What is the Dielectric Constant, Strength, and Dielectric Polarization in Physics?

Asma Mushtaq
What is the Dielectric Constant,

Introduction to Dielectric

In this article I have discussed about what is the dielectric constant and its other parameters. In electromagnetism, a dielectric or a dielectric material or a dielectric medium is a poor conductor or a good insulator but a strong supporter of electrostatic fields. It can be easily polarized by an electric field. When a dielectric is placed in an electric field, like inductor, it does not allow electrons to flow from it due to no availability of free electrons. But instead of shifting of electrons inside the material, these electrons drift a bit. After disturbing their natural equilibrium position, they cause a phenomenon called dielectric polarization. Which we will discuss later.

The amount of polarization which occurs when a potential difference is maintained influences the amount of electrical energy that is stored in electric field. This property of a dielectric is described by dielectric constant of the material. Other important properties of a dielectric that describe its general features include dielectric strength and dielectric loss.

While inspecting the term insulator, we come across the procedure in which a material restricts it free electrons and the free electrons are not allowed to move and thus causing relative resistance. This causes the temperature of the material to rise due to excessive and rapid collisions of electrons. Thus a dielectric can also be defined as a device that stores charge, possess a high specific resistance and a negative temperature coefficient of resistance. The term dielectric was first coined by William Whewell, who later described the perfect dielectric as a material that shows perfect insulation or zero conduction and thus exhibit only the drift or displacement current of free electrons causing the device to store and return electrical energy just like an ideal capacitor.

Important Considerations of Dielectric

A dielectric material releases heat. It possesses the ability to support an electrostatic field, while releasing a minimum amount of energy as heat loss. This heat loss is referred to as dielectric loss. The more the dielectric loss, the more the heat will be dissipated by the dielectric, causing its functionality and efficiency to go down. Thus for an efficient dielectric, the dielectric loss should be minimum.

The extent up to which a substance concentrates the electrostatic lines of flux is called as dielectric constant. The value of dielectric constant is different for different types of materials. Its value is highest for substances such as metal oxides. The moderate values indicate materials that are ceramic, distilled water, paper, mica, and glass. While low dielectric constant materials include a perfect vacuum, dry air and most pure and dry gases.

What is the Dielectric Constant?

The dielectric constant of a material, which is also referred to as permittivity of a material represents the ability of the material to concentrate the electrostatic lines of flux. In more easy words, it represents the ability of a material to store electrical energy in the presence of some applied electric field. All materials possess different values of permittivity as discussed earlier. However, we take the value of free space or vacuum as the reference permittivity which is given as

$$
\varepsilon_o=8.854 * 10^{-12}
$$

Its unit is given as farad per meter. Since permittivity of every material is different from each other therefore the permittivity of any material is given as

ε=ε_o ε_r

Where ε is the absolute permittivity and εr is the relative permittivity which is always greater than one. This ratio shows that when a dielectric is placed in any electric field it stores energy that is more than the energy of the vacuum state.

Dielectric strength

There is a limit to voltage applied on an insulator, beyond which it starts conduction. This voltage is called breakdown voltage. Air is a good example. It acts as an insulator but under certain specific conditions, it can also conduct electric charges. Exceeding the voltage limit can be dangerous as it can cause permanent damage to the device or sometimes complete destruction.

Dielectric loss

It refers to the energy lost as heat during conduction when a variable voltage is applied. These losses happen due to the drift of electrons in the medium. When the material changes polarization, these drifts appear as alternating current flow that may be large but is usually small. As permittivity is different for materials, dielectric losses also differ depending on the nature of materials.

Electric susceptibility

Electric susceptibility of a dielectric determines how easily a dielectric can be polarized when it is introduced in an electric field. This proves quite useful as it determines the electric permittivity of the material and imparts a great deal on other basic and fundamental phenomena. It is defined as the direct relation proportionality between induced dielectric polarization density P and electric field E and is given as

$$
P=\varepsilon_o X_e E
$$
The relation between susceptibility of a medium and relative permittivity is given as
$$
X_e=\varepsilon_r-1
$$
in case of vacuum, this expression becomes zero
$$
\boldsymbol{X}_e=0
$$
The final expression is then given as
$$
D=\varepsilon_o E+P=\varepsilon_o\left(1+X_e\right) E=\varepsilon_o \varepsilon_r E
$$
Dielectric polarization
In the presence an applied electric field, the electron cloud around the atom is distorted. This give rise to dipole. When the electric field is removed, the atom comes back to its original state. The time required to do so is called relaxation time. This happens in an exponentially decay piece of time. A new term electric dipole moment rises which is the measure of separation between the positive and negative poles of the atom. The relationship between dipole moment and electric field give rise to the properties of dielectric material which is given as
$$
\boldsymbol{M}=\boldsymbol{f}(\boldsymbol{E})
$$

Properties of Dielectric Materials

Dielectric properties refer to those fundamental properties that all the dielectric materials have and thus enabling them to restrict the electron flow through them, which results in polarization within the material causing poles to form when an external electric filed is applied

Insulation

Following are the dielectric properties of insulation

  • Breakdown voltage
  • Dielectric parameters
  1. Conductivity
  2. Power factor
  3. Loss angle
  4. Permittivity

Solids

Solids possess following dielectric properties

  • Piezoelectricity
  • Pyro electricity
  • Ferroelectricity
  • Anti-ferroelectricity

Types of Dielectric materials

The type of molecule present in a dielectric determines its type. There are basically two types of dielectric materials

Polar Dielectric

In polar dielectric, the center of mass of both positive and negative do not line on the same point which clearly shows that the molecules are asymmetrical and the dipole moment exists in the material. When electric field is applied, these molecules align themselves in line with the electric field. When the electric field is removed, the net dipole moment again becomes zero.

Examples include water and hydrochloric acid.

Nonpolar Dielectric

In nonpolar dielectric materials, the center of mass of both negative and positive pole coincides and lie on the same point causing the molecule to be symmetrical in shape and thus the value of dipole moment of dielectric material become zero.

Examples are hydrogen, oxygen and nitrogen.

Applications of Dielectric materials

Some common applications of dielectrics are listed below

  • Capacitor or energy storing device is made from dielectrics
  • High permittivity dielectric materials enhance the performance of the semiconductors
  • Liquid crystal displays are made from dielectrics
  • Ceramic dielectric is used in dielectric resonator oscillator
  • In industrial transformers, mineral oils are used as dielectric between their plates as they also assist in cooling process
  • Electrets is a specially processed dielectric material which acts as an electrostatic magnet

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