Electric Susceptibility

Define Electric Susceptibility

The electric susceptibility is generally defined as the constant of proportionality (what can possibly be a matrix); these are related to an Electric Field E to the induced dielectric polarisation density P. Electric susceptibility, which is also known as dielectric susceptibility, is considered to be a dimensionless proportionality constant which is responsible for indicating the degree of polarisation of a dielectric material, this phenomenon happens in response to an applied electric field. Electric susceptibility is directly proportional to the polarisation of a material.

 

Overview

Electric susceptibility is considered to be a quantitative measure to the extent to which an electric field applied to a dielectric constant causes polarisation. When this phenomenon occurs, there is a slight displacement of positive and negative charges within the material. Most of the dielectric materials have similar properties, such as the polarisation P is directly proportional to the electric field strength E; this property is common in every dielectric material. Therefore, the ratio of P and E (P/E) is considered constant. This constant generally expresses the intrinsic value of the material.

In the centimeter-gram-second system (cgs), the electric susceptibility, xₑ, is defined by a ratio that is xₑ = P/E. In the meter-kilogram-second system, electric susceptibility is defined slightly differently because the constant permittivity of a vacuum, ε₀,  gets included here. The expression comes out something like this

xₑ = P/(ε₀E). 

In both systems, electric susceptibility always remains a positively dimensionless number. Due to the slight difference in the definition of cgs and mks, electric susceptibility values of particular materials under the mks system get 4π times more than the cgs system.

 

Electric Susceptibility Formula

The formula of electric susceptibility is derived as follow:

P = ε₀XₑE

Where,

P = It is considered as the polarisation density.

 ε₀ = It is considered as the electric permittivity of free space.

Xₑ = It is considered as the electric susceptibility.

E = It represents the electric field.

The susceptibility is proved to be related to its relative permittivity, also known as dielectric constant εᵣ by:

Xₑ = εᵣ - 1

Therefore in the case of vacuum,

Xₑ = 0

During this time, the electric displacement D also becomes equal to the polarisation density P by:

D = ε₀E + P = ε₀(1 + Xₑ) E = εᵣ

Where,

ε = εᵣε₀

εᵣ = (1 + Xₑ)

 

Dielectric Constant

A dielectric is considered to be a material that has poor electrical conductivity but has the ability to store electric charge in it. It is capable of storing an electric charge because of dielectric polarisation.

The dielectric constant of a material can be defined as the ratio of the permittivity of the substance to the permittivity of the free space.

It shows how capable a material is to hold sufficient electric flux within it.

The dielectric constant is mathematically expressed as 

k = ε/ε₀

Where,

K= Dielectric Constant.

ε = The permittivity of a substance.

ε₀ = The permittivity of free space.

 

Relationship Between Electric Susceptibility and Dielectric Constant

The Dielectric Constant is responsible for indicating the extent to which a particular substance can conduct electricity through it.

Electric susceptibility is responsible for indicating the extent to which a given substance gets polarised when it is kept in an electric field. If the substance gets polarised more than normal, then the substance will start creating an internal field which will, in turn, oppose the external field; this, in turn, will reduce the electric flux present within the material. This is why electric susceptibility affects the electric permittivity of a medium.

So the relationship between dielectric constant and susceptibility conveys that the greater the level of polarisation lower will be the electric permittivity.

 

Relation Between Susceptibility and Dielectric Constant

D = ε₀ (E+P).    ………(1)

Also,

D = εE and P = XeE

Substituting this values in Equation 1 we get,

εE = ε₀E + XeE

ε = ε₀ + Xe

ε / ε₀ = 1 + Xe / ε₀

But,

ε / ε₀ = Dielectric Constant (K)

This is the required relation; clearly, the value for all-dielectric materials is greater than 1.

 

Dielectric Material

The dielectric material is considered to be a non-metallic material. They have high resistance capability, temperature coefficient of resistance negative, and large insulation resistance. In simple words, dielectric materials are considered to be non-conducting materials which do not allow electrical flow to pass through easily. These are poor insulators that store electric charges despite passing them.

If you place a dielectric material in the electric field, the electricity will not flow within that material. Electric charges slightly shift from their average equilibrium positions, which causes dielectric polarisation.

 

Types of Dielectric Material

By considering the type of molecules present in the dielectric materials they can be classified into two categories - polar Dielectric Material and Non-polar Dielectric Material. Let us further discuss these two types of dielectric materials.

Polar Dielectric Material:- Due to the asymmetric shape of the molecules, the possibility of the coincidence between the positive and the negative type of molecules is kept at zero. Dipole moments do exist in this type of dielectric material. 

If an external electrical field is applied to the material, then in such case the charged molecules will assemble themselves in a similar direction of the electric field. When this electric field is removed, random dipole moments will again be observed in this material and the net dipole moment will go to zero. H2O and CO2 are two famous types of polar dielectric substances. 

Non-Polar Dielectric Material:- In this type of dielectric material, both the positive and negative charged molecules of the material will coincide with each other within the non-polar dielectrics. They do not have any permanent dipole moment in their molecules. These molecules show a certain level of symmetry in their structure and form. Gaseous dielectric substances like H2, O2, N2 are common examples of Non-Polar Dielectric Material.

 

Some Examples of Dielectric Materials and their Real-Life Application and Uses 

Examples

  • Some examples of solid dielectric materials can be ceramics, paper, mica, glass, etc.

  • Distilled water and transformer oil can be used as an example of liquid dielectric materials. 

  • Nitrogen (N2), Oxygen (O2), helium, dry air and various kinds of metal and non-metal oxides (CO2) are all types of gaseous dielectric solutions. A perfect vacuum is also dielectric in nature.

Applications:- 

  • Dielectric materials are used in capacitors, as they have the ability to store energy.

  • Dielectric liquids, such as mineral oils, are used in electrical transformers, and they assist in the cooling process. 

  • Used to improve the performance of various semiconductor devices.

FAQs (Frequently Asked Questions)

1. What is the relation between Permittivity and Susceptibility?

According to many physicians, permittivity is considered as the ability to resist the external electric field. This means a substance that has a high permittivity requires a high external electric field in order to get polarised. On the other hand, susceptibility is considered as the ability of a substance to get polarised. Thus, a substance that has a high susceptibility can get polarised very easily. Therefore it is clear that both the quantities are inversely proportional to each other. But if you are considering both the quantities on the basis of mathematical terms, then both are linearly dependent on each other.

2. What is Dielectric Polarization?

At the time of the electric field getting applied to a capacitor, the dielectric material or the electric insulator starts getting polarised. Through this process, the negative charges that are present in the material shift themselves towards the positive electrode and the positive charges present within the material shift towards the negative electrode. Since the charges are not free to move around within the insulator, the polarisation effect that opposes the applied field is seen drawing charges onto the electrodes. This phenomenon helps in storing energy in the capacitor. The above process is known as dielectric polarisation in physics terms.

3. What exactly is the Electric Susceptibility in physics?

In physics (more specifically in Electro- Magnetism), Electric Susceptibility is a proportionality constant that refers to the degree of polarisation of a dielectric material in response to an applied electric field. As most of the proportionality constant, Electric Susceptibility is also a dimensionless unit. It indicates the ability of a given material to polarise in response to the field. Higher the level of Electric Susceptibility, greater the ability of that certain material to polarise in the presence of electric field. This ability reduces the total electric field inside the material. The electric susceptibility of a certain material is denoted by the symbol “χe”.

4. What is difference between insulator and dielectric material

Dielectric constants are also insulators in a sense, but differ then most other types of insulators. The main difference between the insulator and dielectric substance are given below:-

  • Dielectric substances are also basically insulators but these insulators can be polarised in the presence of some external electric field.

  • All dielectric substances are insulators but not all of the insulators are dielectric in nature.

  • Degree of polarisation is measured by a constant called dielectric constant (or relative permittivity). The value of this dielectric constant of dielectric material is quite high compared to the value of the dielectric constant of other insulators, which is very low.

  • Insulators can’t store electricity in any form, but dielectric materials possess the ability to store electric energy due to their polarisation.

5. What are Dielectric Materials? Also, write their applications.

The word dielectric comes from a Greek term of “Di” or “Dia” meaning “Across”. These materials are a very bad conductor of electricity and do not let the electricity (electric current) travel across itself, Dielectric materials are basically insulators. But under certain conditions, these materials do get polarised at the atomic level. Under the peripheral application of electric and magnetic fields, the positive end of the polar molecules present in the dielectric material will slightly shift in the direction of the electric field; this slight shift is very crucial for the working of many modern applications. 

After the electric field is removed from it, the material will come back to its original polarity.

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