Day 4: Dielectrics & Their Molecular Description

• Definition of dielectric material

• Effects of the presence of dielectric material in a capacitor

• The electric field between capacitor plates in the presence of a dielectric

• Molecular description of dielectrics

Dielectrics

• Any insulating material placed between the plates of a capacitor is called a dielectric

• Ex: paper, Mylar, mica, oil, ceramic

Effects of the Presence of a Dielectric

• The presence of a dielectric material increases the capacitance of the capacitor

• is the dielectric constant

• The energy density stored in the electric is now:

typermittiviwhered

ACC 000

2212

021 EE

Dielectric Constants of Various Materials

• The dielectric strength is

the maximum electric field

before breakdown

Effect of the Electric Field when a Dielectric is Inserted

• The electric field in the presence of a dielectric is reduced by a factor of:

• With no dielectric:

• With a dielectric:

• The potential difference drops to:

1

d

VE 00

00 E

Eord

V

d

VEE DD

0VV

Molecular Description of Dielectrics

• An air dielectric capacitor C0, charged to ΔV0, produces charges ±Q on each plate.

00 VCQ

Molecular Description of Dielectrics

• If the capacitor is isolated and a dielectric is inserted, the charge Q is unchanged

00 VdE

Molecular Description of Dielectrics

• When the dielectric is inserted, the electric field between the platescauses the molecules of theinsulator to become polarized

• Some of the electric field linesdo not pass thru the dielectric butrather end on the induced chargeson the surface of the dielectric

Molecular Description of Dielectrics

• The electric field is weaker than

in air by:

• ΔV across the capacitor is also

reduced by:

• Since capacitance must increase

1

1

VCQ

Molecular Description of Dielectrics

• The electric field within the

dielectric can be treated as the

vector sum of the electric field

due to the free charges on the

plates E0 and the electric field

due to the induced charges on the surface of

the dielectric Eind

1

100

0

00

EE

EE

EEEE

ind

indD

Consequences of Dielectrics• Remember that the electric field between the plates is

related to the surface charge density σ

• The induced charge on the dielectric is always less than the free charge on the capacitor plates

11

11

0

00

QQand

A

QwhereEthen

A

QwhereEIf

ind

ind

indind

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