Polarisation — Definition

Definition

Imagine you have a material that doesn't conduct electricity very well, like plastic, glass, or paper. These materials are called dielectrics. When you place a dielectric material in an electric field, something interesting happens at the molecular level. Even though charges can't flow freely like in a conductor, they can still shift slightly or reorient themselves.

Dielectric materials are made up of molecules. These molecules can be broadly classified into two types:

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Non-polar molecules — These molecules, like oxygen ($O_2$), nitrogen ($N_2$), or methane ($CH_4$), have their centers of positive and negative charge coinciding in the absence of an external electric field. This means they don't have a permanent electric dipole moment. Think of them as perfectly balanced spheres of charge. However, when an external electric field is applied, the positive charges (nuclei) are pulled slightly in the direction of the field, and the negative charges (electrons) are pulled slightly in the opposite direction. This separation of charges creates an induced electric dipole moment in each molecule. The molecule effectively becomes a tiny, temporary dipole, aligned with the external field.

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Polar molecules — These molecules, like water ($H_2O$), carbon monoxide ($CO$), or hydrochloric acid ($HCl$), already possess a permanent electric dipole moment even in the absence of an external electric field. This is because their molecular structure is such that the centers of positive and negative charge do not coincide naturally. They are like tiny, pre-existing magnets with a positive and negative end. In the absence of an external field, these permanent dipoles are randomly oriented due to thermal agitation, so their net dipole moment averages out to zero for the entire material. However, when an external electric field is applied, these permanent dipoles experience a torque that tries to align them with the direction of the field. They don't move completely freely, but they do tend to orient themselves along the field lines.

In both cases (non-polar and polar molecules), the result is a collective alignment or induction of electric dipoles within the dielectric material. This phenomenon is called polarisation. When the dipoles align, their positive ends face one way and their negative ends face the other, creating a layer of bound charges on the surfaces of the dielectric.

These bound charges, in turn, create an internal electric field within the dielectric that opposes the external applied field. This opposing field effectively reduces the net electric field inside the dielectric material.

This reduction in the electric field is why dielectrics are used in capacitors – they allow the capacitor to store more charge for a given potential difference.