What is the net charge of an electric dipole?

Despite consisting of two charges, an electric dipole always has a net charge of zero. It is formed by two equal and opposite charges, $+q$ and $-q$. When these are summed, the total charge is $q + (-q) = 0$. This is a crucial point, as it differentiates a dipole's field behavior from that of a single net charge. While its net charge is zero, the separation of these charges means it still produces an external electric field and potential, albeit with a faster fall-off with distance compared to a monopole.

Why is the electric field of a dipole proportional to $1/r^3$ and not $1/r^2$?

The electric field of a single point charge is proportional to $1/r^2$. However, an electric dipole consists of two opposite charges. At a distant point, the electric fields due to the positive and negative charges are nearly equal in magnitude and point in slightly different directions. They tend to largely cancel each other out. The remaining net field is a small difference between two large fields, and this difference happens to fall off more rapidly with distance, specifically as $1/r^3$. This faster decay is a hallmark of a neutral system with separated charges.

What is the significance of the direction of the electric dipole moment?

The direction of the electric dipole moment, defined from the negative charge to the positive charge, is incredibly significant because it dictates how the dipole interacts with an external electric field. When placed in an external field, the dipole experiences a torque that attempts to align its dipole moment vector with the external electric field vector. This alignment tendency is fundamental to understanding the behavior of polar molecules in fields, the working of microwave ovens, and the polarization of dielectric materials.

When is the potential energy of an electric dipole in an external electric field minimum and maximum?

The potential energy ($U = -pE cos heta$) of an electric dipole in a uniform external electric field is minimum when $cos heta = 1$, which means $ heta = 0^circ$. In this state, the dipole moment $vec{p}$ is perfectly aligned with the electric field $vec{E}$, representing a state of stable equilibrium. Conversely, the potential energy is maximum when $cos heta = -1$, meaning $ heta = 180^circ$. Here, the dipole moment is anti-aligned with the electric field, representing a state of unstable equilibrium. Any slight perturbation will cause it to rotate towards the stable equilibrium.

Can a dipole experience a net force in an electric field?

Yes, but only if the electric field is non-uniform. In a uniform electric field, the force on the positive charge ($+qvec{E}$) and the force on the negative charge ($-qvec{E}$) are equal in magnitude and opposite in direction. Thus, the net force on the dipole is zero. However, if the field is non-uniform, the forces on $+q$ and $-q$ will not be equal in magnitude (or might not be perfectly opposite in direction), leading to a non-zero net force on the dipole. This net force will tend to move the dipole towards regions of stronger field if it is aligned with the field, or away from them if anti-aligned.

What is an ideal or point dipole?

An ideal or point dipole is a theoretical concept where the size of the dipole (the separation $2a$) approaches zero, while the magnitude of the charges ($q$) approaches infinity, such that the product $p = q(2a)$ remains finite. This is a limiting case used for mathematical simplification, especially when considering points very far from the dipole. In practical terms, it allows us to use the simplified $1/r^3$ and $1/r^2$ formulas for field and potential without worrying about the exact internal structure of the dipole, as long as the observation distance $r$ is much greater than $2a$.

Electric Dipole

Physics

NEET UG

Version 1Updated 22 Mar 2026

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An electric dipole is a system of two equal and opposite point charges, $+q$ and $-q$ , separated by a small fixed distance, $2a$ . This fundamental configuration gives rise to a characteristic electric field and potential distribution in space. The strength and orientation of an electric dipole are quantified by its electric dipole moment, $vec{p}$ , which is a vector quantity directed from the nega…

Quick Summary

An electric dipole is a system of two equal and opposite point charges, $+q$ and $-q$ , separated by a small fixed distance $2a$ . Its defining characteristic is the electric dipole moment, $vec{p}$ , a vector quantity with magnitude $p = q(2a)$ and direction from $-q$ to $+q$ .

The net charge of a dipole is zero. The electric field due to a dipole falls off as $1/r^3$ (e.g., $E_{axial} = \frac{1}{4piepsilon_0} \frac{2p}{r^3}$ , $E_{equatorial} = -\frac{1}{4piepsilon_0} \frac{p}{r^3}$ for $r gg a$ ), and the electric potential as $1/r^2$ ( $V = \frac{1}{4piepsilon_0} \frac{p cos\theta}{r^2}$ for $r gg a$ ).

When placed in a uniform external electric field $vec{E}$ , a dipole experiences a torque $vec{\tau} = vec{p} \times vec{E}$ that tends to align it with the field. Its potential energy is $U = -vec{p} cdot vec{E}$ .

Dipoles are crucial for understanding polar molecules and dielectric behavior.

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Electric Dipole: — Two equal and opposite charges (

+q, -q

) separated by

2a

Dipole Moment: —

vec{p} = q(2vec{a})

(from

-q

+q

). Unit:

ext{C}cdot\text{m}

Electric Field (Axial): —

vec{E}_{axial} = \frac{1}{4piepsilon_0} \frac{2vec{p}}{r^3}

(along

vec{p}

, for

r gg a

Electric Field (Equatorial): —

vec{E}_{equatorial} = -\frac{1}{4piepsilon_0} \frac{vec{p}}{r^3}

(opposite to

vec{p}

, for

r gg a

Electric Potential: —

V = \frac{1}{4piepsilon_0} \frac{p cos\theta}{r^2}

(for

r gg a

V=0

on equatorial line (

heta=90^circ

Torque in Uniform Field: —

vec{\tau} = vec{p} \times vec{E}

. Magnitude

au = pE sin\theta

. Max at

heta=90^circ

, zero at

heta=0^circ, 180^circ

Potential Energy in Uniform Field: —

U = -vec{p} cdot vec{E} = -pE cos\theta

. Min at

heta=0^circ

(stable), Max at

heta=180^circ

(unstable).

Work Done: —

W = U_{final} - U_{initial}

Net Charge: — Zero.

Force in Uniform Field: — Zero net force.

To remember the direction of the dipole moment and its interaction with the field:

Positive Points Positive (from negative to positive) Torque Tries to Turn (align $vec{p}$ with $vec{E}$ ) Axial Along, Equatorial Exactly Opposite (field direction relative to $vec{p}$ )