Dielectric Polarisation

Polarisation is the induction of an electric field or the rearrangement of charges inside a dielectric material or an insulator when an external electric field is applied to it. The charges are arranged so that the positive charges are attracted towards the negative electrode, and the negative charges are attracted towards the positive electrode. This is because dislike charges attract each other and like charges repel. As the charges in an insulator are immobile, the polarisation causes energy to be stored in a capacitor when there is a dielectric material. 

A material having more polarising power can store more charges. Hence, the charge and energy stored in a capacitor will also be high if we use this material as a dielectric. 

Degree of Polarisation

The degree of polarisation of a material depends on the applied electric field E and dielectric constant K. The relation between polarisation, dielectric constant, and an electric field is represented as

P=ℇ0(K-1)E

The total polarisation of a dielectric is the sum of electronic displacement Pe, permanent dipole orientation Pd, ionic displacement Pi, and space charge displacement Ps.

            Pt=Pe + Pi + Pd + Ps

What is Dielectric?

Dielectrics are non-conducting or insulating materials. They are the poor conductors of electricity. As the charges in a dielectric are immobile, they can maintain the charges when an external field is applied and can get polarised. Some of the examples of dielectric materials are mica, glass, polymers, porcelain, and metal oxides. Dry air is also a dielectric.

Classification Of Molecules

Polar Molecules:

Polar molecules have one end electrically positive and another electrically negative. This happens when a covalent bond combines two molecules of electronegativity difference. The molecules form poles, positive and negative poles. These molecules have a dipole moment, and they align themselves when an external electric field is applied—for example, water molecules, HF molecules, HCl molecules, etc. 

Non-Polar Molecules:

These molecules are formed when there is no electronegativity difference between the combined atoms. The dipole moment of these molecules is also zero. Examples of these molecules are O2, N2, and H2.

Induced Electric Dipole Moment

When an external electric field is applied to a non-polar material, the electrons and protons are arranged so that the protons or positive charges are aligned towards the negative electrode, and the electrons or negative charges are aligned towards the positive electrode. The process continues till the internal pressure is balanced. This arrangement causes an induction of the electric field inside the dielectric material. This field is the induced electric field, and the dipole moment is the induced dipole moment. 

Polarisability

Every material is made up of electrons and protons. When an external electric field is applied to them, they are separated and produce an induced electric field inside them. This ability of a substance is known as polarisability. This property is dependent on the external electric field and the dielectric constant of the material. Polarisability can be used to calculate the dielectric constant and the refractive index of the material. 

The field which is applied externally is independent of temperature and proportional to the induced dipole moment. The direction of the induced dipole moment of the material is the same as that of the applied electric field. 

Polarisability of a solid is defined as the dipole moment per unit volume of the crystal cell, and mathematically it can be written as:

P = ℇ0 α E

where α represents the atomic polarisability and E represents the electric field. Polarisability is represented in m3 in general.

Electric Polarisation

When a dielectric is polarised, a dipole moment is generated in the material. This happens in the presence of an external electric field. The process is known as electric polarisation. Electric polarisability is defined as the dipole moment per unit volume of that material. 

Dielectric Constant

Dielectric constant or relative permittivity is the ratio of the permittivity of the substance to the permittivity of the free space. It can also be written as the ratio of the electric field of a capacitor without dielectric to the electric field of a capacitor with a dielectric. 

Mathematically, it can be written as,

ℇr= ℇ/ℇ0

The greater the dielectric constant, the more will be the charge stored in the capacitor. When the gap between the plates is filled with a dielectric, the magnitude of capacitance of the capacitor increases by a factor of the dielectric constant. 

C = ℇr C0, where C0 denotes the capacitance between the plates in the absence of a dielectric.

Dielectric Strength

The highest electric field strength that an insulating material can sustain without losing its insulating qualities is defined as its dielectric strength.

Sample Question

Question 1: The electric field of a capacitor has a magnitude of 30 V/m. The dielectric constant of the material being inserted between the plates of the capacitor is 3. Find the value of the polarisation. 

Answer 1:

Given:

Dielectric Constant, ℇr= 3

Electric Field, E = 30 V/m

Susceptibility can be calculated as,

Xe= ℇr− 1 = 3 − 1 = 2

Polarisation will be,

P = Xe E

P = 2 × 30

P = 60 C/m2

Hence, the polarisation is 60 C/m2. 

Conclusion

Dielectric polarisation is the process of polarisation or induction of an electric field in the dielectric material when it is introduced to some external fields. We have learned in this article the concept of dielectric materials, their types and polarisation. The topic also discusses the mechanism of polarisation and the terms (dielectric strength, polarisability, dielectric constant) associated with it.