Dielectric and Polarisation: We all know about electric charges and electric fields in space but does the medium effects the strength of the electric field, or is it independent of the medium. The medium consists of molecules that have electrons and protons. How are these affected by the electric field, and on what factors does the strength of the electric field depend? Why does the strength of the electric field inside the bulk of the metal be zero in electrostatics? What does permittivity of space mean?

Let us read the given article to understand dielectric and polarisation better.

Dielectrics

Dielectrics is a substance that, when exposed to an electric field, gets polarised. The charges present in the molecules get away from each other due to the effect of the external electric field, thereby decreasing the strength of the electric field in that medium. In metals, we have electrons.

When a conductor is kept in an external electric field, the electrons experience a force due to the electric field, thereby creating a potential gradient inside the conductor and creating an induced electric field due to this gradient which is in the direction opposite to the external electric field. Due to this induced electric field, the total electric field in the bulk of the conductor is zero in the case of electrostatics

Learn About Factors Affecting Dielectric Polarisation

Types of Dielectrics

Any material, be it a gas, a liquid, or a solid, can be dielectric. Dielectric is classified according to the polarisation at a molecular level.

They are classified as,

1. Polar dielectrics
2. Non-polar dielectrics

Polar Dielectric

For this kind of dielectric, we have the inherent polarity in the molecule. In other words, the centre of mass of the positive element and the centre of mass of the negative element do not coincide. Therefore it has an inherent dipole moment, and when kept in an electric field, it aligns accordingly. Examples of such dielectric are \({\rm{H}}_{2} {\rm{O}},\,{\rm{C O}}_{2}\)

Non-polar Dielectric

For this kind of dielectric, there is no inherent polarity in the molecule. In other words, the centre of mass for the positive elements coincides with the centre of mass of the negative elements. Examples of such dielectrics are, \({{\rm{H}}_2},\,{{\rm{CH}}_4}\)

Effect of Dielectric

Dielectric increases the energy storing capacity of a capacitor. The permittivity of the space is \(\varepsilon_{0}\), whose value is \(8.854 \times 10^{-12}\,{\rm{m}}^{-3} {\rm{k g}}^{-1} {\rm{s}}^{4} {\rm{A}}^{2}\) or \(8.854 \times 10^{-12}\,\mathrm{Fm}^{-1}\) The permittivity for a dielectric is given as . The dielectric constant in terms of permittivity is given by,

\(K=\frac{\varepsilon}{\varepsilon_{0}}\)

Where,

\(\varepsilon\) is the dielectric for the dielectric.

\(\varepsilon_{0}\) is the dielectric of space.

So, the value of the electric field in a dielectric becomes,

\(E=\frac{E_{0}}{K}\)

Where,

\(E_{0}\) is the electric field in space.

\(K\) is the dielectric constant

Therefore, the value of the electric field decreases in a medium other than a vacuum.

Polarisation

When a substance is kept in an electric field. The positive and negative elements of the substance experience some force and due to which they develop a dipole or electric displacement. This reduces the value of the net electric field in the medium.

This phenomenon of electric displacement is known as electric polarisation. The extent of electric polarisation is directly proportional to the permittivity of the material or the value of the dielectric constant of the material. For the conductors, the value of the permittivity is infinity. Therefore the magnitude of the electric field in the conductor is zero in electrostatics.

Permittivity

Permittivity is the measure of the electric polarization of a substance when kept in an electric field. It can also be defined as the measure of the distortion in the atomic charge of that material in an electric field.

Capacitor with Dielectric

The capacitor is a device that is used to store energy. A Parallel plate capacitor is a capacitor that consists of two-conductor plates with opposite charges. The capacitance of the parallel plate capacitor is given by,

\(C=\frac{\varepsilon_{0} A}{d}\)

Where,

\(\varepsilon_{0}\) is the permittivity of the space.

\(d\) is the distance between the plates.

\(A\) is the area of the plates.

If we insert a dielectric in between the plates, then the capacitance of the parallel plate capacitor becomes,

\(C=\frac{K \varepsilon_{0} A}{d}\)

Where,

\(K\) is the dielectric constant of the medium in between the plates.

On comparing, we can see that the capacitance of the parallel plate capacitor has increased \(K\) times.

If the dielectric is partially filled in between the capacitor, then the equivalent capacitor can be considered as the combination of capacitors.

\(C=\frac{\varepsilon_{0} A}{d-\left(t_{1}+t_{2}+t_{3}\right)+\left(\frac{t_{1}}{K_{1}}+\frac{t_{2}}{K_{2}}+\frac{t_{3}}{K_{3}}\right)}\)

Sample Problems

1.

In the given figure, find the total capacitance.

Sol. The given capacitor can be two capacitors in series with plate area \(A\) and distance between the plates is \(\frac{d}{2}\)

The capacitance of the first capacitor is,

\(C_{1}=\frac{2 K_{1} \varepsilon_{0} A}{d}\)

The capacitance of the second capacitor,

\(C_{2}=\frac{2 K_{2} \varepsilon_{\theta} A}{d}\)

For capacitor in series is given by,

\(\frac{1}{C}=\frac{1}{C_{1}}+\frac{1}{C_{2}}\)

Putting in the values, we get,

\(C=\frac{\left(\frac{2 K_{2} \varepsilon_{0} A}{d}\right)\left(\frac{2 K_{1} \varepsilon_{0} A}{d}\right)}{\frac{2 K_{2} \varepsilon_{0} A}{d}+\frac{2 K_{1} \varepsilon_{0} A}{d}}=\frac{2 K_{1} K_{2} \varepsilon_{0} A}{d\left(K_{1}+K_{2}\right)}\)

Applications

Dielectrics are used in capacitors to increase the energy store in them.

Mineral oil, a dielectric, is used in the transformer as a coolant.

High permittivity dielectric materials are used in semiconductor devices.

Porcelain insulators are used for insulating power lines due to their high dielectric strength, as they can bear much higher voltage before the final dielectric breakdown occurs.

Summary

Electric field strength depends on the medium or the permittivity of the medium. Electric field strength is maximum for vacuum. Dielectrics is a substance that, when exposed to an electric field, gets polarised. That is, the charges present in the molecules get away from each other due to the effect of the external electric field, thereby decreasing the strength of the electric field in that medium.

Dielectrics are of two types, that is, polar and non-polar dielectric. Polar dielectrics have an inherent dipole moment that is the centre of mass of the negative and positive elements do not coincide, whereas, for non-polar dielectric, the centre of mass for negative and positive parts coincides. Polarisation is the phenomenon in which the electric charges get displaced when a material is put in an external electric field. The extent of polarization is directly proportional to the value of the dielectric constant. For a conductor, the permittivity is infinite. The value of electric field strength in a conductor is zero.

Frequently Asked Questions

Q.1. What is a dielectric substance?
Ans: Dielectrics is a substance that, when exposed to an electric field, gets polarised. That is, the charges present in the molecules get away from each other due to the effect of the external electric field, thereby decreasing the strength of the electric field in that medium.

Q.2. What is polarisation?
Ans: Polarisation is the phenomenon in which the electric charges get displaced when a material is put in an external electric field.

Q.3. What are two types of dielectrics?
Ans: The dielectric substance can be of two types.
Polar dielectric:
For this kind of dielectric, we have the inherent polarity in the molecule. In other words, the centre of mass of the positive element and the centre of mass of the negative element do not coincide. Therefore it has an inherent dipole moment, and when kept in an electric field, it aligns accordingly. Examples of such dielectric are \({\rm{H}}_{2} {\rm{O}},\,{\rm{C O}}_{2}\)
Non-polar dielectric:
For this kind of dielectric, there is no inherent polarity in the molecule. In other words, the centre of mass for the positive elements coincides with the centre of mass of the negative elements. Examples of such dielectrics are, \({{\rm{H}}_2},\,{\rm{CH}_4}\)

Q.4. What is the permittivity of a conductor?
Ans: The permittivity of a conductor is infinity. We can also verify this as the electric field strength inside of the conductor is always zero in the case of electrostatics.

Q.5. What is the dielectric breakdown?
Ans: Dielectric breakdown is a phenomenon in which the molecules of a dielectric substance get ionized due to the electric potential applied across it, and it starts to conduct electricity.

Q.6. How does the dielectric increase the energy stored by the capacitor?
Ans: The energy stored by the capacitor is given by,
\(E=\frac{1}{2} C V^{2}\)
Where,
\(C\) is the capacitance of the capacitor.
\(V\) is the voltage across the capacitor.
If we insert a dielectric inside a capacitor, then the capacitance will increase.
The capacitance of a parallel plate capacitor is given by,
\(C=\frac{\varepsilon_{0} A}{d}\)
Capacitance, when a dielectric is inserted in between the capacitance, is,
\(C=\frac{K \varepsilon_{0} A}{d}\)
Therefore, the capacitance becomes \(K\), times,
Thus, the energy stored in the capacitor becomes,
\(E_{d}=K\left(\frac{1}{2} C V^{2}\right)\)
Thus, the energy stored becomes \(K\), times.

Now you are provided with all the necessary information on the dielectric and polarisation and we hope this detailed article is helpful to you. If you have any queries regarding this article, please ping us through the comment section below and we will get back to you as soon as possible.