Di Electrics
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Transcript of Di Electrics
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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
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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.
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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.
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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.
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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
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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
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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).
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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
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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.
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The process of producing electric
dipoles which are oriented along
the field direction is called
Polarization in dielectrics.
P=NαE.
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Polarization in dielectrics
Electronic polarization.
Ionic Polarization.
Orientational Polarization.
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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.
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Electronic polarization
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Electronic polarization
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.
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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.
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The dipoles can not rotate; their
direction is fixed.
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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.
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The Na+ ions moved a bit to the
right, the Cl– ions to the left
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The distance between the ions
increases by d
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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.
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It depends on temperature T
It decreases with T.
αo(T)=µm2/3KBT.
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Orientational polarization
ELECTRIC FIELD IS NOT APPLIED.
MOLECULAR DIPOLES IN RAMDOM DIRECTIONS
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Electric dipoles in Electric field
ELECTRIC FIELD IS APPLIED
E
MOLECULAR DIPOLES ORIENTED IN FIELD DIRECTION.
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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 ).
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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 .
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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
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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.
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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).
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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
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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
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Engaging Peers, Inspiring Careers!
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• IIT-JEE/AIEEE preparation resources
• Coaching centre packages
• Engineering Major and Minor projects
• Seminar reports
• Paper presentations
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-By FaaDoOEngineers.com