Polarization density (also known as "electric polarization," or simply just "polarization") decides the response of an object towards an electric field. It is also responsible for the changes that an electric field experiences due to any material placed within the region of its influence.
Different materials exhibit different electric polarization – that is, their interaction with the electric field gives rise to different forces in nature. As an essential principle of electromagnetism, electric polarization reasonably explains the displacement of charged elementary particles under the influence of an electric field.
Definition of electric polarization
Before you go through the definition of polarization you must first understand the concept of the dielectric, which plays a significant role in explaining polarization density and various other phenomena in physics. In general, a material that acts as a poor conductor of electricity but supports an electric field, and is easily polarized, can be considered a dielectric. Some examples of dielectric materials are plastic, glass, and porcelain.
Now, when a dielectric material is polarized in an electric field, separation of positive and negative charge occurs inside the system. The measure of this separation within a system is referred to as the dipole moment.
Electric polarization of a substance is defined as the electric dipole moment induced per unit volume of a dielectric material. It is basically a vector field that defines the density of permanent or induced electric dipole moments in a dielectric material. When a dielectric material gains an electric dipole moment, it is considered polarized.
Polarization is often compared with magnetization, which explains the response of materials inside a magnetic field. Polarization density is denoted by P, and its SI unit is Coulomb per square meter (C/m2).
How is electric polarization measured?
When a material is placed in an electric field, the bound charged elements of the dielectric material might be expected to move freely, but they can’t because they are held together by molecular and atomic bonds. However, the applied electric field can cause a slight change in the position of positively and negatively charged particles, so when electrons are slightly pushed towards the positive atom nuclei, one side of the atom changes a bit negative while the other turns slightly positive and this displacement of charge leads to electric polarization.
The positively charged particles move in the direction of the applied field and the negatively charged particles move in the opposite direction. The polarization density (P) of the material is then given as:
P = ∆p/∆V
∆p = dipole moment occurred due to displacement of charge.
∆V = volume of the dielectric material
Since the dipole moment shows variation at different points within the dielectric, the electric polarization for an infinitesimal volume dV of dielectric is:
P = dp/dV
Induced polarization produced in a given volume of a material due to the action of an electric field is measured in terms of dielectric constant (κ), it is also defined as the relative permittivity of a material. This represents the ability of a material to concentrate electrostatic lines of flux, or, the ability of a material to store electrical energy in the presence of an electric field.
κ = E/Eo
E = dielectric permittivity of the material
Eo = vacuum permittivity
Permittivity impacts the Coulomb force that exists between two point charges in a material.
Types of dielectric polarization
Piezoelectric materials such as quartz experience polarization not because of the electric field but due to mechanical stress. Similarly, the mechanical strain, or strain gradient, is the cause of polarization in many solid dielectric materials, and this phenomenon is known as the flexoelectric effect.
There are mainly four categories in which dielectric polarization is classified:
1. Electric polarization
Also known as atomic polarization, electric polarization occurs due to the separation of the center of positive charge and the center of negative charge in the atoms of a material in the presence of an electric field. The induced dipole moment, in this case, is found directly proportional to the applied field.
2. Ionic polarization
There are some dielectric materials such as NaCl and LiBr in which atoms are held together by ionic bonds. Induced polarization occurs in such materials when an external electric field is applied and the cations and anions are displaced in opposite directions, giving rise to a net dipole moment.
However, in the absence of an electric field, net polarization inside the dielectric material comes out to be zero because positive and negative ions cancel out each other and no net dipole moment is produced.
3. Orientational polarization
Orientational polarization arises when there is a permanent dipole moment in the material. Materials such as HCl and H2O will have a net permanent dipole moment because the charge distributions of these molecules are skewed. So, when an electric field is applied, the molecules start to rotate in order to align in the direction of the field, and polarization results.
Orientational polarization is also affected by temperature, thermal agitation occurring in dipolar molecules results in the cancellation of the net dipole moment (when no electric field is applied). This is also the reason why the selection of dielectric material for electronic or optical applications is sometimes made on the basis of their temperature dependency.
4. Space charge polarization
Space-charge polarization occurs when an external field causes an accumulation of charge at an interface between two materials, or between two regions within a material. The electric field will cause a charge imbalance because of the insulating properties of the dielectric material.
It generally occurs in compound dielectric materials, such as between electrodes connected to a dielectric substance. Surprisingly, interface polarization is also able to exert an influence on free charges, unlike ionic or orientational polarization.
Interesting facts about polarization
Polarization is not just limited to dielectrics but also observed in electromagnetic radiation, such as light rays. It is an integral part of electromagnetism and has various applications depending on its medium and nature:
- Time-domain reflectometry is a technique that uses electric polarization as a means to detect contaminants in soil. This process is inspired by the induced polarization method used in mineral and groundwater exploration.
- Piezoelectric transducers can convert sound energy into an electric field and are used in applications such as microphones, medical imaging equipment, electric guitars, and non-destructive industrial testing procedures.
- Polarization methods are used to determine the symmetry or asymmetry (chirality) in organic compounds.
- Stress analysis tests in plastic manufacturing units are performed using polaroid filters, and polarization also holds importance in the study of earthquakes and transverse waves.
- Polarized 3D glasses enable you to experience movies in 3D to create an illusion of three-dimensional imagery by restricting the light that reaches each eye. The use of polarized sunglasses is also common, such lenses are usually treated with a chemical applied in a vertical pattern, which blocks horizontal light to eliminate glare.
The primary role of dielectric polarization is to explain the connection between the macroscopic properties of a material and its microscopic attributes. Without understanding this principle, many mysteries of classical electrodynamics stay unresolved.