This means there is a layer of negative charges adjacent to the positive plate of the capacitor and a layer of positive charges adjacent to the

negative plate.

For this reason the entire dielectric material can be said to be polarized (to have positive and negative regions). This polarization decreases the

net charge in close proximity to the surface of each capacitor plate. The net charge of each plate is now the charge on the plate surface minus

the charge on the adjacent face of the dielectric. Less effective charge on each plate surface means the overall field between the plates is

weaker.

26.8 - Dielectrics

Dielectric: An insulating substance placed

between the plates of a capacitor to increase its

capacitance.

Dielectric constant (ț): Measures reduction in

electric field caused by a dielectric. It equals the

ratio of the field in a vacuum to the field in the

dielectric.

Dielectrics are insulating materials used in capacitors to increase their capacitance.

Effective dielectrics make possible the manufacture of small, high-farad capacitors.

The materials discussed in this section are linear dielectrics. The dielectric field of a

linear dielectric is linearly proportional to the strength of the external electric field.

The dielectric constant, represented by ț(Greek letter kappa), is a property of a

material. It equals the ratio of two electric fields: the electric field inside a capacitor with

a vacuum separating the plates and the field strength for the same capacitor charge

with the dielectric present. This is stated in Equation 1. The greater the dielectric field

within a dielectric, the more it diminishes the field caused by the plates, and the greater

the value of the dielectric constant.

Dielectrics increase the capacitance of capacitors. As is stated in Equation 2, the

capacitance with the dielectric present equals the capacitance without it times the

dielectric constant.

The table in Equation 3 lists some dielectric constants. As you can see, the dielectric

constants for vacuum and air are quite close. Strontium titanate, a substance used in

commercial capacitors, has a far greater dielectric constant, especially at low

temperatures, than a vacuum or air.

Dielectrics also can be classified by their dielectric strength. The dielectric strength

characterizes the field strength at which the dielectric becomes a conductor and charge

will flow through it. This is not a desirable effect, so capacitors are often labeled with

their maximum safe field strength (in V/m). Like many stress properties, the dielectric

strength of a material can be difficult to determine precisely, so the stated dielectric

strength should be an approximate, conservative value. The dielectric strengths of

selected materials are also shown in the table.

Lightning is a very visible example of the breakdown of a dielectric í air í between

oppositely charged objects, such as two clouds or a cloud and the nearby surface of the

Earth. When the electric field between them becomes strong enough, the separating

atmosphere becomes a conductor and current flows through it as a lightning bolt.

Dielectric

Insulator that increases capacitance

Dielectrics and electric fields

Dielectric diminishes electric field

Definition of dielectric constant

ț = Evacuum/Edielectric

ț = dielectric constant

E = field in vacuum or dielectric

(^482) Copyright 2000-2007 Kinetic Books Co. Chapter 26