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ENGINEERING PHYSICS II
DIELECTRICS
UNIT IV
DIELECTRICS
4.1 INTRODUCTION
Solids which have an energy gap of 3 eV or more are termed as insulators. In these materials, it is
almost not possible to excite the electrons from the valence band to the conduction band by an applied
field. Generally, dielectrics are also called as insulators, thereby poor conductors of electricity.
However, they allow some of the electrons at abnormally high temperatures, causing a small flow of
current.
Dielectrics are non-metallic materials of high specific resistance ρ, negative temperature coefficient
of resistance (-α) and large insulation resistance. Insulation resistance will be affected by moisture,
temperature, applied field and age of dielectrics.
4.2 FUNDAMENTAL DEFINITIONS AND PROPERTIES
1. Electric polarization
The process of producing electric dipoles inside the dielectrics by an external electric field is called
polarization in dielectrics.
2. Polarization vector (𝑷)
If the strength of the electric field E is increased the strength of the induced dipole also increases.
The induced dipole moment is proportional to the intensity of the electric filed.
µ = αE
Where, α is the constant of proportionality, called the Polarizability.
If µ is the average dipole moment per molecule and N is the number of molecules per unit volume,
the polarization vector 𝑃 is defined as dipole moment per unit volume of the dielectric material.
P=N μ
3. Electric Displacement Vector (D)
Electric Displacement Vector or electric induction (D) is a quantity which is used for analyzing
electrostatic fields in the presence of dielectrics, which is given by
D=
Q
4πr2
…(1)
We know electric field intensity
E=
Q
4π∈r2
...(2)
From (1) and (2) we get
D =∈ E
D =∈o ∈r E
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… (3)
ENGINEERING PHYSICS II
DIELECTRICS
D =∈o (1 + χe )
Since ∈𝑟 = 1 + 𝜒𝑒
Where, 𝜒𝑒 is the electrical susceptibility.
4. Relation between P and E
𝐷 =∈𝑜 𝐸 +∈𝑜 𝜒𝑒 𝐸
We know
Since P=∈𝑜 𝜒𝑒 𝐸, we have
𝐷 =∈𝑜 𝐸 + 𝑃
... (4)
Equating (3) and (4)
∈𝑜 ∈𝑟 𝐸 =∈𝑜 𝐸 + 𝑃
𝑃
𝐸
=∈𝑜 (∈𝑟 − 1)
…(5)
5. Electrical Susceptibility (𝝌𝒆 )
The polarization vector (P) is proportional to the applied electric field (E), for field strengths that
are not too large. So we can write
𝑃∝𝐸
𝑃 =∈𝑜 𝜒𝑒 𝐸
…(1)
𝜒𝑒 is a characteristic of every dielectric and which is called electrical susceptibility.
𝜒𝑒 =
P
Since
E
𝑃
∈𝑜 𝐸
=∈𝑜 (∈𝑟 − 1)
Therefore,
𝜒𝑒 =
∈o (∈r − 1)
∈o
𝜒𝑒 = ∈𝑟 − 1
…(2)
6. Dielectric constant (𝜺𝒓 )
Dielectric constant (ε r) is the measure of the polarization produced in the material. It is the ratio
between absolute permittivity (ε) and the permittivity of free space (ε 0).
i.e.,
∈
∈𝑟 = ∈
𝑜
∈𝑟 is a dimensionless quantity and it is a measure of polarization in the dielectrics. The value of
∈𝑟 = 1 for air or vaccum.
For solids, ∈𝑟 > 1, for glass it is 4 to 7, for diamond is 5.68, for silicon it is 12, for germanium
it is 16, for ethanol it is 24.3 and for water at 0˚C ∈𝑟 = 87.8.
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ENGINEERING PHYSICS II
DIELECTRICS
4.3 ACTIVE AND PASSIVE DIELECTRICS
Active Dielectrics
Dielectrics which can be easily adapt itself to store the electrical energy in it is called active
dielectrics. Ex: Piezo electrics, Ferro electrics, Pyro electrics.
It is used in production of Ultrasonics.
Passive Dielectrics
Dielectrics which restrict the flow of electrical energy in it are called passive dielectrics.
Ex: Glass, mica, plastic
It is used in production of sheets, pipes etc.
4.4 POLAR AND NON-POLAR MOLECULES
POLAR MOLECULES
The molecules have permanent dipole moments even in the absence of an applied field is called
polar molecules. These molecules do not have symmetrical structure and do not have centre of
symmetry.
Ex: H2O, N2O, HCl, NH3 etc.
Effect of electric field
In the absence of electric field
In the absence of electric field the polar molecule posses some dipole moment. since, these
dipoles are randomly oriented they cancel each other and the net dipole moment will be very less (app
zero).
In the presence of electric field
When an external electrical field is applied the dipoles in the dielectrics will align themselves
parallel to the field direction and produces a net dipole moment.
NON POLAR MOLECULES
The molecules which do not have permanent dipole moments is called Non Polar molecules.
These molecules have symmetrical structure and they have centre of symmetry
Ex: N2, H2, O2, CH4, CO2
Effect of electric field
When a non-polar molecule is placed in an external electric field, a force is exerted on each
charged particle within the molecule. (i.e.,) the positive particles are pushed along the field direction
and the negative charges are pushed opposite to the field direction. Hence the positive and negative
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ENGINEERING PHYSICS II
DIELECTRICS
charges are separated by some distance from their equilibrium positions, creating a dipole and
therefore a net dipole moment will be produced in non-polar molecules.
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