Tag: physics

Questions Related to physics

The dielectric constant $k$ of a medium can be defined as $k=M/N$, where $N$ is the capacity of a parallel plate capacitor. When the space between plates is filled with air, $M$ is ?

dielectric substances and polarization dielectrics and polarisation electrostatic potential and capacitance electrostatics physics

  1. the charge of the capacitor

  2. the capacity of a parallel plate capacitor with a dielectric between the plates

  3. the dielectric intensity in the space between the plates

  4. the P.D. across the plates of the condenser, with a dielectric between them

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Correct Option: C

$64$ 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 substances and polarization dielectrics and polarisation electrostatic potential and capacitance electrostatics physics

  1. $4:1$

  2. $1:4$

  3. $1:64$

  4. $64:1$

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Correct Option: B
Explanation:
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 $1:4$

The force of attraction between two charges separated by certain distance in air is F1. If the space between the charges is completely filled with dielectric of constant 4 the force becomes F2. If half of the distance between the charges is filled with same dielectric the force between the charges is F3. Find F1:F2:F3 is

dielectric substances and polarization dielectrics and polarisation electrostatic potential and capacitance electrostatics physics

  1. 16 : 9 : 4

  2. 9 : 36 : 16

  3. 4 : 1 : 2

  4. 36 : 9 :16

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Correct Option: A

Water possesses a high dielectric constant, therefore

dielectric substances and polarization dielectrics and polarisation electrostatic potential and capacitance electrostatics physics

  1. it always contains ions

  2. it is an universal solvent

  3. can dissolve covalent compounds

  4. can conduct electricity

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Correct Option: A

Two charges placed at a distance experience a force of x N.The force between the charges when each charge is doubled, distance between them is halved and charge placed in a medium of dielectric constant 2.52 is.

dielectric substances and polarization dielectrics and polarisation electrostatic potential and capacitance electrostatics physics

  1. 634 X

  2. 63.4 X

  3. 40.3 X

  4. 0.403 X

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Correct Option: C

Two fixed charges separated by a distance $d$ experience a force $F$. A dielectric medium of thickness $\dfrac{d}{4}$ and dielectric constant $4$ is introduced in the space between them. Find the new force between the charges.

dielectric substances and polarization dielectrics and polarisation electrostatic potential and capacitance electrostatics physics

  1. $\dfrac{F}{4}$

  2. $\dfrac{F}{3}$

  3. $F$

  4. $\dfrac{16F}{25}$

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Correct Option: A
Explanation:
The force between the 2 charged particles is inversely proportional to the permittivity.

Therefore if the permittivity increases by 4 times, then obviously the force decreases by 4 times.

Therefore the new force is given by, $\dfrac{F}{4}$

The relative permittivity of water is 81. If $\varepsilon _0$ and $\varepsilon$ are permittivities of vaccum and water respectively Then:

dielectric substances and polarization dielectrics and polarisation electrostatic potential and capacitance electrostatics physics

  1. $\varepsilon _0 = 9\varepsilon _w$

  2. $\varepsilon _0= 81\varepsilon _w$

  3. $\varepsilon _w = 9\varepsilon _0$

  4. $\varepsilon _w = 81\varepsilon _0$

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Correct Option: D
Explanation:

The relative permittivity of water id defined as:

$\varepsilon _{rel}=\dfrac{\varepsilon _w}{\varepsilon _0}$

Substituting the values:
$81=\dfrac{\varepsilon _w}{\varepsilon _0}$

So, $\varepsilon _w=81\ \varepsilon _0$

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

dielectric substances and polarization dielectrics and polarisation electrostatic potential and capacitance electrostatics physics

  1. 2

  2. 1

  3. 4

  4. 3

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Correct Option: A

The capacitance of an air filled parallel plate capacitor is $8 pF$. The separation between the plates is doubled and the space between the plates is then filled with wax giving the capacitance a new value of $80 \times 10^{-12}$ farads. 
The dielectric constant of wax is

dielectric substances and polarization dielectrics and polarisation electrostatic potential and capacitance electrostatics physics

  1. $5.0$

  2. $10.0$

  3. $8.0$

  4. $4.2$

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Correct Option: A

The dielectric strength of air is nearly ___ V/mm

dielectric substances and polarization dielectrics and polarisation electrostatic potential and capacitance electrostatics physics

  1. 1

  2. infinite

  3. 1000

  4. 3000

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Correct Option: A