---
title: "Define electric dipole and polarization"
subject: "Electromagnetism"
chapter: 2
key: "semester_3/electromagnetism:define-dipole-and-polarization"
source: https://www.bctengineer.com/q/semester_3/electromagnetism/define-dipole-and-polarization
site: BCT Engineering
---

# Define electric dipole and polarization

Electromagnetism · Chapter 2: Electric field

- Asked in: 2070 Chaitra, 2 marks
- Video: [Polarization and Dipoles](https://www.youtube.com/watch?v=KSJZ72BI6uY) — ASCPhysicsAndAstronomy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electromagnetism/define-dipole-and-polarization
- Whole chapter: https://www.bctengineer.com/md/semester_3/electromagnetism/chapter-2

## Question

Define electric dipole and polarization.

## Short answer

An electric dipole consists of two equal opposite charges ±Q separated by a small distance d; its dipole moment is p = Qd (C·m), directed from −Q to +Q. Polarization P is the dipole moment per unit volume: P = lim Δv→0 (Σp)/Δv, and in linear dielectrics P = ε₀(εᵣ−1)E.

## Answer

#### Electric dipole
An **electric dipole** consists of two equal and opposite point charges, +Q and −Q, separated by a small distance d. Its **dipole moment** is the vector

```text
p = Q d   (C·m), directed from −Q to +Q
```
Far from the dipole, the potential is V = p cosθ/(4πε₀r²) and the field falls off as 1/r³. A dipole placed in a field E experiences a torque T = p × E.

#### Polarization
**Polarization** P is the dipole moment per unit volume of a dielectric:

```text
P = lim(Δv→0) (Σ pᵢ)/Δv    (C/m²)
```
When an electric field is applied to a dielectric, its atoms or molecules form (or align) electric dipoles, so the material becomes polarized. For a linear, isotropic dielectric,

```text
P = ε₀ χ_e E = ε₀(ε_r − 1)E
D = ε₀E + P = ε₀ε_r E
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. An electric dipole consists of:
   A. Two equal like charges
   B. One charge and a conductor
   C. Two unequal charges far apart
   D. Two equal and opposite charges separated by a small distance
   Answer: D. Equal and opposite charges +Q and −Q, separated by d. (Key point: Dipole definition)

2. Polarization is:
   A. Charge per unit volume
   B. Force per unit charge
   C. Dipole moment per unit volume
   D. Dipole moment per unit area
   Answer: C. P = lim Σp/Δv, C/m². (Key point: Polarization definition)

3. For a linear isotropic dielectric:
   A. P = ε₀ε_r E
   B. P = ε₀(ε_r − 1)E and D = ε₀E + P
   C. P = D/ε₀
   D. D = P − ε₀E
   Answer: B. P = ε₀χ_e E and D = ε₀E + P = ε₀ε_r E. (Key point: Relation)

## Related in Electromagnetism

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- [Polarization and E in both regions](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Adielectric-boundary-x-polarization-e) — Electromagnetism, chapter 2

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