📘 Chapter 26: Dielectrics and Polarisation (Class XII)

🔷 1. Introduction

In electrostatics, materials that do not conduct electricity but respond to electric fields are known as dielectrics. When placed in an electric field, these materials exhibit a phenomenon called polarisation.

This chapter explains how dielectrics behave and how internal charge rearrangement modifies the electric field.

🔷 2. Dielectrics

A dielectric is an insulating material that does not allow free movement of charges but can be polarized under the influence of an electric field.

🔹 Examples:

  • Glass
  • Mica
  • Plastic
  • Air

🔷 3. Polarisation

Polarisation is the process in which positive and negative charges inside a dielectric slightly separate when subjected to an electric field.

This creates tiny dipoles within the material.

📦 4. Important Results (Must Remember)

  • Dielectrics: Insulators that can be polarized
  • Polarisation: Separation of charges inside material
  • Polarisation vector: P = dipole moment per unit volume
  • Induced field opposes external field
  • Electric field reduces inside dielectric

🔷 5. Types of Polarisation

  • Electronic polarisation
  • Ionic polarisation
  • Orientation polarisation

Each type depends on structure of material.

🔷 6. Polarisation Vector

Polarisation is defined as:

P = Dipole moment / Volume

It represents strength of dipole formation inside material.

🔷 7. Effect on Electric Field

When dielectric is placed in external field:

  • Induced field is created
  • This field opposes external field
  • Net electric field reduces

E = E₀ − Einduced

🔷 8. Bound Charges in Dielectrics

Polarisation creates:

  • Surface bound charges
  • Volume bound charges

These charges are responsible for internal electric effects.

🔷 9. Dielectric Constant

Dielectric constant (K) is defined as:

K = E₀ / E

It shows how much field is reduced inside material.

🔷 10. Physical Interpretation

Dielectrics do not allow free movement of charges, but they respond to electric fields by creating internal dipoles. This reduces the effective field and stores energy.

Polarisation is the key mechanism behind dielectric behavior

🧠 11. Solved Conceptual Questions

🔹 Q1

What is dielectric?

Answer:

An insulating material that can be polarized.

🔹 Q2

What is polarisation?

Answer:

Separation of charges inside dielectric.

🔹 Q3

What happens to field inside dielectric?

Answer:

It decreases

🔹 Q4

What is dielectric constant?

Answer:

Ratio of external to internal field.

🔹 Q5

What type of charges appear in dielectric?

Answer:

Bound charges.

🔷 12. Advanced Conceptual Insight

Dielectric behavior is explained using molecular structure and electric dipole theory. It is fundamental to capacitor design, electromagnetic theory, and material science.

Polarisation plays a key role in energy storage and field modification.

🔷 13. Applications

  • Capacitors
  • Insulation systems
  • Electronic devices
  • Energy storage systems

🔷 14. Summary

Dielectrics are insulating materials that become polarized in electric fields. This polarization reduces the internal field and plays a crucial role in electrostatics and electronics.

✨ End of Chapter 26: Dielectrics and Polarisation ✨