In the earlier article, I have explained the electronic polarization. Today we will discuss the ionic polarization.
Ionic polarization. As the name suggests, ionic polarization occurs in ionic materials. It occurs when an electric field is applied to an ionic material then cations and anions get displaced in opposite directions giving rise to a net dipole moment.
Example : Polyatomic gases
In absence of electric field E, the distance between the ions is d but in presence of electric field, distance between the ions increases
The dipole moment p for each ion pair is equal to the product of charge on each ion and relative displacement.
That is p = qd
Thus ionic polarization is given as
Pe =n αiE
where αi is constant of proportionality known as ionic polarizability constant.
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.
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),