Dielectrics and polarisation

What is a dielectric?

In plain words: a metal has free electrons that can run to its surface and cancel a field inside it completely. A dielectric has none to spare. Its charges can only shift a little within each molecule, or the molecules can turn. Each molecule becomes a small dipole, and together they weaken the field inside the material.

Conductor and dielectric in a field

• Conductor: the free charge carriers move until the field of the induced charges cancels the external field inside. In the static situation the net field in the conductor is zero (NCERT page 65; see free charges in conductors).
• Dielectric: charges cannot move freely. Instead the external field induces dipole moments by stretching or re-orienting the molecules. The combined effect of all the molecular dipoles is a net charge on the surfaces of the dielectric, whose field opposes the external field (page 65).
• Unlike in a conductor, this opposing field does not cancel the external field; it only reduces it. How much depends on the nature of the dielectric (page 65).

Polar and non-polar molecules

• Non-polar molecule: the centres of positive and negative charge coincide, so it has no permanent (intrinsic) dipole moment. Oxygen (O₂) and hydrogen (H₂) are examples: their symmetry gives them no dipole moment (NCERT page 65).
• Polar molecule: the two centres are separated even with no external field, so it has a permanent dipole moment. NCERT’s examples are HCl and water (H₂O) (page 65; see dipoles in molecules).
• A note on NCERT’s wording: NCERT calls HCl “an ionic molecule” (page 65). HCl is better described as a polar covalent molecule: hydrogen and chlorine share a pair of electrons, pulled towards the more electronegative chlorine, as in HF (NCERT Chemistry Part I (Class 11), Section 4.3.6, pages 110–111; see polarity of bonds). Its bond is only partly ionic, unlike an ionic compound such as NaCl. It is polar, as NCERT says.

Polarisation and electric susceptibility

• Non-polar dielectric in a field: the positive and negative charges of each molecule are pushed in opposite directions. They stop when the external force is balanced by the restoring force of the molecule’s internal fields. Each molecule now has an induced dipole moment, and the dielectric is polarised (NCERT page 66).
• Linear isotropic dielectrics: NCERT takes only the simple case in which the induced dipole moment is along the field and proportional to its strength. Materials for which this holds are called linear isotropic dielectrics (page 66).
• Polar dielectric in a field: with no field, thermal agitation points the permanent dipoles in random directions, so the total dipole moment is zero. A field makes them tend to line up with it, giving a net dipole moment along the field. How far they line up depends on two opposing effects: the dipole’s potential energy in the field, which tends to align it, and thermal energy, which disrupts the alignment. Induced dipoles may add to this, but for polar molecules alignment is generally the larger effect (page 66).

Either way, polar or non-polar, the dielectric gains a net dipole moment along the field. NCERT names this quantity:

The dipole moment per unit volume is called polarisation and is denoted by P.

NCERT Physics Part I (Class 12), Section 2.10, page 66

For a linear isotropic dielectric, E is the total macroscopic field inside the material, not the field E₀ of the external sources alone (NCERT Eq. (2.37), page 66):

• χ_e, the electric susceptibility, is a constant characteristic of the dielectric. It can be related to the molecular properties of the substance, which NCERT does not pursue (page 66).
• Units: polarisation is a dipole moment per unit volume, C m ÷ m³ = C m⁻² (NCERT table, page 78). ε₀ is in C² N⁻¹ m⁻² and E in N C⁻¹, so ε₀E is in C m⁻², the unit of P. So χ_e is a pure number.

Bound charges on a polarised slab

Field reduction below assumes fixed source charge. If a slab fills a capacitor held at fixed voltage, extra free charge flows from the battery and the field remains V/d.

NCERT takes a wide rectangular dielectric slab, neglecting edge effects, in a uniform external field E₀ parallel to two of its faces. The field polarises it uniformly (NCERT pages 66–67).

• Every small volume element Δv has a dipole moment PΔv along the field. Δv is macroscopically small but holds a very large number of molecular dipoles (page 67).
• Inside, Δv has a dipole moment but no net charge, because the positive end of one dipole sits next to the negative end of its neighbour (page 67).
• At the two faces normal to the field there is a net charge: the positive ends are left unneutralised at the right face and the negative ends at the left face. The slab acts like two charged surfaces with densities +σ_p and −σ_p (page 67).
• The field of these surface charges opposes E₀, so, with the external free-charge sources fixed, the total field in this slab is smaller than with no dielectric present (page 67).
• The charge ±σ_p comes from bound charges in the dielectric, not free charges (page 67). NCERT builds on this slab to show that a dielectric raises a capacitor’s capacitance (Section 2.13, pages 69–70; see capacitance with a dielectric).
• Exam note (beyond the NCERT text): combined with NCERT Section 2.13, σ_p = P (Example 3 below) gives the dielectric constant as K = 1 + χ_e.

Solved examples

Common mistakes to avoid

Quick revision summary

• Dielectrics are non-conductors with no (or negligibly few) charge carriers; a field stretches or turns their molecules into dipoles.
• Non-polar molecules (O_₂, H_₂) get induced dipoles; polar molecules (H_₂O, HCl) have permanent dipoles that line up against thermal agitation.
• Polarisation P is the dipole moment per unit volume, in C m^⁻²; for a linear isotropic dielectric P = ε_₀χ_ₑE, with χ_ₑ the electric susceptibility.
• A polarised slab carries bound charges ±σp on the faces normal to the field and none in the bulk; for uniform P, σp = P.
• The bound charges’ field opposes the external field, so the field inside a dielectric is reduced, not cancelled.

Questions students ask

What is the difference between a dielectric and an insulator?

A dielectric and an insulator are the same kind of material, a non-conductor with no or very few free charge carriers (NCERT page 65); the two names stress different behaviour. ‘Insulator’ is the word for a material that does not let charge flow, so that charge put on it stays where it is (NCERT Chapter 1, page 3), while ‘dielectric’ is used when we look at how it is polarised in an electric field and weakens the field inside it, as between the plates of a capacitor (NCERT pages 65–67). NCERT itself writes “the insulator (dielectric)” when it says what a capacitance depends on (page 68), and glass, plastic, paper and mica are both.

Why does water have such a high dielectric constant?

Water has a high dielectric constant mainly because its molecules are polar: each has a permanent dipole moment, 1.85 D, that a field can turn into line, besides the small induced dipole every molecule gets (NCERT pages 65–66; NCERT Chemistry Part I (Class 11), Table 4.5, page 112). NCERT Chemistry gives water’s value as about 80 (Unit 6, page 189); beyond the NCERT text, it is 78.5 at 25 °C, against only 2.23 for carbon tetrachloride, whose molecules have zero dipole moment (Table 4.5) and give only the induced effect (US National Bureau of Standards, Circular 514, 1951).

Are dielectrics and electric polarisation in the JEE Main and NEET syllabus?

Yes, dielectrics and electric polarisation are in both syllabuses: the JEE Main syllabus lists “Conductors and insulators, dielectrics and electric polarization, capacitors and capacitance” in Unit 11, Electrostatics, and the NEET (UG) 2026 syllabus lists “Dielectrics and electric polarization” in its own Unit 11. In NCERT Class 12 Physics Part I the topic is Section 2.10 (pages 65–67), and it leads to the effect of a dielectric on capacitance in Section 2.13 (pages 69–71). Neither syllabus names the electric susceptibility, but NCERT defines it through P = ε₀χ_eE (NCERT Eq. (2.37), page 66).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Why do O₂ and H₂ molecules have no permanent dipole moment?

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   Because of their symmetry, the centres of positive and negative charge coincide (NCERT page 65).

2. Q2. The field inside a linear isotropic dielectric with χ_e = 4.0 is 2.0 × 10⁴ V/m. Find the polarisation.

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   P = ε₀χ_eE = 8.854 × 10⁻¹² × 4.0 × 2.0 × 10⁴ ≈ 7.1 × 10⁻⁷ C m⁻² (NCERT Eq. (2.37), page 66).

3. Q3. What is the total charge on a polarised dielectric slab that was neutral to begin with?

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   Zero. The bulk has no net charge, and the face charges +σ_p and −σ_p are equal and opposite (NCERT page 67).