Electromagnetism, Science

Types of polarization

by amsh

The four types of polarization which occur in dielectrics are:

(I) Electronic polarization,

(II) Ionic polarization,

(III) Orientation or dipole polarization,

(IV) Space charge or interfacial polarization

Let us discuss them one by one:

(I) Electronic polarization. Electronic polarization occurs due to displacement of the centre of the negatively charged electron cloud relative to the positive nucleus of an atom by the electric field.

Example. Monoatomic gases exhibit only electronic  polarization.

The shifting of electron cloud results in dipole moment and dipole moment p is defined as the  product of the charge and shift distance,

that is                                         p = qd

Dipole moment p is also directly proportional to electric field strength, E thus p is proportional to E

Or p = αeE                                                                               (1)

where αe is constant of proportionality known as electronic  polarizability constant.

Electronic polarization Pe is given as

Pe =np

where n is number of molecules per unit volume.

By putting value of p from equation (1) in above equation, we get

Pe =n αeE

This polarization is independent of temperature.

Note:  In next article, I will discuss ionic polarization. This article is referred from my authored book. In case of any problem in this article or any other physics article,  kindly post in the comment section.

Let us derive the relation between polarization vector (P), displacement (D) and electric field (E):

In the last article of polarization, we have discussed about the effect on dielectric placed in an external electric field E0 and there will be electric field due to polarized charges, this field is called electric field due to polarization (Ep). (You can see the figure in that article).

Rewrite equation (1) of that article, that is:

E = E0 – Ep (1)

Polarization vector, P = P is equal to the bound charge per unit area or equal to the surface density of bound charges (because surface charge density is charge per unit area),

Thus P = qb/A = σp (2)

Where qb is bound charge and σp is surface density of bound charges.

P is also defined as the electric dipole moment of material per unit volume.

P = np

where n is number of molecules per unit volume.

Displacement vector, D= D is equal to the free charge per unit area or equal to the surface density of free charges,

Thus D = q/A = σ (3)

where q is free charge and σ is surface density of free charges.

As for parallel plate capacitor (already derived in earlier articles):

E = σ /ε0 (4)

Ep = σp /ε0 (5)

By substituting equations 4 and 5 in equation 1, we get

E = σ /ε0 – σp /ε0

Or ε0E = σ – σ0

By putting equations 2 and 3 in above equation, we get

ε0E = D – P

or D = ε0E + P

This is the relation between D, E and P.

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