Definition: Dielectric is insulating (non-conducting) materials which transmits electric effect without conducting we know that in every molecule, there is a positively charged atoms and a negatively charged electron cloud surrounding it. The two different charged sections have their own centers of particle. The centre of positive atom is the centre of mass of positively charged protons in the center. The centre of negative atom is the centre of mass of negatively charge in the molecules.
Dielectrics are of two types:
(I) Polar dielectrics
(II) Non-Polar dielectric
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(I) POLAR DIELECTRICS
Like water, Alcohol, CO2, HCl, NH3 etc. are created of polar atoms/molecules.
In polar atoms when no electric field is applied centre of positive charges does not coincide with the centre of negative charges.
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A polar molecule has permanent electric dipole moment in the absence of electric space also. But a scalar dielectric has net dipole moment is zero in the absence of electric field because polar molecules are randomly oriented as shown in figure.
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In the result of electric space polar molecules tends to line up in the direction of electric field, and the substance has finite dipole moment.
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(II) NON-POLAR DIELECTRIC
Like N2, O2, Methane, Benzene etc. are build of non-polar atoms/molecules. In non-polar particles, when no electric field is applied the centre of positive charge coincides with the centre of negative charge in the particle. Every atom has zero dipole moment in its normal state.
When electric field is applied, positive charge experiences a force in the direction of electric field and negative charge experiences a force in the direction opposite to the field i.e., molecules converts induced electric dipole.
The total surface is decreased in that part hence If E = Original electric field and E' = Reduced electric field. Then 
where K is called dielectric constant K is also known as relative permittivity of the material or SIC (Specific Inductive Capacitance)
The value of K is always bigger than one. For vacuum there is no polarization and hence
E = E' and K = 1.
Field due to charge on one plate on the other is E = σ/2εo, hence the field force F = QE
The energy stored in a capacitor is not localized on the charges or the plates but is distributed in the space. A as in part of a parallel plate capacitor field is only between the plates i.e. in a volume, the so called energy density.
Hence Energy density = 
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