[Physics Class Notes] on Electric Polarization Pdf for Exam

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 u_i 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 u_i = α_i 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.