Dielectrics : Classification, Dielectric Polarisation and Dielectric Constant With Definition

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What is a Dielectric Material and What are its Classifications?

Now we discuss about the materials which do not conduct electricity and can be inserted between the plates of a capacitor. The substances which have no free electrons cannot conduct electricity. They are called insulators or dielectrics. When they are placed in an electric field, charges are induced on their surfaces.

Classification of Dielectrics

Dielectrics are classified into two groups according to the posi-tion of charge within their molecules:

1. non-polar substance and
2. polar substance.

Non-polar substance: A substance in which the centre of negative charges (electrons) coincides with that of positive charges (protons) in each of its molecules, is called a non-polar substance [Fig.].

In the absence of external electric field, these molecules do not possess any permanent electric dipole moment. Thus they are called non-polar molecules.

In the presence of an external electric field, a relative displace-ment occurs between the centres of positive and negative charge distributions [Fig.], Thus a non-polar molecule when sub-jected to an electric field, acquires an electric dipole moment. These types of dipoles are called induced dipoles.

Example: Methane (CH₄).

Polar substance: A substance in which the centre of neg-ative charges (electrons) does not coincide with that of positive charges (protons) in each of its molecules, is called a polar sub-stance [Fig.],

So, even in the absence of external electric field, each of these molecules possesses a permanent electric dipole moment. So they are called polar molecules.
Example: water (H₂O), ammonia (NH₃).

Generally the dipole moments of different molecules of a polar substance are randomly directed. So the resultant dipole moment of a polar substance is zero. But when subjected to an electric field, each molecule of a polar substance experiences a torque and tends to fall in line with the direction of field lines of the external electric field [Fig.]. As a result, the sample of the polar substance acquires a resultant electric dipole moment.

So polar and non polar molecules behave in a similar manner when subjected to an external electric field.

Polarisation of a Dielectric

A conductor in an external electric field: If a conductor is placed in an external electric field the free electrons of the conductor orient themselves in a direction opposite to the electric field.

This transfer continues until finally the induced electric field balances the external electric field. In that case, no further displacement of charges takes place i.e., an equilibrium has been reached. So the resultant electric field (\(\vec{E}\)) inside a conductor is zero [Fig.].

A dielectric in an electric field: If a dielectric is placed in an external electric field (\(\vec{E}_0\)), the dipoles align themselves along the lines of force. So an electric field (\(\vec{E}_p\)) is generated inside the dielectric whose direction is opposite to that of the applied external field (\(\vec{E}_0\)). \(\vec{E}_p\) is less than \(\vec{E}_0\). As a dielectric has no free electrons, the external field (\(\vec{E}_0\)) is not completely balanced by the internal field (\(\vec{E}_p\)) set up inside the dielectric. So at any point inside a dielectric the resultant intensity (\(\vec{E}\)) is less than the external field intensity (\(\vec{E}_0\)) but it does not become zero as in a conductor [Fig.(c)],

The alignment of the molecules of a dielectric, which behave like electric dipoles under the influence of an external field, is known as electric polarisation. It is observed that, one face of the dielectric acquires a net positive charge and the other, negative. This is because the charges in between the two dotted lines [Fig.(b)] neutralise each other’s effect, thus leaving unbalanced negative charge on the left face and positive charge on the right face of the dielectric.

In Fig.(a), the random arrangement of the molecules of a dielectric has been shown in absence of any external electric field. In Fig. (b), alignment of the molecules along the field lines under the influence of the external field has been shown. Fig.(c) shows that, inside a dielectric, electric field intensity reduces due to electric polarisation. The resultant intensity of the electric field inside the dielectric is given by \(\vec{E}\) = \(\vec{E}_0\) – \(\vec{E}_p\).
The electric polarisation is directly proportional to the resul-tant electric field in the dielectric.

Dielectric Constant

The capability of storing charge of a capacitor, i.e., its capaci-tance can be increased by using suitable dielectric substance between its two plates. For example, air, paraffin, glass, sulphur, mica, paper, etc. are the dielectric substances used as intervening medium in a parallel plate capacitor. The increase in the capacitance of a capacitor depends on a property of dielectric materials, termed as dielectric constant (re).

Definition: Dielectric constant of a material is the ratio of the capacitance of a capacitor filled with the given dielectric material to the capacitance of a similar capacitor without any medium.

So, dielectric constant,

Dielectric constant is also known as specific inductive capacity (SIC).
The capacitor without any medium between its two plates has only vacuum between the plates.

By the statement that dielectric constant of glass is 8.5 , we mean that the capacitance of a capacitor will increase 8.5 times if glass is used as dielectric instead of vacuum. Naturally, the dielectric constant of vacuum is 1 . The dielectric constant of dry air is 1.000586(≈1).