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.

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Basic Knowledge of Polarisation of Dielectrics

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Important Questions

Two identical charged spheres are suspended by strings of equal lengths. The strings make an angle of with each other. When suspended in a liquid of density 0.8 , the angle remains the same. lf density of the material of the sphere is 1.6 , the dielectric constant of the liquid is:

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Define dielectric constant of a medium. What is its . unit?

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Dielectric constant for a metal is:

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What are polar and non polar dielectric ,give one example each other ?Define di-electric constants.

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Define the relative permittivity of a medium.

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The ratio of the forces between two small spheres with constant charges, in air and in a medium of dielectric constant , is

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The dielectric constant of an insulator can be

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small drops of mercury each of radius and change coalesce to from a big drop. The ratio of the surface charge density of each small drop with that of big drop is:

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Answer carefully:

(a) Two large conducting spheres carrying charges and are brought close to each other. Is the magnitude of electrostatic force between them exactly given by , where is the distance between their centres?

(b) If Coulomb's law involved 1/ dependence (instead of 1/),would Gauss' law be still true ?

(c) A small test charge is released at rest at a point in an electrostatic field configuration. Will it travel along the field line passing through that point?

(d) What is the work done by the field of a nucleus in a complete circular orbit of the electron? What if the orbit is elliptical?

(e) We know that electric field is discontinuous across the surface of a charged conductor. Is electric potential also discontinuous there?

(f) What meaning would you give to the capacitance of a single conductor?

(g) Guess a possible reason why water has a much greater dielectric constant (= 80) than say, mica (= 6).

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A plane electromagnetic wave in a non magnetic dielectric medium is given by with distance being in meter and time in seconds. The dielectric constant of the medium is:

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