Essentials of Materials

Science,McGraw-Hill Book

Company, New York, 1976.)

wherePis thepolarization, or the increase in charge density above that for a vacuum

because of the presence of the dielectric; or, from Figure 12.31c,P=Q′/A, whereA

is the area of each plate. The units ofPare the same as forD(C/m^2 ).

The polarizationPmay 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 atomic or molecular dipoles with

the externally applied fielde. For many dielectric materials,Pis proportional toe

through the relationship

P= 0 (r−1)e (12.32)

Polarization of a

dielectric medium—

dependence on

dielectric constant

and electric field

intensity

in which caseris independent of the magnitude of the electric field.

Table 12.6 lists the several dielectric parameters along with their units.