Dielectric Polarization in Polar and Nonpolar Material and Dielectric Constant

What is a Dielectric Material?

A dielectric material is a type of insulator that becomes polarised when it comes in contact with an electric field. Although it is not a good conductor of electricity, it can easily support an electrostatic field. Dielectrics are used in components such as capacitors and radios. When configured properly, it can be used to store energy too. Most of these materials are solid in nature and some fluids and gases exhibit dielectric properties. A few examples of dielectrics are:

  • Dielectric Gas – Dry Air
  • Solid Dielectrics – Mica, Ceramic, Plastic and Glass
  • Dielectric Liquid – Distilled Water

In the next few sections, let us discuss Dielectric Polarization in Polar and Nonpolar Material.

Characteristics of Polar and Non-Polar Molecules

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 the electric field. The charges in dielectric material do not move but only shifts slightly from the equilibrium position resulting in the dielectric polarization. In our article on Polar and Non-Polar Material: Dielectric Material and Dipole Moment, we examined the nature of 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 nonpolar molecule.

Polar MoleculeCalculating Dielectric PolarisationApplications of Dielectric Material

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 until 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 the 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 returned 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 remain 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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Calculating 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 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. The dielectric constant is the ratio of the capacitance ‘C’ of the capacitor with the dielectric medium to the capacitance ‘C0’ of the capacitor in a vacuum. This can also be written in terms of the ratio of charges ‘Q’ (with the dielectric) to ‘Qo’ (in a vacuum).

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

Applications of Dielectric Material

The dielectric material is largely used in the manufacturing of capacitors. Dielectric due to their unique capacity for electrical polarization. We have already discussed the phenomenon of the 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 between the plates of the capacitor.

2 Comments

  1. explain the polarisation in nonpolar molecule shortly

    1. Consider a nonpolar molecule with equally distributed charges. When the nonpolar molecule is in an electric field, the charges of the molecule experience forces in opposite directions and this is due to their opposite polarities. The electron cloud in the nonpolar molecule displaces in the direction of attraction. Until an electric balance is obtained within the molecules, the attraction goes on. Therefore, we can say that in the presence of an electric field, nonpolar molecules can experience induced dipole moment.

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