Dielectrics Polarization

8 MCQs9-step worked example
Source: NCERT ElectrostaticsOfficial key: NTA-verifiedLast updated: 10 Oct 2026

Try this first

Which pair consists of polar molecules?
  1. A.O₂ and H₂
  2. B.H₂ and HCl
  3. C.O₂ and H₂O
  4. D.HCl and H₂O
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Answer: D. Based on NCERT Class 12 Physics, Chapter 2, page 65: HCl and H₂O are polar, with separated centres of positive and negative charge; O₂ and H₂ are non-polar.

A is wrong: A is wrong because O₂ and H₂ are the NCERT examples of non-polar molecules, whose symmetry gives no dipole moment.

B is wrong: B is wrong because H₂ is non-polar; only HCl in this pair is polar.

C is wrong: C is wrong because O₂ is non-polar; only H₂O in this pair is polar.

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Dielectrics Polarization, explained for NEET

The trap: treating a dielectric like a conductor. In a conductor the induced charges cancel the external field completely inside. In a dielectric the induced field opposes the external one but only reduces it. NCERT Class 12 Physics, Chapter 2, page 65 says so directly, and a student who writes a net field of zero inside a dielectric has used the conductor rule.

What a dielectric is. Dielectrics are non-conducting substances with no, or a negligible number of, charge carriers (page 65).

Two kinds of molecule (page 65). In a non-polar molecule the centres of positive and negative charge coincide, so there is no permanent dipole moment; O₂ and H₂ are examples. In a polar molecule the centres are separated even with no field, so it has a permanent dipole moment; HCl and H₂O are examples.

Two routes to the same result (page 66):

  • Non-polar: the field displaces positive and negative charges in opposite directions until the restoring force balances it. The molecule develops an induced dipole moment and the dielectric is polarised.
  • Polar: with no field the permanent dipoles point in random directions because of thermal agitation, so the total is zero. A field tends to align them, and a net dipole moment appears along the field.

In either case a dielectric develops a net dipole moment in an external field (page 66).

Polarisation. The dipole moment per unit volume is called polarisation, P (page 66). Its unit follows: C m ÷ m³ = C m⁻². For linear isotropic dielectrics, P = ε₀ χₑ E, where χₑ is the electric susceptibility (page 66).

Bound charges. In a polarised slab each interior volume element has no net charge, but the surfaces normal to the field carry induced charge ±σ_p, which is bound, not free. These surface charges are what reduce the field (page 67).

Permittivity. The dimensionless ratio K = ε/ε₀ is the dielectric constant (page 70).

Bridge to NEET. Questions ask which molecules are polar, what P means, and how to compute P or ε. Distractors drop ε₀ or use K as if it were ε.

Watch out: the field inside a dielectric is reduced, not zero.

How do you solve a Dielectrics Polarization question? A worked example

  1. 1

    Given

    A linear isotropic dielectric slab with electric susceptibility χₑ = 3.0 and volume 2.0 × 10⁻⁶ m³ is in a uniform field of 5.0 × 10³ N/C. Take ε₀ = 8.85 × 10⁻¹² C² N⁻¹ m⁻².

  2. 2

    Required

    (a) The polarisation P. (b) The total dipole moment of the slab.

  3. 3

    Concept

    For a linear isotropic dielectric, P = ε₀ χₑ E, and polarisation is the dipole moment per unit volume, so the total moment is P times the volume (NCERT Class 12 Physics, Chapter 2, page 66).

  4. 4

    Formula

    P = ε₀ χₑ E; p_total = P × volume.

  5. 5

    Substitution

    (a) P = 8.85 × 10⁻¹² × 3.0 × 5.0 × 10³
    (b) p_total = P × 2.0 × 10⁻⁶

  6. 6

    Calculation

    (a) P = 1.3275 × 10⁻⁷ ≈ 1.3 × 10⁻⁷ C m⁻²
    (b) p_total = 1.3275 × 10⁻⁷ × 2.0 × 10⁻⁶ = 2.655 × 10⁻¹³ ≈ 2.7 × 10⁻¹³ C m

    The unrounded value of P is used in part (b) so that rounding does not accumulate. The numbers given carry two significant figures, except ε₀, which carries three.

  7. 7

    Final answer

    P = 1.3 × 10⁻⁷ C m⁻²; total dipole moment = 2.7 × 10⁻¹³ C m.

  8. 8

    Common trap

    Leaving out ε₀ and writing P = χₑE = 3.0 × 5.0 × 10³ = 1.5 × 10⁴. That number is χₑE, which does not have the units of a dipole moment per volume. The correct value carries the factor ε₀ and is of order 10⁻⁷ C m⁻².

  9. 9

    Similar NEET-style question

    A linear isotropic dielectric with χₑ = 4.0 is in a field of 2.0 × 10³ N/C. What is its polarisation? (Answer: P = ε₀ χₑ E, with χₑE = (4.0 × 2.0) × 10³ = 8.0 × 10³ N/C, so P = 8.85 × 10⁻¹² × 8.0 × 10³ = 7.08 × 10⁻⁸ ≈ 7.1 × 10⁻⁸ C m⁻².)

    ---

Can you answer these Dielectrics Polarization MCQs?

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

Which pair consists of polar molecules?

Show answer and why every option is right or wrong

Answer: D. Based on NCERT Class 12 Physics, Chapter 2, page 65: HCl and H₂O are polar, with separated centres of positive and negative charge; O₂ and H₂ are non-polar.

Why A is wrong: A is wrong because O₂ and H₂ are the NCERT examples of non-polar molecules, whose symmetry gives no dipole moment.

Why B is wrong: B is wrong because H₂ is non-polar; only HCl in this pair is polar.

Why C is wrong: C is wrong because O₂ is non-polar; only H₂O in this pair is polar.

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MCQ 2Easy RecallPractice

The dipole moment per unit volume of a dielectric is called:

Show answer and why every option is right or wrong

Answer: A. Based on NCERT Class 12 Physics, Chapter 2, page 66: the dipole moment per unit volume is called polarisation, denoted by P.

Why B is wrong: B is wrong because the susceptibility χₑ is the constant in P = ε₀ χₑ E, characteristic of the medium; it is not itself a moment per volume.

Why C is wrong: C is wrong because permittivity ε relates to how the medium responds in K = ε/ε₀; it is not a dipole moment per volume.

Why D is wrong: D is wrong because the dielectric constant K = ε/ε₀ is a dimensionless ratio.

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MCQ 3Easy RecallPractice

When a dielectric is placed in an external field, the field produced by the induced charges:

Show answer and why every option is right or wrong

Answer: C. Based on NCERT Class 12 Physics, Chapter 2, page 65: the field due to induced charges opposes the external field, but in a dielectric it only reduces it; it does not cancel it.

Why A is wrong: A is wrong because the induced field opposes the external one.

Why B is wrong: B is wrong because exact cancellation is the property of a conductor, whose free carriers can move without limit; a dielectric has no such carriers.

Why D is wrong: D is wrong because the dielectric does have bound charges, and their displacement (polarisation) produces a field.

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MCQ 4Direct ApplicationPractice

For a linear isotropic dielectric with χₑ = 2.0, the field is 1.0 × 10⁴ N/C. Take ε₀ = 8.85 × 10⁻¹² C² N⁻¹ m⁻². The polarisation is:

Show answer and why every option is right or wrong

Answer: B. P = ε₀ χₑ E = 8.85 × 10⁻¹² × 2.0 × 1.0 × 10⁴ = 1.77 × 10⁻⁷ ≈ 1.8 × 10⁻⁷ C m⁻² (NCERT Class 12 Physics, Chapter 2, page 66).

Why A is wrong: A is wrong because 8.9 × 10⁻⁸ is ε₀E alone; it leaves out the factor χₑ.

Why C is wrong: C is wrong because 2.0 × 10⁴ is χₑE, which drops the factor ε₀ and so is not a polarisation in these units.

Why D is wrong: D is wrong because 2.7 × 10⁻⁷ comes from adding 1 to χₑ before multiplying; the formula uses χₑ itself.

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MCQ 5Direct ApplicationPractice

A dielectric has K = 5.0. Take ε₀ = 8.85 × 10⁻¹² C² N⁻¹ m⁻². Its permittivity ε is:

Show answer and why every option is right or wrong

Answer: A. K = ε/ε₀, so ε = Kε₀ = 5.0 × 8.85 × 10⁻¹² = 4.4 × 10⁻¹¹ C² N⁻¹ m⁻² (NCERT Class 12 Physics, Chapter 2, page 70).

Why B is wrong: B is wrong because 5.0 is K itself; it is dimensionless and is not the permittivity in C² N⁻¹ m⁻².

Why C is wrong: C is wrong because 1.8 × 10⁻¹² is ε₀/K, dividing where K must multiply ε₀.

Why D is wrong: D is wrong because 4.4 × 10⁻¹³ is the correct value with the power of ten slipped by two.

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MCQ 6Direct ApplicationPractice

A sample of 1.0 × 10²² polar molecules, each with a permanent dipole moment of 6.0 × 10⁻³⁰ C m, is in a region with no external field at room temperature. Its total dipole moment is:

Show answer and why every option is right or wrong

Answer: D. Based on NCERT Class 12 Physics, Chapter 2, page 66: without a field the permanent dipoles are oriented randomly because of thermal agitation, so the total dipole moment is zero.

Why A is wrong: A is wrong because 6.0 × 10⁻⁸ is the number of molecules times one moment, which would need every dipole to point the same way.

Why B is wrong: B is wrong because 3.0 × 10⁻⁸ takes half of that sum; the random orientation gives a total of zero, not half.

Why C is wrong: C is wrong because 6.0 × 10⁻³⁰ C m is the moment of one molecule, not of the sample.

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MCQ 7CalculationPractice

A linear isotropic dielectric slab with χₑ = 1.0 is in a uniform field of 2.0 × 10⁴ N/C. The slab has volume 4.0 × 10⁻⁶ m³. Take ε₀ = 8.85 × 10⁻¹² C² N⁻¹ m⁻². The total dipole moment of the slab is:

Show answer and why every option is right or wrong

Answer: B. P = ε₀ χₑ E = 8.85 × 10⁻¹² × 1.0 × 2.0 × 10⁴ = 1.77 × 10⁻⁷ C m⁻². Total moment = P × volume = 1.77 × 10⁻⁷ × 4.0 × 10⁻⁶ = 7.1 × 10⁻¹³ C m (NCERT Class 12 Physics, Chapter 2, page 66).

Why A is wrong: A is wrong because 1.8 × 10⁻⁷ is only the polarisation, the moment per unit volume; it still has to be multiplied by the volume.

Why C is wrong: C is wrong because 7.1 × 10⁻⁷ takes the volume as 4.0 (m³ number with the 10⁻⁶ dropped, as if it were in cm³).

Why D is wrong: D is wrong because 4.4 × 10⁻² is P divided by the volume, dividing where the volume must multiply.

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MCQ 8CalculationPractice

A dielectric is placed in an external field of 8.0 × 10³ N/C, and the net field inside it is found to be 5.0 × 10³ N/C. By what fraction does the dielectric reduce the external field?

Show answer and why every option is right or wrong

Answer: C. The induced field is 8.0 × 10³ − 5.0 × 10³ = 3.0 × 10³ N/C, which opposes the external field and only reduces it (NCERT Class 12 Physics, Chapter 2, page 65). The fractional reduction is 3.0 / 8.0 = 0.375 ≈ 0.38.

Why A is wrong: A is wrong because 0.63 is the fraction that remains, 5.0 / 8.0, not the fraction removed.

Why B is wrong: B is wrong because 1.6 is 8.0 / 5.0, the factor by which the external field exceeds the net field.

Why D is wrong: D is wrong because a reduction of 1.0 would mean complete cancellation, which is the conductor result; in a dielectric the field is only reduced.

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What to remember before solving Dielectrics Polarization questions

8 NCERT lines
Key Fact

Dielectrics are non-conducting; the induced field opposes the external field but only reduces it

Dielectrics are non-conducting substances. In contrast to conductors, they have no (or negligible number of ) charge carriers. Recall from Section 2.9 what happens when a conductor is placed in an external electric field. The free charge carriers move and charge distribution in the conductor adjusts itself in such a way that the electric field due to induced charges opposes the external field within the conductor. This happens until, in the static situation, the two fields cancel each other and the net electrostatic field in the conductor is zero. In a dielectric, this free movement of charges is not possible. It turns out that the external field induces dipole moment by stretching or re-orienting molecules of the dielectric. The collective effect of all the molecular dipole moments is net charges on the surface of the dielectric which produce a field that opposes the external field. Unlike in a conductor, however, the opposing field so induced does not exactly cancel the external field. It only reduces it.

-- NCERT Class 12 Physics, Ch. 2, p. 65
Key Fact

Non-polar molecules (O2, H2) have coincident centres, no permanent dipole; polar molecules (HCl, H2O) do

The molecules of a substance may be polar or non-polar. In a non-polar molecule, the centres of positive and negative charges coincide. The molecule then has no permanent (or intrinsic) dipole moment. Examples of non-polar molecules are oxygen (O2) and hydrogen (H2) molecules which, because of their symmetry, have no dipole moment. On the other hand, a polar molecule is one in which the centres of positive and negative charges are separated (even when there is no external field). Such molecules have a permanent dipole moment. An ionic molecule such as HCl or a molecule of water (H2O) are examples of polar molecules.

-- NCERT Class 12 Physics, Ch. 2, p. 65
Key Fact

A non-polar molecule develops an induced dipole moment; the dielectric is polarised

In an external electric field, the positive and negative charges of a non- polar molecule 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 (due to internal fields in the molecule). The non-polar molecule thus develops an induced dipole moment. The dielectric is said to be polarised by the external field.

-- NCERT Class 12 Physics, Ch. 2, p. 66
Key Fact

Polar molecules: random orientation (zero total) until the field aligns them; alignment vs thermal agitation

A dielectric with polar molecules also develops a net dipole moment in an external field, but for a different reason. In the absence of any external field, the different permanent dipoles are oriented randomly due to thermal agitation; so the total dipole moment is zero. When an external field is applied, the individual dipole moments tend to align with the field. When summed overall the molecules, there is then a net dipole moment in the direction of the external field, i.e., the dielectric is polarised.

-- NCERT Class 12 Physics, Ch. 2, p. 66
Key Fact

A polarised slab has induced surface charges +-sigma_p (bound, not free) and no volume charge; they reduce the field

Anywhere inside the dielectric, the volume element Dv has no net charge (though it has net dipole moment). This is, because, the positive charge of one dipole sits close to the negative charge of the adjacent dipole. However, at the surfaces of the dielectric normal to the electric field, there is evidently a net charge density. As seen in Fig 2.23, the positive ends of the dipoles remain unneutralised at the right surface and the negative ends at the left surface. The unbalanced charges are the induced charges due to the external field. Thus, the polarised dielectric is equivalent to two charged surfaces with induced surface charge densities, say sp and –sp. Clearly, the field produced by these surface charges opposes the external field. The total field in the dielectric is, thereby, reduced from the case when no dielectric is present. We should note that the surface charge density ±sp arises from bound (not free charges) in the dielectric.

-- NCERT Class 12 Physics, Ch. 2, p. 67

Dielectrics Polarization: NEET previous year questions (PYQs) with answers

25 questions in Electrostatics

No question in our NEET 2020–2025 set targets this topic directly.

All 25 past-paper questions from Electrostatics →

More in Electrostatics: 3 exam traps and mistakes · 11 formulas · 6 question patterns from its other lessons.

Sources

NCERT refs: Class 12 Physics Chapter 2, p.65

Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.

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