Dielectrics and Their Polarisation

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In electrostatics, materials can be broadly classified into conductors and insulators. Conductors allow free movement of electric charges, whereas insulators (also called dielectrics) do not.
Dielectrics and Their Polarisation-Insulators
Insulators
Dielectrics play a crucial role in capacitors, electronic circuits, and various electrical devices because they can store and influence electric fields without conducting electricity.
The study of dielectrics and their polarisation is essential to understand how insulating materials behave in an electric field and how they affect capacitance and energy storage.

What is a Dielectric?

A dielectric is a non-conducting substance that can be polarised by an electric field. Unlike conductors, dielectrics do not have free electrons; instead, their electrons are bound to atoms or molecules.
Examples:
Solid dielectrics: Glass, mica, ceramic, plastic.
Liquid dielectrics: Distilled water, transformer oil.
Gaseous dielectrics: Air, nitrogen.
Dielectrics and Their Polarisation-Transformer
Transformer
Dielectrics and Their Polarisation-Distilled water
Distilled water

Polarisation in Dielectrics

When a dielectric is placed in an external electric field E, the positive and negative charges inside its atoms or molecules experience equal and opposite forces. This leads to a small displacement of charge centers, creating electric dipoles. This process is called polarisation.

Definition:

Polarisation is the process of alignment or orientation of the dipole moments of molecules in a dielectric material under the influence of an external electric field.

Dipole Moment and Polarisation Vector

Dipole Moment (p): For a dipole with charges +q and -q separated by distance 2a,    p = q x 2a
Direction: From negative to positive charge.
Polarisation Vector (P): The dipole moment per unit volume of the dielectric.
P = Net dipole moment / Volume
Units: C·m-2 (Coulomb meter per square meter).

Types of Polarization

There are mainly three types

(a) Electronic Polarisation

Occurs in all dielectrics.
The external electric field displaces the electron cloud of an atom slightly relative to its nucleus.
Induced dipole moment is proportional to the electric field:
p = αeE  Where αe is the electronic polarisability.

(b) Ionic Polarisation

Found in ionic solids like NaCl.
In an electric field, cations and anions shift slightly in opposite directions, creating dipoles.
More significant than electronic polarisation in ionic compounds.

(c) Orientational Polarisation

Found in polar molecules (e.g., H2O, HCl) which already have a permanent dipole moment.
In the absence of a field, these dipoles are randomly oriented due to thermal motion.
An electric field tends to align them partially along its direction.

Effect of Dielectrics on Electric Field

When a dielectric is inserted between the plates of a capacitor:
The electric field inside is reduced due to the polarisation charges induced on the dielectric surfaces.
The reduction factor is expressed by the dielectric constant (relative permittivity) K:   E = E0 / K
Where E0 is the electric field without the dielectric.

Dielectric Constant (Relative Permittivity)

The dielectric constant K is the ratio of the capacitance with dielectric C to the capacitance without dielectric C0:
K = C / C0    It is also given by: K = € / €0
Where:
€ = permittivity of dielectric.
€0= permittivity of free space (8.854 10-12 C²·N-1 m-2).
A higher K means better ability to reduce the electric field and increase capacitance.

Relation between Polarisation and Electric Field

The polarisation P in a linear dielectric is proportional to the applied electric field:  P = €0XeE
Where Xe is the electric susceptibility of the dielectric.

Bound Charges

Due to polarisation, bound charges appear:
Surface bound charge density:
ᵟb(sigma) = P . ή
Where ή is the unit normal vector to the surface.
Volume bound charge density:  pb = -▼. P
These bound charges produce an electric field opposing the applied field.

Importance and Applications

Capacitors: Insertion of a dielectric increases capacitance, allowing more energy storage for the same voltage.
Electrical insulation: Dielectrics prevent unwanted current flow in high-voltage equipment.
Energy storage: Used in batteries and energy storage devices.
Electronic devices: Essential in transistors, ICs, and communication systems.
Microwave and RF applications: Certain dielectrics are used as substrates for antennas and circuits.
Microwave

Summary Table

Property
Symbol
Unit 
Significance
Permittivity
€0
C2 . N-1. m-2
Measures ability to permit electric field lines     
Dielectric Constant    
K
Dimensionless
Relative measure compared to vacuum                     
Polarization
 P
C . m-2
Dipole moment per unit volume                           
Electric Susceptibility
Xe
Dimensionless
Proportionality constant in P = €0XeE

Conclusion

Dielectrics are indispensable in electrical and electronic engineering. Their ability to become polarised under an electric field reduces the net field inside them, increases capacitance, and enables efficient energy storage without conduction losses.
Polarisation mechanisms electronic, ionic, and orientational helps in selecting the right dielectric material for different applications, from high-voltage insulation to microelectronics.
A dielectric is a non-conducting material that can be polarised in the presence of an electric field. Examples include glass, mica, plastic, and distilled water. They do not allow free flow of electric charges but influence the electric field in which they are placed.   
 
Polarisation is the process in which the positive and negative charge centers of atoms or molecules in a dielectric shift slightly in opposite directions under an external electric field, creating induced dipole moments.
 
The dielectric constant K (relative permittivity) is the ratio of the capacitance of a capacitor with a dielectric to the capacitance without it:
K = C / C0 = € / €0
It measures how effectively a dielectric reduces the electric field inside it.
 
Electronic Polarization: Displacement of the electron cloud relative to the nucleus.
Ionic Polarization: Relative shift of positive and negative ions in ionic solids.
Orientational Polarisation: Alignment of permanent dipoles in polar molecules along the field.
 
Inserting a dielectric reduces the effective electric field between the plates, allowing the capacitor to store more charge for the same voltage, thus increasing its capacitance by a factor of K (the dielectric constant).
 
For a linear dielectric,
P = €0XeE
Where P is polarisation, €0 is the permittivity of free space, Xe is electric susceptibility, and E is the applied electric field.
 
Dielectrics are used in capacitors, electrical insulation, energy storage devices, electronic circuits, microwave technology, and antenna substrates due to their ability to store energy and control electric fields.

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