Dielectrics and Polarisation
Dielectrics are non-conducting substances, they have no charge carriers or no free electrons.
If an external field is applied, it turns out that charges are induced on the surface which in turn produces a field and opposes the external field.
The opposing field does not exactly cancel the external field but only reduces it. Dielectrics are insulators, plain and simple.
The two words refer to the same class of materials, but are of different origin and are used preferentially in different contexts.
Applications
Dielectrics in capacitors serve three purposes
- To keep the conducting plates from coming in contact, allowing for smaller plate separations and therefore higher capacitances;
- To increase the effective capacitance by reducing the electric field strength, which means you get the same charge at a lower voltage;
- To reduce the possibility of shorting out by sparking (more formally known as dielectric breakdown) during operation at high voltage.
To understand this concept, one has to look into the charge distribution of a dielectric in its molecular level. The molecules of a dielectric are classified as polar or non-polar.
The "Polar molecules" is one in which the centre of gravity of protons (positive charges) and electrons (negative charges) do not coincide. They have an asymmetrical distribution of charge and have permanent dipole moments in the range 10-30 cm.
Examples : H2O, CO2, NO2 etc.
Types of dielectrics
1. Non-Polar Dielectrics
When the centre of positive charge coincides with the centre of negative charge in a molecule. For example - Nitrogen, Oxygen, CO2 etc.
2. Polar Dielectrics
When the centre of positive and negative charges do not coincide because of the asymmetric shape of the molecules. For example - NH3, HCL etc.
Polarisation
In both cases (polar or non-polar) a dielectric develops a net dipole moment in the presence of an external field. The dipole moment per unit volume is called polarisation and is denoted by P.
For linear isotropic dielectrics
P = XeE
The constant characteristic of the dielectric, which is known as the electric susceptibility of the dielectric medium. It is possible to relate to the molecular properties of the substance.