Dielectric Polarization in Polar and Nonpolar Material and Dielectric Constant

Insulators and Dielectric Material

Insulators are materials with very poor conductivity. Insulators are not very good at conducting either heat or electricity owing to the absence of loosely bound or freely moving charges in the atoms of the insulator. When insulators are placed in an electric field, practically no current flows in them, unlike in metals. Instead in some insulators, electric polarization occurs. A dielectric is an insulator that undergoes electric polarization on the application of electric field. The charges in dielectric material does not move but only shifts slightly from the equilibrium position resulting in the dielectric polarization. In the previous article on: Polar and Non-Polar Material: Dielectric Material and Dipole Moment’, we examined the nature of the bonds that decide the polarity of the molecules. Electronegativity and the structure of the atoms in the molecules decide whether it is a polar or a nonpolar molecules.

Dielectric Polarization

Non-Polar Molecule

A nonpolar molecule refers to a molecule without a dipole. The charges in a nonpolar molecule are equally distributed. In spite of the lack of a dipole, a dielectric nonpolar material introduced in an electric field will be affected. In an electric field, the positive and the negative charges in a nonpolar molecule experience forces in opposite directions as a result of their opposite polarities. This force causes the electron cloud of a nonpolar molecule to be displaced in the direction of the attraction. This displacement goes on till the attraction by the electric field is balanced by the internal forces of the molecule. Thus, in the presence on an electric field, even a nonpolar molecule experiences induced dipole moment.

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This dipole moment is induced in the direction of the field and is directly proportional to the strength of the electric field the nonpolar material is subject to. Both polar and nonpolar molecule experience polarization on exposure to electric field but the difference between a nonpolar and a polar molecule is that, nonpolar molecules are induced with a dipole by current whereas polar molecules have permanent dipoles. Due to the induced nature of polarity, on the removal of the electric field, a nonpolar material loses its polarity are returns to its original state.

Polar Molecule

Polar molecules undergo Dipolar Polarization which is also referred to as Orientation Polarization. A polar molecule on the other hand is already blessed with electric dipoles and this dipole is not induced. This dipole exists due to the bonds and the structure of a polar molecule. But we cannot utilize this already existing dipole moment right away. Due to thermal agitation, the dipoles in a polar material are oriented randomly. Therefore the dipole moment of the molecules in the material cancels out resulting in a net dipole moment of zero. We need to apply an electric field here as well, albeit for different purposes.

When an electric field is applied, the individual dipole moments align themselves in the direction of the electric field. This means that the bonds, their nature and their orientation remains constant and the polar molecule only rotates about its axis minutely to align itself. This alignment when summed up over all the molecules leads to a net dipole moment in the direction of the electric field. The extent to which the polar molecules get polarizes and align themselves is related to two factors; the strength of the external field and the thermal energy that breaks this alignment.

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Dipolar Polarization. The already existing dipoles rotate to align with the electric field. It is also known as Orientation Polarization.

Thus, irrespective of whether a material is polar or nonpolar, the application of an electric field results in the creation of a net dipole moment across the material. The dipole moment per net unit volume is called Polarisation. For an ideal dielectric material,

P = χe E

Where χe represents the characteristic property of the dielectric material known as the Electric Susceptibility of the dielectric medium. P here represents Polarization due to the applied electric field ‘E’. Another important parameter is the Relative Permittivity ‘ϵr’ also known as the Dielectric Constant. The dielectric constant refers to the ability of a dielectric medium to store electrical energy in an electric field. Dielectric constant is the ratio of the capacitance ‘C’ of the capacitor with the dielectric medium to the capacitance ‘C0’ of the capacitor in vacuum. This can also be written in terms of ratio of charges ‘Q’ (with dielectric) to ‘Qo’ (in vacuum).

\(ϵ_r\) = \(\frac{Q}{Q_0}\) = \(\frac{C}{C_0}\)

Applications of Dielectric Material

Dielectric material is large used in the manufacturing of capacitors. Dielectric due to their unique capacity for electrical polarization. We have already discussed the phenomenon of induced dipole in nonpolar materials and the alignment of the dipole in the polar material. The dielectric sample contains a very large number of dipoles distributed throughout its body. When subjected to an electric field, the positive end of one dipole sits next to the negative of the neighbouring dipole, the positive end of which sits next to the negative terminal of the other dipole and so on. They form a chain of alternating positive and negative polarities throughout the material.

This also the case with nonpolar materials and this alignment leads to the creation of an electric field. This electric field which is set up due to the net dipole moment of the material opposes the external electric field which reduces the electric field built up by a capacitor during charging. This means that we can now accumulate more charges to bring back the electric field to its original intensity. This means that the capacity of a capacitor is effectively increased by introducing a dielectric medium in between the plates of the capacitor.


Practise This Question

Which of following  can not be polarised

[Kerala PMT 2001]