What is Dielectric constant?

Water constitutes about 70% of earth.  Some important distinguishing properties of water are high molar concentration, small dissociation constant and high dielectric constant.

The dielectric constant is an indicator of the total of electric potential energy contained in a given amount of material under the influence of an electric field in the context of induced polarization. It is defined as the ratio of the material's dielectric permittivity to that of a vacuum or dry air.

Water's dielectric properties are critical for understanding its behavior as a solvent and electromagnetic radiation absorber. Detailed understanding of these properties is needed in a variety of fields, including biophysics, climate science, remote sensing, and microwave engineering.

Concept

A dielectric is a product that has low electrical conductivity but can hold an electrical charge (due to Dielectric polarization). As a result of exhibiting only displacement current, it is suitable for constructing a capacitor, which stores and returns electrical energy. A substance's dielectric constant is established as that measure of the substance's permittivity to the permittivity of free space.

Let there be parallel plates facing towards each-other and put close to each-other without touching. Then dielectric is represented as C₀ = Q/V, where Q is the charge and V is the potential difference. When a dielectric material is inserted between two plates, it allows it to accumulate more charge on it.

Expression for Dielectric

Dielectric of a material is given by following expression

$$\kappa =\frac{\epsilon }{{\epsilon }_o}$$

Where $\kappa$ is the dielectric constant, $\epsilon$ is the permittivity of the material and ${\epsilon }_o$ is the permittivity of the free space.

Note: Permittivity of free space is the capability of vacuum to allow electric fields. And can be related with electric field and capacitance.

Factors affecting Dielectric Constant

Frequency: One of the considerations influencing the dielectric constant is the frequency of the applied voltage. The value of the dielectric constant becomes non-linear as the frequency of the applied voltage increases.

Applied voltage: Whenever a direct current voltage is generated, the value of the dielectric constant decreases, whereas an alternating current voltage increases the amount of the dielectric constant.

Temperature: Whenever the temperature is low, it is hard to organize the molecules in a dielectric substance. As the temperature rises, dipoles in dielectric materials become more dominant, resulting in an increase in the dielectric constant. These were referred to as the transition temperature. As the temperature increases past the transition temperature, the dielectric constant gradually decreases.

Heating effect: Dielectric loss occurs as the dielectric material is heated. Dielectric loss is known as the dispersion of energy in the form of heat as molecules in a substance shift when subjected to an alternating current voltage. This occurs as the substance consumes electrical energy.

Material and ambience effect: The dielectric constant is also affected by the material's composition and morphology. The dielectric constant is also affected by substrate deterioration and weathering.

Polarization of Dielectric Materials

Polarization of dielectric material is described as the process of creating electrical dipoles within the dielectric by varying the applied electric field. There are following types of polarization mechanisms Electronic polarization, Orientationally polarization, Ionic polarization and Space-charge polarization

Classification and Usage

They are of two types- active and passive dielectric. It is based on the energy storage capability of various materials. The distinction amongst active and passive dielectrics would be that active dielectrics are dielectrics that adjust readily for the storage of electrical energy, while passive dielectrics are dielectrics that limit the storage of electrical energy. Glass is an example of a passive dielectric, while piezoelectric is an example of an active dielectric.

Usage in capacitor

This is a critical parameter for characterizing a capacitor. A capacitor is an electrical device used to store electric power. This is commonly constructed by sandwiching a dielectric insulating layer between two metal conducting plates. The dielectric property is crucial to the operation of a capacitor. The dielectric material layer determines how well the capacitor can hold energy. It is critical to choose the proper dielectric material. As a result, it could also be described as the ratio of the electric field without the need for a dielectric (E_o) to the net field with such a dielectric (E). In the given expression E_o is always larger or equivalent to E.

$$\kappa =\frac{E_o}{E}$$

Dielectric Breakdown

Dielectric behaves as a perfect insulator up to a certain limit of applied electric field, once we cross that limit of electric field, dielectric begins to conduct and it is said to have reached a breakdown point.

There are three types of breakdown namely, Avalanche breakdown, thermal breakdown and defect breakdown.

Avalanche breakdown

When a dielectric is exposed to strong electric fields, electrons in the valence band gain enough energy to surpass the broad energy barrier gap and become enthralled by the conduction band in a high electric field, mobile electrons are strongly accelerated, and collisions excite more electrons to the conduction band. As a consequence, an increasing number of electrons are released into the conduction band, resulting in an explosion of conduction electrons. The substance eventually becomes extremely conductive and is said to have entered the breakdown region.

Thermal breakdown

When a high frequency alternating current field is applied to a dielectric, energy leakage occurs, and this energy must be dissipated as heat energy. if dissipation is ineffective due to low thermal conductivity. Because of the dielectric's conductivity, the material heats up and can trigger the dielectric to melt. This is referred to as thermal breakdown.

Defect breakdown

This form of breakdown occurs in dielectric materials with defects such as cracks and pores. Gases can accumulate in cracks and pores. At high electric fields, there is a local electric field at minor cracks, and he pores would be so large that gas discharge will occur, resulting in breakdown of a dielectric

Problem

Question: Calculate permittivity, if the dielectric constant of water is 100.

Solution: The dielectric constant is given by

$$\kappa =\frac{\epsilon }{{\epsilon }_o}$$

$$\kappa =\frac{\epsilon }{8.85*{10}^{-12}}$$

$\epsilon$ = $8.85*{10}^{-10}$

Solving we have, permittivity of medium is $8.85*{10}^{-10}.$

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for      

• Bachelor of Science in Physics
• Master of Science in Physics    
• M.Tech