Di Electrics

Engaging Peers, Inspiring Careers!

FaaDoOEngineers.com

India’s No.1 website for:

• IIT-JEE/AIEEE preparation resources

• Coaching centre packages

• Engineering Major and Minor projects

• Seminar reports

• Paper presentations

• EBOOKS

• Resumes/CVs, and so much more…

-By FaaDoOEngineers.com

INTRODUCTION

Dielectrics represent a class of materials which,

although insulators, exhibit a number of effects

when placed in an electric field.

A good example is their effect on capacitors.

A capacitor has capacitance C0 when the space

between its two conductors is a vacuum, filling

this space with a dielectric increases the

capacitance to a new value Cm. The ratio

Cm/C0=r is known as the relative permittivity

of the dielectric.

LECTURE-1


When the atoms or molecules of a

dielectric are placed in an external electric

field, the nuclei are pushed with the field

resulting in an increased positive charge on

one side while the electron clouds are

pulled against it resulting in an increased

negative charge on the other side.


This process is known as polarization and a

dielectric material in such a state is said to be

polarized. There are two principal methods by

which a dielectric can be polarized: stretching

and rotation.

Stretching an atom or molecule results in an

induced dipole moment added to every atom or

molecule.


Polarizability

It can be defined as induced dipole

moment per unit electric field.

i.e. µ= αE

Where α is the proportionality

constant called Polarizability.


Polarization vector

The dipole moment per unit volume of

the dielectric material is called

polarization vector.

If µ is the average dipole moment per

molecule and N is the number of

molecules per unit volume then the

Polarization vector P=N µ


Electric flux density (D)

The flux density or electric displacement

D at a point in a material is given by D=єr

є0E.

Where E is the electric field strength, є0 is

the dielectric constant and єr is relative

permitivity of the material.

The 3 vectors D,E and P are related by the

equation D= є0 E+P

P= є0 (єr -1)E-By FaaDoOEngineers.com

Electric susceptibility(אe)

The polarization vector can be

written as P= אє0 א eE

Where the constant א e is the

electric susceptibility.

א e=(єr -1).


Dielectric constant (єr ) Lecture- 2

Dielectric constant (єr ) is the ratio

between the permitivity of the

medium and the permitivity of free

space.

i.e єr = є/ є0.

Єr has no units.

єr =C1/C. -By FaaDoOEngineers.com

Electric Polarization Lecture- 3

If a material contains polar molecules, they will generally be in random orientations when no electric field is applied.

An applied electric field will polarize the material by orienting the dipole momentsof polar molecules.

This decreases the effective electric fieldbetween the plates and will increase the capacitance of the parallel plate structure.


http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/dipole.html

The process of producing electric

dipoles which are oriented along

the field direction is called

Polarization in dielectrics.

P=NαE.


Polarization in dielectrics

Electronic polarization.

Ionic Polarization.

Orientational Polarization.


Electronic polarization

Electronic polarization represents the distortion of the electron distribution or motion about the nuclei in an electric field.

The positive charge in the nucleus and the center of the negative charges from the electron "cloud" will thus experience forces in different direction and will become separated. We have the idealized situation shown in the image below.


http://www.answers.com/topic/distortion


The separation distance d will have

a finite value because the

separating force of the external

field is exactly balanced by the

attractive force between the centers

of charge at the distance d.


Ionic Polarization

In the absence of electric field,

The polarization of a given volume,

however, is exactly zero because for every

dipole moment there is a neighboring one

with exactly the same magnitude, but

opposite sign.


The dipoles can not rotate; their

direction is fixed.


When field is applied

In an electric field, the ions feel forces in opposite directions. For a field acting as shown, the lattice distorts a little bit

The Na+ ions moved a bit to the right, the Cl–ions to the left.

The dipole moments between adjacent NaCl -pairs in field direction are now different and

there is a net dipole moment in a finite volume now.


The Na+ ions moved a bit to the

right, the Cl– ions to the left


The distance between the ions

increases by d


Orientational polarization.

The polarization arising due to the

allignment of already existing but

randomly oriented dipoles in the polar

substance is called the Orientational or

dipolar polarization.

It is denoted by αo.


It depends on temperature T

It decreases with T.

αo(T)=µm2/3KBT.


Orientational polarization

ELECTRIC FIELD IS NOT APPLIED.

MOLECULAR DIPOLES IN RAMDOM DIRECTIONS


Electric dipoles in Electric field

ELECTRIC FIELD IS APPLIED

E

MOLECULAR DIPOLES ORIENTED IN FIELD DIRECTION.


Internal fields in solids. Lecture- 4

The total electric field at the site of the atom within the dielectric is called the local field or the internal field.

It is also called the Lorentz field.

We have P=NαEi.

Ei=[ є0 (єr -1)E]/N α.

Ei=E+(ГP/ є0 ).


Claussius-Mosotti relation Lecture-5

It gives the relation between the microscopic

polarizability and the macroscopic dielectric

constant.

Clasius Mossotti equation is given by (єr -

1)/( єr +2)= N α/3 є0 .


Dielectrics in alternating fields

Lecture-6

According to Maxwell’s theory of wave

propagation V=√1/ єµ.

C= √1/ є0µ0.

Hence C/V=n=√ єr µr.

If the materials are non magnetic, µr=1


n=√ єr( or) єr =n2.

Then the Clasius Mossotti relation

becomes (n2-1)/( n2 +2)= N α/3 є0 .

This is known as Lorentz-Lorentz relation.


Lecture-7

In case of the alternating fields, we write E=E (t) and P=P (t) to indicate that both E and P vary with time t.

There will be some time lag between the response P (t) and the cause E (t).

If the applied field E (t) is oscillatory, then P (t) is also oscillatory.

If E (t) is given by E (t)=E0coswt, then P (t)=P0cos(wt+δ).


The ferroelectricity Lecture- 8

Some dielectrics become spontaneously

polarized when their temperature is equal to

critical temperature.

This phenomena is called the ferroelectricity.It is

not because of it is possessed by the ferrous

materials but because its origin and

characteristics are same as those of ferro

magnetism.

The critical temperature of the polar dielectrics

is called the ferroelectric curie temperature.-By FaaDoOEngineers.com

PIEZOELECTRICITY

When crystals are subjected to electric field,

their geometrical dimensions are altered. This

phenomenon is called electrostriction.

If crystals are subjected to mechanical stress,

electrical charges will be induced on the surfaces

of the crystals. This phenomenon is called

piezoelectricity.

When an electric stress (voltage) is applied, the

material becomes strained. This phenomenon is

known as inverse piezoelectric effect.-By FaaDoOEngineers.com

Engaging Peers, Inspiring Careers!

FaaDoOEngineers.com

India’s No.1 website for:

• IIT-JEE/AIEEE preparation resources

• Coaching centre packages

• Engineering Major and Minor projects

• Seminar reports

• Paper presentations

• EBOOKS

• Resumes/CVs, and so much more…


Di Electrics

Documents

Transcript of Di Electrics