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Dielectrics-Polar and Non-polar Dielectric Molecules

Physics

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Dielectric

Dielectrics are non-conducting substances which are the insulating materials and are bad conductor of electric current.
Mica, Plastics, Glass, Porcelain and Various Metal Oxides and even dry air is also example of dielectric.

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Induced surface charge density and polarization in dielectric

Induced surface charge density is given by:

where 
Polarisation
Unit vector along the outward normal surface

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Dielectric constant

The ratio of permittivity of a medium to the permittivity of free space is known as the dielectric constant or relative permittivity of the medium. It is denoted by or .

Note:
where for free space

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Dielectric Constant

n parallel plate capacitor, when dielectric slab is placed between the two plates then 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:

K = Dielectric constant

E = Net Field

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Effect of dielectric on capacitance

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 = KC'
Where C= New capacitance after introduction of dielectric
C' = C
apacitance with no dielectric between the plates.

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Polar dielectric

If in any dielectric substance the centre of positive charge and centre of negative charge of each molecule do not coincide, then such type of dielectric material is called polar dielectric.
Ex : water and HCl 

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Non-polar dielectric

If in a substance the centre of positive charge and centre of negative charge of each molecule coincide with each other, because of symmetry of molecules, then these substances are called non-polar dielectric substances.
Ex: etc.