Q.Write the definition of dielectric constant of a medium.
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A dielectric is an insulating material whose molecules develop a net dipole moment (POLARISATION) when placed in an external field. POLAR molecules (like water) already have a permanent dipole moment due to their asymmetric shape, which an applied field tends to ALIGN; NON-POLAR molecules (like CO2) have zero dipole moment normally, but an applied field INDUCES a temporary dipole by pulling their positive and negative charge centres apart. …
When a dielectric medium is placed between a capacitor's plates in place of vacuum, it becomes polarised and reduces the net field, which raises the capacitance — the dielectric constant is the standard measure of exactly how many times greater the medium's permittivity (or the resulting capacitance) is compared to vacuum's. …
Dielectric constant tells us how many times the permittivity (or capacitance) of a medium is compared to that of free space/vacuum.
When a dielectric (insulating) medium is placed between the plates of a capacitor instead of vacuum, its capacitance increases because the dielectric gets polarised and reduces the net electric field between the plates.
The dielectric constant (also called relative permittivity), denoted K or εr, of a medium is defined as:
K=ε0ε
where ε is the absolute permittivity of the medium and ε0 is the permittivity of free space (8.85×10−12F/m).
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Showing the 12 most recent of 15 on this concept.
- CBSE 2026Set A1 markMCQQ.When a dielectric is placed in an external electric field, the effective electric field inside the dielectric (A) Decreases (B) Increases (C) Becomes zero (D) Remains same
›Reveal solutionSolution
A dielectric polarises and creates an opposing internal field, so the net field falls to E₀/K.
When a dielectric is placed in an external field E0, its molecules polarise, developing bound surface charges. These produce an internal field Ep opposite to E0.
The net (effective) field inside becomes …
- CBSE 2026Set A1 markMCQQ.Dielectric constant of water is (A) 80 (B) 1 (C) 0 (D) ∞
›Reveal solutionSolution
Water's dielectric constant is about 80.
The dielectric constant (relative permittivity) κ=ε/ε0 measures how much a medium reduces the electric field between charges compared with vacuum. Water is highly polar, so it is a strong dielectric with κ≈80. This is why Coulomb forces between ions are …
- CBSE 2026Set ANNUAL1 markMCQQ.In the following dielectric materials are(a) Conductor(b) Super conductor(c) Semiconductor(d) Non-conductor
›Reveal solutionSolution
A dielectric is an insulating (non-conducting) material whose molecules get polarised in an external electric field.
Dielectric materials have no free (mobile) charge carriers; under an external electric field their bound charges only shift slightly, creating induced dipoles (polarisation), but no …
- CBSE 2026Set ANNUAL1 markQ.The maximum electric field that a dielectric medium can withstand without break-down is called its ______.
›Reveal solutionSolution
This maximum field a dielectric can sustain before it breaks down (starts conducting) is called its dielectric strength.
Every insulating (dielectric) material can withstand only up to a certain electric field; beyond that value, the material's bound electrons are ripped free, it starts to conduct, and the insulating property fails (electrical breakdown). This ma …
- CBSE 2026Set ANNUAL1 markQ.What is the dielectric constant of a metal?
›Reveal solutionSolution
A metal behaves as a conductor with zero internal electric field for any applied field, which is the limiting case K -> infinity in the dielectric-constant formula.
The dielectric constant (relative permittivity) K of a material is defined by K = E0/E, where E0 is the field applied in vacuum and E is the reduced field actually present inside the material once it polarises to oppose the applied field. In a conductor (like a metal), free electrons redistribute instantly so that the NET electric field inside the bulk of the conductor is always exactly zero, however strong the external field E0 is. Putting …
- CBSE 2025Set A1 markMCQQ.Dielectric constant of air —(i) Infinity(ii) Zero(iii) One(iv) Two
›Reveal solutionSolution
Dielectric constant is defined relative to vacuum/air, so air's own dielectric constant is (very nearly) exactly 1.
The dielectric constant (relative permittivity) K of a medium is defined as K = ε/ε₀, the ratio of the permittivity of the medium to the permittivity of free space (vacuum). Since air is very close to vacuum electrically, its permittivity ε_air ≈ ε₀, giving K_air = ε₀/ε₀ = 1 (strictly about 1.0006, taken as 1 for all practical/board purposes). This means air, l …
- CBSE 2024Set A1 markMCQQ.The dielectric constant of a metal is (A) -1 (B) 0 (C) 1 (D) infinity
›Reveal solutionSolution
A conductor allows charges to move freely, reducing the internal field to zero → dielectric constant → ∞.
The dielectric constant K = E₀/E, where E is the field inside the medium and E₀ the applied field. In a conductor the free charges rearrange so that the internal field E becomes zero.
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- CBSE 2024Set A1 markMCQQ.The electric dipole moment per unit volume of a substance is called -(a) Electric field(b) Polarisation(c) Potential(d) Electric capacitance
›Reveal solutionSolution
Polarisation P is the dipole moment per unit volume induced in a dielectric.
When a dielectric (insulating) material is placed in an external electric field, the field displaces the positive and negative charge centres of each molecule, creating induced dipole moments (or aligning already-present permanent dipoles). The net dipole moment developed per unit volume of the material is called the polarisation, denoted P.
For a linear isotropic dielectric, P=χeE, where χe is the electric susceptibility of the medium and E is the applied field.
- Electric field (option a) is force per unit charge, not a dipole-moment density. …
- CBSE 2024Set ANNUAL1 markMCQQ.The value of dielectric strength for air is -(a) 3 x 10^6 V/m(b) 3 x 10^8 V/m(c) Zero(d) Infinity
›Reveal solutionSolution
Dielectric strength is the maximum electric field a dielectric can withstand without electrical breakdown; for air this standard value is 3 × 10^6 V/m.
Dielectric strength is defined as the maximum electric field that an insulating (dielectric) medium can tolerate without its molecules ionising and the medium starting to conduct (breaking down). Every dielectric has a characteristic breakdown field. For dry air at atmospheric pressure, this value is a stan …
- CBSE 2024Set ANNUAL1 markMCQQ.Dielectric constant for a metal is(a) zero(b) infinite(c) 1(d) 10
›Reveal solutionSolution
Dielectric constant K measures how much a material reduces an external field inside it; inside a conductor the net field is always zero, which corresponds to K -> infinity.
For a dielectric placed in an external field E0, the field inside reduces to E=E0/K, where K is the dielectric constant. In a conductor (like a metal), free electrons rearrange themselves so as to make the net electric field inside exactly zero in electrostatic equilibrium, i.e. E=0 fo …
- CBSE 2023Set BS1 markMCQQ.The dielectric constant (k) of silver is:i) k=0ii) k=+1iii) k=−1iv) k=∞ (infinity)
›Reveal solutionSolution
A conductor behaves like a medium of infinite dielectric constant, so for silver k=∞.
The dielectric constant k=EE0, where E0 is the applied field and E is the net field inside the material. In a conductor such as silver, free electrons rearrange themselves until the field inside becomes zero, i.e …
- CBSE 2022Set I1 markMCQQ.Dielectric strength for air is (A) 3 × 10^6 Vm^-1 (B) 4 × 10^6 Vm^-1 (C) 5 × 10^6 Vm^-1 (D) 10^6 Vm^-1
›Reveal solutionSolution
Dielectric strength of air ≈3×106 V/m.
The dielectric strength of a medium is the maximum electric field it can withstand before it breaks down and starts to conduct (sparks/discharge). For dry air at standard conditions this value is about
3×106 V/m=3 MV/m=30 kV/cm.
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