Dielectrics and Electric Polarization

Generally speaking, materials that are classified as dielectrics are those that conduct electricity with low efficiency. When an electric field is supplied to a dielectric material, it can be easily polarised. Applied electric fields can polarise dielectrics (also known as dielectric materials or dielectric media) in electromagnetism. Electric charges do not flow through a dielectric material as they do in an electrical conductor, but instead, they shift slightly from their equilibrium positions, causing dielectric polarisation. Positive charges move in the direction of the field, while negative charges move in the opposite direction because of dielectric polarisation. As a result, the dielectric itself experiences a decrease in its magnetic field due to an internal electric field. Weakly-bonded molecules in a dielectric get polarised and reorient to align their symmetry axes with the field, resulting in a polarised dielectric.

Terminology

A dielectric is a material with a high polarisability, and low electrical conduction. The relative permittivity is a quantity used to describe the high polarisability. An insulator is a material that acts as an electrical barrier, while a dielectric is a material that can store energy (utilising polarisation). For example in a capacitor, the dielectric substance acts as an electrical insulator between the metal plates. The applied electric field polarises the dielectric, increasing the surface charge of the capacitor for the given electric field strength.

Classification of Dielectrics

Polar molecules- The dielectrics known as “polar molecules” are those in which the chances of positive and negative molecules ever colliding are 0 percent. It is because they’re all asymmetrical. Examples are CO2, H2O, and NO2. The electric dipole moment of these molecules will move in any direction if there is no electric field. The average dipole moment is, therefore, a value of 0. However, the assemblage of molecules occurs in a direction parallel to the electric field if an external one is present.

Non-polar molecules-  Non-polar molecules, unlike polar molecules, do not have a zero point at the centre of the positive and negative charges. The molecule is then devoid of a dipole moment that is both permanent and intrinsic. Other examples include O2, N2, and H2.

Induced Electric Dipole Moment

A nonpolar molecule’s protons and electrons flow in opposite directions when an external electric field is applied. If the internal forces are not balanced, this process will continue indefinitely. As a result, two separate charging stations are created. We name them the Induced Electric Dipole because they are polarised. There is an Induced Electric Dipole Moment, which is the dipole moment.

Properties of Dielectric Materials

William Whewell coined the term dielectric. Two terms, “Dialectic” and “Electric,” are combined to form the term. A perfect dielectric has no electrical conductivity. Instead, like an ideal capacitor, dielectrics store and dissipate electrical energy. Electric susceptibility, dielectric polarisation, dispersion, dielectric relaxation, and tuning are only a few of the characteristics of a dielectric material.

Some more properties are-

• The dielectric materials have a significant energy gap
• Insulation resistance is strong, and the temperature coefficient of resistance is negative
• The resistivity of dielectric materials is very high
• The electrons’ attraction to the parent nucleus is quite strong
• As there are no free electrons in these materials, their electrical conductivity is inferior

Applications of Dielectrics

The following are a few uses for dielectrics:

• Capacitors employ them to store energy
• High permittivity dielectric materials are utilised to improve the performance of semiconductor devices
• Liquid crystal displays (LCDs) make use of dielectrics
• Oscillators using ceramic dielectric resonators are known as DROs
• Strontium-Barium-Iron Thin films of titanate, a dielectric, are employed in microwave tunable devices because of their great tunability and low leakage currents
• A barrier between the substrate and the outside world is provided using perylene in industrial coatings
• Mineral oils are utilised as a liquid dielectric in electrical transformers and also help to cool them down throughout the manufacturing process
• High-voltage capacitors employ castor oil to boost their capacitance
• Similarly, as magnetism, Electrets, a dielectric substance that has been carefully treated, operate as an electrostatic analogue

Dielectric Properties of Insulation

Following are the dielectric properties of insulation:

• Breakdown voltage

• Dielectric parameters such as:

Conductivity 

Power factor

Loss angle

Permittivity

Electric Polarisation

An electric field causes a little shift in the relative positive and negative charge positions in an insulator or dielectric in the opposite directions. An electric field bends the electron cloud around positive atomic nuclei in a direction that is opposite to the field’s direction. A little charge separation results in an atom with a positive side and a negative one. Electrostatic polarisation is responsible for some of the polarisation in some materials, such as water molecules, that are permanently polarised by chemical forces. A measure of polarisation is electric dipole moment, which is the distance between the slightly displaced centres of positive and negative charge multiplied by the quantity of the other charge.

Dipolar Polarisation

To put it another way, anything that can cause nuclei to be bent in an unbalanced direction will exhibit dipolar polarisation, whether it comes from the molecules themselves or can be created by using another molecule (distortion polarisation). In the absence of an external electric field, an orientation polarisation results from the 104.45° angle between the asymmetric bonds between the oxygen and hydrogen atoms in the water molecule. The macroscopic polarisation of these dipoles is the result of their assembly. An external electric field must rotate the polarisation direction to maintain orientation polarisation. Still, the distance between charges within each permanent dipole, related to chemical bonding, does not change. In this rotation, the torque and surrounding viscosity of molecules influence the timescale. As a result, dipolar polarisation loses their ability to respond to high-frequency electric fields. Fluids have a molecular rotational rate of around one radian per picosecond. Therefore, this loss happens at about 1011 Hz (in the microwave region). The friction and heat caused by the delay in responding to the change in the electric field Molecular dipole moment changes when an external electric field is supplied at less infrared frequencies. It is because the molecules are bent and stretched. Vibrational polarisation disappears above infrared wavelengths, inversely proportional to the amount of time it takes molecules to bend.

Ionic Polarisation

In ionic crystals, the relative displacements of positive and negative ions are responsible for forming the polarisation that occurs in the crystal (for example, NaCl). It means that the distributions of charges around an atom are more likely to be positive or negative depending on the number of different kinds of atoms present in the crystal. If the atoms’ lattice or molecular vibrations cause their relative displacement, the centres of positive and negative charges will be shifted, too. Displacement symmetry affects these centres’ positions. Polarisation occurs in molecules and crystals when the centres do not match. Ionic polarisation is the name given to this type of polarisation. The ferroelectric effect and dipolar polarisation are both caused by ionic polarisation. Order-disorder phase transition describes the ferroelectric transition generated by the alignment of the orientations of permanent dipoles in a specific direction. A displacive phase transition occurs when ionic polarisation in crystals creates a change in the crystal’s structure.

Conclusion

A tiny amount of displacement occurs in the dielectric, with positive charges moving in the electric field’s direction and negative charges moving in the polar opposite direction. The dielectric’s electric field is reduced due to the charge separation or polarisation. The presence of the dielectric substance influences other electrical phenomena. While the amount of energy stored per unit volume of a dielectric medium in an electric field is more than in a vacuum, the force between two electric charges in a dielectric medium is smaller. As a result, a dielectric-filled capacitor has a higher capacitance than a vacuum-filled capacitor. Macroscopic notions like dielectric constant, permittivity, and electric polarisation are used to characterise the impact of the dielectric on electrical processes.