Two identical charged spheres are suspended by strings of equal lengths. The strings make an angle of \$30^{0}\$ with each other. When suspended in a liquid of density 0.8 \$\mathrm{g}\mathrm{c}\mathrm{m}^{-3}\$, the angle remains the same. lf density of the material of the sphere is 1.6 \$\mathrm{g}\mathrm{c}\mathrm{m}^{-3}\$, the dielectric constant of the liquid is:

From fig,\$tan\theta= qE/mg\$\$tan15^o=\dfrac{k q^2}{d^2 mg}\$\$tan15^o=\dfrac{k q^2}{1.6 Vgd^2 }\$ .....[\$V\$ is the volume]...........(1)When system is suspended in liquid,\$tan15^o=\dfrac{k q^2}{K(mg- \rho Vg)d^2 }\$ \$tan15^o=\dfrac{k q^2}{K(1.6-0.8)Vgd^2 }\$ ......(2) from (1) and (2) we get,\$\dfrac{k q^2}{K(1.6-0.8)Vgd^2 }\$\$=\dfrac{k q^2}{1.6 Vgd^2 }\$ \$\therefore K=2=\$Dielectric constant of liquid.

A plane electromagnetic wave in a non magnetic dielectric medium is given by \$\bar{E} = \bar{E_0} ( 4 \times 10^{-7} \times - 50 t)\$ with distance being in meter and time in seconds. The dielectric constant of the medium is:

Given equation of wave \$\vec{E} = \vec{E}_0 (4 \times \vec{10} x - 50 t)\$Comparing with general equation of wave,\$\vec{E} = \vec{E}_0 (kx - wt),\$we get,\$w = 50\ rad/s.\$ \$k = 4 \times 10^{-7} m^{-1}\$Thus velocity of wave, \$v = \dfrac{w}{k} = \dfrac{50}{4 \times 10^{-7}} = 1.25 \times 10^8 m/s\$so, refractive index of medium,\$\mu = \dfrac{e}{v} = \dfrac{3 \times 10^8}{1.25 \times 10^8} = 2.4\$using, \$u^2 = km.ke\$ [km and ke are magnetic and dielectric di - constants]as medium is non diamagnetic, \$km = 1\$\$\Rightarrow ke = \mu^2 = (2.4)^2 \Rightarrow ke = 5.76\$

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>>Class 12
>>Physics
>>Electrostatic Potential and Capacitance
>>Dielectrics and Polarisation
>> $Basic Knowledge of Polarisa...$

Basic Knowledge of Polarisation of Dielectrics

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

NON POLAR DIELECTRICS:Are made of non-polar molecules.The center of mass of positive charges coincides with the center of mass of the negative charges, in the molecule.In its normal state (i.e., in the absence of any external force, esp electric field), each molecule has zero dipoles andhas a symmetric shape.POLAR DIELECTRICS:Are made of polar molecules.The center of mass of positive charges does not coincide with the center of mass of the negative charges, in the molecule.In its normal state (i.e., in the absence of any external force, esp electric field), each molecule has some intrinsic permanent dipole andhas an asymmetric shape.The dielectric constant is the ratio of the permittivity of a substance to the permittivity of free space. It is an expression of the extent to which a material concentrates electric flux, and is the electrical equivalent of relative magnetic permeability.

Q: Define dielectric constant of a medium. What is its \$S.I\$. unit?

The Dielectric constant (K) of the medium is the ratio of the permittivity of the substance (\varepsilon) to permittivity of the free space \$\left( \varepsilon _ { 0 } \right) .\$i.e \$\mathrm { K } = \varepsilon / \varepsilon _ { 0 }\$As Permittivity of medium and permittivity of free space both have same units(F/m ie Farad/meter) dielectric constant becomes dimensionless quantity.

Q: Define the relative permittivity of a medium.

The amount by which the force between the charge (in presence of medium) is relatively less than that in a vacuum, is called the dielectric constant of that medium or relative permittivity (\$\epsilon_r\$)\$\epsilon_r=\dfrac{\text{Force between charges in air or vaccum}}{\text{Force between charges in medium}}\$\$\therefore\,\,\varepsilon_r=\dfrac{\varepsilon}{\varepsilon_0}\$

Q: \$64\$ small drops of mercury each of radius \$r\$ and change \$q\$ coalesce to from a big drop. The ratio of the surface charge density of each small drop with that of big drop is:

Formulae:\$\dfrac{\sigma_{small} }{\sigma_{big} }=\dfrac{q}{Q} \times \dfrac{R^2}{r^2}\$\$=\dfrac{q}{nq}\times \dfrac{(n^{\frac{1}{3}}r)^2}{r^2}\$\$=n^{-\frac{1}{3}}\$\$=64^{-\frac{1}{3}}\$\$=\dfrac{1}{4}\$hence the ratio is \$1:4\$

Q: Dielectric constant for a metal is:

Permittivity of metals is very high comparable to the permittivity of free space. So dielectric constant for metal is infinite.

Q: Two parallel plate capacitor of capacitances C and 2C are connected in parallel and charged to a potential difference V . If the batteryis disconnected and the space between the plate of the capacitor of capacince C is completely filled with a material of dielectric constant K, then the potential difference across the capacitor will be come

The original capacitance of the parallel combination of C and 2 C is = C +2C = 3C Total charge Q = 3CV When the capacitor C is filled with a dielectric, its capacitance becomes KC. Therefore, after the capacitor is filled with dielectric the capacitance of the combination, C' = KC + 2C = C (K + 2). However, the charge remains the same, i.e. Q = 3CV Therefore, the potential difference across the capacitors will be

Physics

definition

Polarisation

When an electric field is applied there is small displacement of bound charges creating small electric dipoles within the dielectric. This phenomenon is called Polarization.
The polarisation of a region is defined as the dipole moment per unit volume.
For linear isotropic dielectrics, it is given by

P = χ_{e} E

where

χ_{e}

is the dielectric susceptibility of the dielectric medium

definition

Electric susceptibility

Polarisation in a dielectric is given by:

P = χ_{e} E

where

χ :

Electric susceptibility of the medium

E :

External electric field
Electric susceptibility is related to the molecular properties of the substance.
It is also given by:

χ = ε_{o} (K - 1)

where

K :

dielectric constant of the medium

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Important Questions

Two identical charged spheres are suspended by strings of equal lengths. The strings make an angle of $3 0^{0}$ with each other. When suspended in a liquid of density 0.8 $g c m^{- 3}$ , the angle remains the same. lf density of the material of the sphere is 1.6 $g c m^{- 3}$ , the dielectric constant of the liquid is:

Medium

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

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 $K$ , is

Hard

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$64$ small drops of mercury each of radius $r$ and change $q$ coalesce to from a big drop. The ratio of the surface charge density of each small drop with that of big drop is:

Medium

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Dielectric constant for a metal is:

Medium

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Two parallel plate capacitor of capacitances C and 2C are connected in parallel and charged to a potential difference V . If the battery
is disconnected and the space between the plate of the capacitor of capacince C is completely filled with a material of dielectric constant K, then the potential difference across the capacitor will be come

Easy

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A plane electromagnetic wave in a non magnetic dielectric medium is given by $\overset{ˉ}{E} = \overset{ˉ}{E_{0}} (4 \times 1 0^{- 7} \times - 50 t)$ with distance being in meter and time in seconds. The dielectric constant of the medium is:

Medium

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definition

Polarisation

definition

Electric susceptibility

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

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

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

Define the relative permittivity of a medium.

The dielectric constant K of an insulator can be

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

Dielectric constant for a metal is:

A plane electromagnetic wave in a non magnetic dielectric medium is given by Eˉ=E0​ˉ​(4×10−7×−50t) with distance being in meter and time in seconds. The dielectric constant of the medium is:

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

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

The dielectric constant $K$ of an insulator can be

$64$ small drops of mercury each of radius $r$ and change $q$ coalesce to from a big drop. The ratio of the surface charge density of each small drop with that of big drop is:

A plane electromagnetic wave in a non magnetic dielectric medium is given by $\overset{ˉ}{E} = \overset{ˉ}{E_{0}} (4 \times 1 0^{- 7} \times - 50 t)$ with distance being in meter and time in seconds. The dielectric constant of the medium is: