Dielectrics and Polarisation MCQ Quiz in తెలుగు - Objective Question with Answer for Dielectrics and Polarisation - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

Interfacial polarization:

• Interfacial polarization is the phenomenon caused by the presence of impurities or fillers.
• It increases dielectric constant.
• Interfacial or space charge polarization occurs when there is an accumulation of charge at an interface between two materials or between two regions within a material because of an external field.
• This can occur when there is a compound dielectric, or when there are two electrodes connected to a dielectric material.
• Under the application of applied field some of defects may migrate through the material towards two electrodes that has opposite polarity to their charge in this phenomenon. 

CONCEPT:

Conductors and dielectrics

• A conductor has free electrons.
  • When a conductor is placed in an external electric field, these free electrons move and align themselves to create an induced charged density σ which produces an induced electric field such that the net field inside the conductor is zero. 
• Dielectrics are non-conducting substances. They do not contain free electrons.
  • When a dielectric is placed in an external electric field, its molecules align themselves in such a way that charges are induced on the surfaces of the dielectric with having charge density σp, and thus an electric field is created inside the dielectric.
  • This induced electric field Ein has a value less than that of the external electric field.
• Both these fields have opposite directions.
• Therefore the magnitude of the net electric field inside a dielectric is less than that of the external electric field.
• The direction of the net electric field inside the dielectric is the same as that of the external electric field.

EXPLANATION:

Polar and non-polar molecules

• In a non-polar molecule, the centers of positive and negative charges coincide.
  • The molecule then has no permanent dipole moment. 
• In a polar molecule, the centers of positive and negative charges are separated.
  • Such molecules have a permanent dipole moment.

In an external electric field

• In a nonpolar molecule,
  • The positive and negative charges are displaced in opposite directions.
  • The displacement stops when the external force on the constituent charges of the molecule is balanced by the restoring force
  • The non-polar molecule thus develops an induced dipole moment.
  • This dipole moment is in the direction that of the external electric field. 
  • The dielectric is said to be polarised by the external field.
• Therefore option 3 is correct.

Concept:

• Dielectric is a material that does not have a free charge carrier for conduction. They however contain positive and negative charges which are bound together. 
• They can be easily polarized which means charges inside the material will show response to an electric field applied externally.
• Dielectric materials are used to store energy. These materials exist in solid, liquid, and gaseous forms. Some examples of dielectric materials are:
  • Solid Dielectrics – Plastic, Ceramic, Glass, and Mica
  • Dielectric Liquid – Distilled Water.
  • Dielectric Gas – vacuum, nitrogen, Dry Air,  and helium.

• Polarizability- It is the tendency of an atom's electron cloud to be distorted from its normal shape by the application of an external electric field.

• 

• It is denoted by\(\alpha\).
• When we placed Dielectric in an external field E, the positive and negative charges are displaced from their equilibrium position as shown in the above diagram. This process occurs throughout the volume of the dielectric.
• This result in the formation of a large number of dipoles with each having some dipole moment along the direction of the electric field and
• The whole phenomenon is called polarization.
• The polarization vector is given as-

                          \(\vec{P}={{\varepsilon }_{0}}{{\chi }_{e}}\vec{E}\)

       Where,

• \({\vec{P}}\) is the polarization vector
• \({\vec{E}}\) is the electric field vector
• \({{\varepsilon }_{0}}\) is the permittivity of air and
• \({{\chi }_{e}}\) is susceptibility

Calculation:

Given-

\({{\chi }_{e}}=6\)

\(E=15\text{ N/C}\)

For air \({{\varepsilon }_{0}}=1 C^2/Nm^2\)

The polarization vector is given as-

  \(\vec{P}={{\varepsilon }_{0}}{{\chi }_{e}}\vec{E}\)..(i)

Where,

\({\vec{P}}\) is the polarization vector

\({\vec{E}}\) is the electric field vector

\({{\varepsilon }_{0}}\) is the permittivity of air and

\({{\chi }_{e}}\) is susceptibility

On substituting the given values in the above equation (i), we get

\(P=(1\times 6\times 15\text{ )C/}{{\text{m}}^{2}}=90\text{ C/}{{\text{m}}^{2}}\)

\(\therefore P=90\text{ C/}{{\text{m}}^{2}}\)

Hence, Option-4 is the correct answer.

Properties of Dielectric material

• The band energy gap in the dielectric materials is very large.
• The coefficient of temperature for resistance is negative.
• They have high resistivity.
• They have a strong attraction between electrons and the parent nucleus.
• They have a very low electrical conductivity of these materials.
• That's why they are basically electrical insulators.

Correct option-2

Concept:

Dielectric

• Dielectric is a material that does not have a free charge carrier for conduction. They however contain positive and negative charges which are bound together. 
• They can be easily polarized which means charges inside the material will show response to an electric field applied externally.
• In Dielectric material, the external field will compel the molecules to align the individual dipole moment along the direction of the applied field.
• They are used to store energy. These materials exist in solid, liquid, and gaseous forms. Some examples of dielectric materials are:
  • Solid Dielectrics – Plastic, Ceramic, Glass, and Mica
  • Dielectric Liquid – Distilled Water.
  • Dielectric Gas – vacuum, nitrogen, Dry Air,  and helium.

Explanation:

• According to the statement given in the question, dielectrics is a material that is a perfectly electrical insulator because it has a lack of free charges for conduction.
• Although they charge carriers but in a bounded form which can only show response to an electric field applied externally.
• In the case of a conductor, they have a lot of free charge carriers which makes electrical conduction better.
• Superconductors are materials that show perfect electrical conductivity at a very low temperature.

Hence, Option- 2 is the correct answer.

Properties of Dielectric material

• The band energy gap in the dielectric materials is very large.
• The coefficient of temperature for resistance is negative.
• They have high resistivity.
• They have a strong attraction between electrons and the parent nucleus.
• They have a very low electrical conductivity of these materials.
• That's why they are basically electrical insulators.

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. As the dielectric constant increases, the electric flux density increases, if all other factors remain unchanged. This enables objects of a given size, such as sets of metal plates, to hold their electric charge for long periods of time, and/or to hold large quantities of charge. Materials with high dielectric constants are useful in the manufacture of high - value capacitors.

CONCEPT:

Dielectric constant

• Dielectric is a material that has poor electrical conductivity but has the capability to retain or store electric charge in it.
  • The dielectric constant of a given material is defined as the ratio of the permittivity of the material (ϵ) to the permittivity of the free space (ϵ0).
  • This ratio is also known as the relative permittivity of the dielectric.
• The dielectric constant, 

\(⇒ K = \frac{ϵ}{ϵ_0}\)

• The relative permittivity of a dielectric, 

\(⇒ ε _r = \frac{ϵ}{ϵ_0}\)

• The dielectric constant K of a material is defined as

\(⇒ K=\frac {σ}{σ - σ_p}\)

where σ is the induced charge density in the case of a conductor and σp is the induced charge density in the case of a dielectric in the same external electric field.

• A dielectric develops a net dipole moment in the presence of an external field. The dipole moment per unit volume is called polarization and is denoted by P.

\(⇒ P= ε_0 χ_e E\)

where χe is a constant. It is the characteristic of the dielectric and is known as the electric susceptibility of the dielectric medium.

Electric susceptibility

• Electric susceptibility can also be expressed in terms of dielectric constant K as

\(⇒ χ_e= K-1\)

CALCULATION: 

Given: external electric field E = 1 Nm^-1 and dielectric constant K = 2, 

• A dielectric develops a net dipole moment in the presence of an external field. The dipole moment per unit volume is called polarization and is denoted by P.

​\(⇒ P= ε_0 χ_e E\)

\(⇒ P= ε_0 χ_e E = ε_0 (K-1) E = 8.85 \times 10^{-12} \times (2-1)\times 1= 8.85 \times 10^{-12} Cm^{-2}\)

• Therefore option 2 is correct

Concept:

• Polarization: Electronic materials have permanent electric dipole moments; the vector field expression of concentration of stable electric dipole moments is called the electric potential polarization.
• Polarization occurs when an electric field twists the negative cloud of electrons around the positive atomic nuclei in a direction reverses the field.

P = q d

\(\text{P}=\frac{\text{q }\!\!~\!\!\text{ d}}{\text{V}}=\frac{\text{q}}{\text{A}}\frac{\text{d}}{\text{d}}=\frac{\text{q}}{\text{A}}\)

• The SI unit of polarization is C / m². Generally, the area is very small so it can be taken as C / cm².

P = α E

Where α is a constant called the polarizability.

• The polarizability is determined experimentally and it depends on the properties of the atom.

Where P is polarization, q is a charge, V is volume, d is a dimension of material, and A is area.

Explanation:

• A dielectric material gets polarized when it is placed in an electric field.
• The field produces due to the polarization of material to minimize the effect of the external field. Hence, the electric field inside a dielectric decrease when it is placed in an external electric field.

P = α E

Where α is a constant called the polarizability.

So polarization is the linear function of the applied electric field.

CONCEPT:

Dielectric:

• A dielectric is a substance that does not allow the flow of charges through it but permits them to exert electrostatic forces on one another through it.
• A dielectric is essentially an insulator that can be polarised through the small localized displacement of charges.

EXPLANATION:

The main purpose of dielectrics in capacitors is as follows:

• Dielectrics are used to keep the conducting plates from coming in contact by allowing only a small separation between the plates. Therefore option 1 is correct.
• Dielectrics are used to increase effective capacitance by decreasing the electric field. Therefore option 2 is incorrect.
• Dielectrics are used to reduce the possibility of shorting out by sparking at high voltage. Therefore option 3 is incorrect.
• Dielectrics act like the little conducting bridge between the plates of a capacitor. Therefore option 4 is incorrect.

Concept:

When two dielectric slabs are inserted between the plates of a parallel plate capacitor, the total capacitance is determined based on whether they are connected in series or parallel.

Capacitance (C): The ability of a system to store charge per unit voltage.

Dielectric Constant (K): The factor by which the capacitance increases when a dielectric medium is introduced.

Series Combination: When different dielectrics are arranged in layers along the electric field, their capacitances add reciprocally.

Formula for Capacitors in Series:
1/C = 1/C₁ + 1/C₂

Calculation:

Given:

Dielectric constants: K₁ and K₂

Thickness of first slab: d/4

Thickness of second slab: 3d/4

Plate area: A

Permittivity of free space: ε₀

Capacitance of the first slab:

C₁ = (4K₁ ε₀ A) / d

Capacitance of the second slab:

C₂ = (4K₂ ε₀ A) / 3d

Since the slabs are in series, the total capacitance C is given by:

1/C = 1/C₁ + 1/C₂

Substituting the values:

1/C = d / (4K₁ ε₀ A) + 3d / (4K₂ ε₀ A)

Simplifying:

C = (4A ε₀ K₁ K₂) / (3K₁ + K₂) d

∴ The correct answer is (4A ε₀ K₁ K₂) / (3K₁ + K₂) d.

Concept:

• Polarization: Electronic materials have permanent electric dipole moments; the vector field expression of concentration of stable electric dipole moments is called the electric potential polarization.
• Polarization occurs when an electric field twists the negative cloud of electrons around the positive atomic nuclei in a direction reverses the field.

P = q d

\(\text{P}=\frac{\text{q }\!\!~\!\!\text{ d}}{\text{V}}=\frac{\text{q}}{\text{A}}\frac{\text{d}}{\text{d}}=\frac{\text{q}}{\text{A}}\)

• The SI unit of polarization is C / m². Generally, the area is very small so it can be taken as C / cm².

P = α E

Where α is a constant called the polarizability.

• The polarizability is determined experimentally and it depends on the properties of the atom.

Where P is polarization, q is a charge, V is volume, d is a dimension of material, and A is area.

Explanation:

• A dielectric material gets polarized when it is placed in an electric field.
• The field produces due to the polarization of material to minimize the effect of the external field. Hence, the electric field inside a dielectric decrease when it is placed in an external electric field.

P = α E

Where α is a constant called the polarizability.

So polarization is the linear function of the applied electric field.