Electric Polarization

Dhristi JEE 2022-24

Definition of Electric Polarization

Electric polarization is a part of the study of classical electromagnetism. If one has to define electric polarisation, it can be said that electric polarization (or polarization density or just polarization) is a vector field that defines the density of permanent or induced electric dipole moments in a dielectric material. Polarization is said to be completed when the dielectric is placed in an external electric field and gains an electric dipole moment.


Thus dielectric and polarization definition can be stated as ‘the electric dipole moment induced per unit volume of the dielectric material.’ It also explains the response of material on the applied electric field and how the material changes the electric field. It can be thus used to calculate the forces that come out due to these interactions. 


It is also compared with magnetization which measures the relative response of a material to a magnetic field in magnetism. The unit for measurement is coulombs per square meter and polarization is represented by a vector P. 

Dielectric Polarization Significance

The displacement of bound charged elements of dielectric material occurs due to the external electric field application. These elements cannot freely move around the material because they are bound to the molecules. Elements with a negative charge are displaced opposite and those with a positive charge move towards the field. An electric dipole moment is formed though the molecules are neutral in charge. 


Let’s say that a volume element ∆V in the material with a dipole moment ∆p, the polarization density P can be described as


P =  \[\frac {\Delta p}{\Delta V}\]


Usually, the dipole moment ∆p varies from point to point within the dielectric therefore the polarization density P inside an infinitesimal volume dV with an infinitesimal dipole moment do is


P = \[\frac {dp}{dV}\]


Qb is indicated for the bound charge of the result of polarization. ‘Dipole moment per unit charge’ is the definition that is now widely accepted. 

How to Explain Dielectric Polarization?

In an insulator or dielectric, the slight change in position of negative and positive charge takes place in opposite directions that are caused by an external electric field. The electrical polarization occurs when, because of the electric field, the negative electrons are pushed towards the positive atom nuclei surrounding it. This distortion of charges results in one side of the atom becoming a little negative and the opposite side becoming a little positive. 


However, in some chemically bound molecules like water molecules, polarization partially takes place due to the rotation of molecules into the same line under the influence of the electric field. 

Electric Polarization in Dielectrics 

Now we will define electric polarization and the effects of the application of electric fields in molecules. There are polar and nonpolar molecules. Let’s consider that Pi Pi is the induced electric polarization and ui is the induced dipole moment. Now, the induced dipole moment is directly proportional to the strength of the electric field applied (E) . Hence, ui α E. Hyperpolarization occurs within the molecules if the electric force is very low, so we have to say that ui = αi F. Here, αi is the proportionality constant. Thus, this is the induced polarizability constant of the polarizing molecules,


Thus, the induced polarization of dielectric material in chemistry means the amount of induced moment in the polarized molecule when the unit electric field of the current strength is applied. 

Unit and Dimension of Polarizability 

The electric polarization constant has the dimension of volume and is derived from the definition and polarizing formula. Unit of dipole moment obtained from Coulomb’s law can be stated as esu X cm and force unit as esu cm-2.


As the atom size, ionization energy, and atomic number increase, the polarizability of the atom increases. 

Dielectric and Polarization 

Dipolar polarization can be achieved by inducing an electric field in the molecules which can exhibit uneven distortion of the nuclei (distortion polarization). The ‘orientation polarization’ happens because of a permanent dipole (arising from 104.45 deg angle), for eg, oxygen and hydrogen atom in water.  

Effect of Temperature on Polarization –

The orientation polarization is zero because of the fixed polarized chemical bond and inability to orient in a fixed direction. Strong intermolecular forces oppose the free rotation of the polarized molecules like in a condensed system. This is the reason why molecules in carbon dioxide, ethane, propane, methane, nitrogen, hydrogen do not vary with temperature. 


However, molecules in many substances like benzyl alcohol, methyl chloride, hydrochloric acid, nitrobenzene, etc, are temperature-dependent and vary with varying temperatures. 

Clausius Mossotti Equation –

A relation between the polarizability of substances and the dielectric constant of the non-polar medium between the two plates is derived from electromagnetic theory. The distortion of 1 mole of the polarizing substance by a unit electric field gives rise to induced polarizability constant. Hence, the electric polarization constant formula – 


Dielectric constant (D) =  \[\frac {C}{C_0}\]


where 

C = capacitance of the condenser having the polarized substance and 

C0 = vaccum.

 

Therefore the dielectric is a dimensionless quantity shown with the unit of vacuum.   

Dielectric loss 

When a dielectric material is put through an AC voltage, the insulating material absorbs and dissipates electrical energy in the form of heat. This dissipation of this energy is known as dielectric loss.

FAQs on Electric Polarization

1. What are the types of Dielectric Polarization?

There are four types of dielectric polarization.


Electronic Polarization – Electronic polarization of dielectrics is the outcome of displacement of negative and positive charges in the dielectric field. 


Ionic Polarization –The ionic polarization takes place only in those dielectric materials where atoms are having ionic bonds. The distance between the separated atoms due to the external electric field is larger. 


Orientation Polarization – This polarization occurs even without the electric field. The polar dielectric gets random when the external field is applied and thus the dipole moment is zero.

It is also affected by the thermal agitation occurring in dipolar molecules, which results in the cancellation of the net dipole moment when no electric field is applied. 


Space Charge Polarization - It occurs due to the accumulation of an electric charge at an interface between two materials, such as between electrodes connected to a dielectric substance. The electric field causes an imbalance in charge because of the insulating properties of the dielectric material. To respond to the applied field, the ions disperse over a considerable distance.

2. What is polarization static electricity?

Electricity is the interaction between electric charges. The same charges will repel each other whereas the opposite charges will bond. The force that bonds the charged substances is called the electric force. When the object’s electric force is exerted on other charged objects there is a formation of an electric field.

3. What are dielectric breakdown and dielectric strength?

When a dielectric is applied with electrical field strength, it surpasses a critical value, and a large current flows through it. The dielectric loses its insulating property and starts conducting. This is called dielectric breakdown. 


Basically, it’s an insulating material’s failure to prevent the flow of current under applied electrical stress. The breakdown voltage is the point at which this failure takes place. The electrical field strength at which the dielectric breakdown happens is called dielectric strength.


The types of breakdown mechanisms are intrinsic breakdown, chemical, and electrochemical breakdown, defect breakdown, discharge break down, thermal breakdown, and avalanche breakdown.

4. Why does orientational polarization happen?

Orientational polarization happens even without the electric field. It takes place when there is a permanent dipole moment in the material. 


Materials such as hydrochloric acid (HCL) and water (H2O) have a net permanent dipole moment because the distribution of charge of these molecules is skewed. So, when an electric field is applied to these materials, the molecules rotate so that they can align in the direction of the field. This activity results in polarization. 


This kind of polarization is also affected by temperature. When no electric field is applied, the thermal agitation occurs in dipolar molecules, resulting in the cancellation of the net dipole moment. This is why the selection of dielectric material for electronic applications is made on the basis of their dependency on temperature.

5. What are the daily life applications of dielectric polarization?

Dipole polarization has various applications depending on its medium and nature.

  • To detect contaminants in soil using the time-domain reflectometry technique.

  • Piezoelectric transducers convert sound energy into an electric field. These are used in medical imaging, electric guitars, microphones, and non-destructive industrial testing procedures.

  • To determine the symmetry or asymmetry in organic compounds.

  • Stress analysis tests in plastic manufacturing units.

  • Study of earthquakes and transverse waves.

  • Polarized 3D glasses are used to watch 3D movies. It is used to create an illusion of three-dimensional imagery by restricting the light from reaching the eyes. 

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