Mumbai university > FE > SEM 1 > Applied Physics 1

Marks: 5M

Year: Dec 2013

Dielectric Polarization

• When a Dielectric material is subjected to an external electric field, the polar molecules rearrange themselves along the electric field giving rise to net dipole moment.

• This phenomenon is called Dielectric Polarization.

• The induced dipole moment per unit volume of the dielectric is called Polarization vector (P).

Dielectric Susceptibility

• It is a dimensionless proportionality constant that indicates the degree of polarization of a dielectric material in response to an applied electric field.

• The greater the electric susceptibility, the greater the ability of a material to polarize in response to the field, and thereby reduce the total electric field inside the material.

The Relation

Consider a parallel plate capacitor with a dielectric medium of constant $“ε_r”$ placed between the two plates

Polarization of Dielectric Medium between Two Capacitor Plates If σ is the Charge per unit area of the surface of the plate, then by using Gauss’ Theorem, we have,

$$E_0 = \frac{σ}{ε_0}$$

Also,

$$E_i = \frac{σ_p}{ε_0}$$

Where, $σ_p$ is the charge per unit area of the surface of the slab

The resultant electric field (E) is given by,

$$ E = E_0 - E_i = \frac{σ}{ε_0} - \frac{σ_p}{ε_0}$$

$$ ∴ ε_0 E = σ - σ_p$$

The magnitude of polarization $P = σ_p$, And the Electric Flux Density $D = σ = ε_0 ε_r E$

$$ ∴ ε_0 E = P - ε_0 ε_r E$$

$$ ∴ P = ε_0 (ε_r - 1)E$$

Or,

$$ P = ε_0 χE$$

Where,

$$χ = (ε_r-1)$$

…And is called Dielectric Susceptibility.