Explore Detailed Notes for Dielectrics and Polarisation Class 12 Physics

A dielectric is an insulating material. It doesn’t conduct electricity, but can respond to an electric field. 

This material behaves just like any insulator that restricts free electron movement. But you'll find bound charges in it. The electrons and nuclei of the bound charges go through subtle and slight shifts relative to their positions, when the material comes in contact with an electric field. 

Polarisation explains the physical behaviour of how dielectrics respond to an electric field.

This field is external to the medium, here. If we consider the external field, and it’s applied to a dielectric, read what happens below. 

• The negative charges, ie., the electrons, get pulled slightly to the opposite side of the field. 
• Positive charges residing in the nuclei move along the field, ever so slightly. 

So, since you can figure, there’s a separation happening here. The outcome of this separation leads to the creation of electric dipoles within the dielectric.

Further, each dipole is also contributing to small dipole moments.

The combined effect of tiny dipole moments in Class 12 Physics is kind of essential to understand the shift from the microscopic world to the macroscopic world. That is where the concept of polarisation comes in. 

The small dipole moments create a net dipole moment per unit volume inside a dielectric, and that’s polarisation.  

Polarisation, P = Dipole moment per unit volume

This basic formula of polarisation gives us the calculation of the total electric dipole moment that’s spread inside a single unit of volume, or in other words, we get to know how strong the polarisation is in a dielectric.
To learn more about the electric polarisation in dielectrics, you need to continue reading below. 

Not all dielectric materials have electric dipole moments inside them. There are two categories to understand at the molecular level about the polarised or non-polarised nature of such insulators when introduced to an electric field. 

Non-Polarised Dielectric Materials 

Non-polar dielectrics have no molecules that create electric dipole moment permanently on their own. It means that the positive and negative charges of these substances stay together unless there is an external electric field to which they are introduced.
Common examples of non-dielectrics are Oxygen, Nitrogen, Hydrogen, and Carbon Dioxide.

So how do they create dipole moments?

You have to introduce them to an external electric field to see the effects below.

• The centres of positive and negative charges shift 
• The dipole moment created is temporary until the external field exists
• Only then does the material become polarised
  

Mathematically, this relationship is that the induced dipole moment per molecule is in direct proportion to the electric field. 

Polarised Dielectric Materials 

Polar dielectrics are those insulating substances that have permanent dipole moments in random orientations, without an external field required to make them polarised.

Water or H2O and sulphur dioxide or SO2 are popular examples you can think of. The net dipole moment is zero, as the orientations are random.

When there’s no electric field,

Net dipole moment of a polarised dielectric = 0

So, what would happen if you add this type of polarising material to an external field?

• The dipoles align perfectly along the electric field.
• Net polarisation is achieved, while more dipoles align and less of them oppose the field’s force.

We saw above that not all types of dielectric materials have permanent electric dipole moments. You should also know that not every dielectric will behave at the microscopic level in the same way, even when they are already categorised into polar and non-polar dielectrics.
Briefly, you can review this table outlining three types of polarisation. 

Type of Polarisation	How Polarisation Happens	Example of types of polarisation
Electronic Polarisation	When the electron clouds (negative charges) start to distort around the nuclei	These are your non-polar atoms like He, Ar
Ionic Polarisation	When the negative and positive ions see some shift or displacement at a relative scale	NaCl, KBr, etc. are pretty well-known in this ionic polarisation type
Orientational (Dipolar) Polarisation	Permanent dipoles align with the field	Water, HCl are common examples

One important aspect to know when talking about the mechanism of any of these three types of polarisation. The goal is to oppose the electric field, at a microscopic scale. That ends up reducing the overall net electric field inside the dielectric material. 

With the types of dielectrics and polarisations covered, you should look into how you can apply physics and maths behind how matter responds to an electric field at the microscopic level.

Now, for simplicity’s sake or for the ease of calculations, we take a dielectric medium as a simple material such as air or glass where the dipole moment induced is along the electric field. 

These are known as linear isotropic dielectrics, better known as uniform dielectric materials.

• What does linear mean? The polarisation increases directly in proportion to the electric field, meaning that if the electric field increases, polarisation increases. P ∝ E. 
• What’s Isotropic in linear isotropic dielectrics? In every direction of the electric field, the response of the field is the same. 

The 2.37 equation no. from your NCERT book gives this expression to measure polarisation density of the material when placed in an electric field on Page 66.

P = ε_0𝜒_eE

The chi or 𝜒 symbol is the electric susceptibility, which is a dimensionless constant or proportionality that’s introduced to measure the density of polarisation in a dielectric medium when electric field is applied. 

We also see the epsilon naught (ε_0) here. That’s the permittivity of the medium, telling us how much empty space the material itself can allow or support electric field lines to exist.

When a dielectric is placed in a field, the charges don’t behave the same way. Some are bound to the material, while others roam free.

One analogy to think here would be bringing a charged sphere near a plastic ruler. This charged sphere has free electrons. While the plastic ruler is neutral and a dielectric, it will, for the time being, experience polarisation.

The part of the ruler near the sphere will be negatively charged, while the other part that’s away will have positive charges.

These charges have a temporal shift in positions to balance the electric field.

Which of these charges are free to move, and which ones are bound between the sphere and the dielectric material?

This gives us two types of charges:

• Free Charges: These are charges in the external field. In the case of the sphere that’s charged, it creates the electric field. 
• Bound Charges: These are the charges already inside the dielectric. They can have small displacements as long as they are placed in the electric field. Using the same example from above, you can say that the plastic ruler has bound charges. 

For CBSE boards, you can learn these properties of dielectrics. 

1. Dielectric materials do not conduct electricity on their own. They do not have free charge carriers. 
2. Electric field inside a dielectric is never truly zero. It’s only reduced due to internal polarisation. 
3. Dielectric gets polarised only when placed in an electric field, where the molecules and atoms shift in opposite directions. 
4. When polarisation occurs in dielectrics, it leads to the creation of bound charges, which are not free to move. 
5. The direction of polarisation under the influence of electric field, depends whether the charges are positive or negative. Polarisation direction in a dielectric goes along the electric field direction if they are positive charges. For negative charges, the direction of polarisation is opposite to the field.