What is dielectrics?
Dielectrics or insulators are bad conductors of electricity due to the unavailability of
free electrons. Ex: Plastic, rubber, mica, glass, wood, ceramic etc
(i) Dielectric/insulators have very large energy gap. So they are bad conductors of electricity.
(ii) There are no free electrons to carry current.
(iii) They have negative temperature of co-efficient of resistance.
(iv) They have high specific resistance.
(v) If electric field is applied then they can be polarized.
(vi) Dielectrics are widely used in the capacitors to store energy.
(a) ELECTRIC DIPOLE:
Two equal and opposite charges are separated by a small distance is called an electric dipole.
Consider two equal and opposite charges of magnitudes + q and - q are separated by a small distance
Ex. Electric dipoles are HCl, CO2 and Water etc.
(b) ELECTRIC DIPOLEMOMENT (μ):
If two equal and opposite charges are separated by a small distance, then dipole moment μ arises between the dipole. Mathematically, it is the product of magnitude of charge and distance between the charges. It is a vector quantity pointing from a negative charge towards positive charge.
µ = q X d
(c) POLARIZABILITY (𝜶):
The induced dipole moment is directly proportional to the intensity of electric field.
its unit is F-m2or Coulomb-meter or Debye.
1Debye = 3.3x10-30 cm
(d) POLARISATION VECTOR
If 𝜇 is the average dipole moment per molecule and N is the number of molecules per unit volume, the polarization vector is defined as dipole moment per unit volume of the dielectric material.
(e) DIELECTRIC SUSCEPTIBILITY (χe):
The electric susceptibility (χe) of a dielectric is a measure of how easily it polarizes in response to an electric field. When a dielectric material is placed in an electric field E, then polarization takes place. The polarization vector is proportional to the electric field E.
𝑃⃗ 𝛼 𝐸
𝑃⃗ = 𝜒𝑒 𝐸,
where χe is called electric susceptibility.
Units: No units
(f) DIELECTRIC PERMITTIVITY (ε):
It is defined as the ability of the material to permit the passage of electric field lines through it. The permittivity of any dielectric material can be represented as ε = εr ε0. Here εr is the relative permittivity or dielectric constant of the material and ε0 is the permittivity of free space and is equal to 8.85 × 10-12 F/m.
It is a dimensionless quantity.
(g) DIELECTRIC CONSTANT OR RELATIVE PERMITTIVITY (𝜀𝑟):
The ratio of permittivity of medium to permittivity of free space is called as dielectric constant or relative permitivity. It has no units
The dielectric constant can be derived from electric flux density (D) and applied electric field (E). The electric flux density is proportional to the applied electric field (E).
𝐷 𝛼 𝐸
𝐷 = 𝜀 𝐸 (in medium)
where 𝜀 is the proportionality constant, called permittivity of medium.
𝐷 = 𝜀𝑜 𝐸 (in a free space)
(h) Electric flux density/ Electric displacement (D)
The electric flux density or electric displacement (D) at a point in a material is expressed as,
D = εr εo E (1)
where E is the electric field strength, εr is the relative permittivity and εo is permittivity of free space.
If electric field is applied, then polarization occurs
D = εo E + P (2)
Eq. (1) in (2), εr εo E = εo E + P
εr εo E - εo E = P
εo E (εr - 1) = P (3)
(h) POLARIZATION IN DIELECTRICS:
The process of producing electric dipoles when an electric field is applied to a dielectric is known as dielectric polarization (or )the phenomenon of splitting of positively charged nucleus and negatively charged electron cloud is also known as dielectric polarization.
When an electric field is applied to the dielectrics, then field exerts a force on each positive charge in its own direction, as a result the positive charges are displaced in the Direction of field while negative charges are displaced in the opposite direction.
Consequently, the displacement of these charges produces electric dipoles throughout the dielectric material. This process is known as polarization in dielectrics.