Dielectrics and Electric Polarization

Dielectrics

Dielectrics are non-conducting substances. In contrast to conductors, they have no or the negligible number of charge carriers. In a dielectric, free movement of charges is not possible. It turns out that the external field induces dipole moment by stretching or re-orienting molecules of the dielectric. The collective effect of all the molecular dipole moments is net charges on the surface of the dielectric which produce a field that opposes the external field.

The difference in behaviour of a conductor and a dielectric in an external electric field.

Types of dielectrics

Dielectrics can be classified as:

· Polar Molecules

· Non-Polar Molecules

Polar Molecules:

Polar Molecules are those type of dielectric in which the possibilities that the positive and negative molecules will coincide with each other is null or zero. In other words, When the centre of positive and negative charges do not coincide because of the asymmetric shape of the molecules, e.g., NH_3,HCl, etc.

Non-Polar Molecule:

Unlike polar molecules in non-polar molecules the center of positive charge and negative coincide, that is it is not zero. The molecule then has no permanent (or intrinsic) dipole moment. When the centre of positive charge coincides with the centre of negative charge in a molecule, e.g., Nitrogen, Oxygen, CO₂ etc.

The non-polar molecule thus develops an induced dipole moment. The dielectric is said to be polarized by the external field. Substances for which this assumption is true are called linear isotropic dielectrics. The induced dipole moments of different molecules add up giving a net dipole moment of the dielectric in the presence of the external field.

The non-polar molecule thus develops an induced dipole moment. The dielectric is said to be polarized by the external field. Substances for which this assumption is true are called linear isotropic dielectrics. The induced dipole moments of different molecules add up giving a net dipole moment of the dielectric in the presence of the external field.

Induced Electric Dipole Moment

When in a non-polar molecule, all the protons are pulled in the direction as of electric field and electrons are pulled in opposite direction as of electric field, when an external electric field is applied. Due to the presence of electric field, this process continues unless the internal forces balance them. Due to this two centers of charge are created; the molecules are known as Polarized and is known as Induced Electric Dipole. The dipole moment is known as Induced Electric Dipole Moment.

Electric Polarisation

When a dielectric slab is placed in an electric field, then the dipole moment is gained by the molecule and the dielectric is said to be polarised.

The Electric Polarization is dipole moment per unit volume of a dielectric material.

The polarization is denoted by P.

Polarization

Polarization process

Dielectric Constant

When Dielectric slab is placed between parallel plate, the ratio of the applied electric field strength to the strength of the reduced value of electric field capacitor is called Dielectric Constant that is:

E is always less than or equal to E_.

where E_o is dielectric and E is net field

The larger the dielectric constant, the more charge can be stored. Completely filling the space between capacitor plates with a dielectric increases the capacitance by a factor of the dielectric constant:

C = κ C_o

where C_o is the capacitance with no dielectric between the plates.

Dielectric Polarization

Dielectric polarization occurs when a dipole moment is formed in an insulating material because of an externally applied electric field. When a current interacts with a dielectric (insulating) material, the dielectric material will respond with a shift in charge distribution with the positive charges aligning with the electric field and the negative charges aligning against it. By taking advantage of this response, important circuit elements such as capacitors can be made.