An electric dipole is one of the simplest arrangements in electrostatics. It consists of two equal and opposite electric charges placed at a small distance from each other. Though small and simple, dipoles are very important in understanding how electric fields behave, especially in molecules and materials.
A electric dipole is a system formed by two equal and opposite point charge placed at a small distance apart.
What is an Electric Dipole?
An electric dipole is formed when:
There are two point charges.
These charges are equal in magnitude but opposite in sign (for example, +q and -q).
They are separated by a distance 2a.
We denote this system as a dipole because it has two poles: one positive and one negative.
Dipole Moment
The most important quantity associated with an electric dipole is its dipole moment.
It is a vector quantity.
Symbol: p
Defined as:
p = q × 2a
Where:
q is the magnitude of the charge,
2a⃗ is the vector pointing from the negative charge to the positive charge.
So, the dipole moment always points from the negative charge to the positive charge.
SI unit: Coulomb-meter (C·m)
Electric Field Due to a Dipole
A dipole creates an electric field in the surrounding space. The field depends on where we are observing it:
1.On the axial line (the line passing through both charges):
This is also called the end-on position.
Electric field at a point at distance r from the center of dipole on the axial line is:
Eaxial = 1 / 4πε0 ⋅ 2pr3
2. On the equatorial line (the line perpendicular to the dipole axis and passing through the center):
Also called the broadside-on position.
Electric field at a point at distance r on the equatorial line:
Eequatorial = 1 / 4πε0 ⋅ pr3
(But directed opposite to the direction of dipole moment)
3. At a general point:
The net electric field is the vector sum of the fields due to both charges. In most cases, this requires vector addition.
Torque on a Dipole in a Uniform Electric Field
When an electric dipole is placed in a uniform electric field, it experiences a torque.
The torque tries to rotate the dipole to align it with the electric field.
Torque formula:
τ = p × E
Magnitude:
τ = pE sinθ
Where:
p is dipole moment,
E is electric field strength,
θ is angle between p and E.
Equilibrium Conditions:
Stable equilibrium: when p is parallel to E (θ = 0∘)
Unstable equilibrium: when p is opposite to E (θ = 180∘)
Potential Energy of a Dipole in an Electric Field
The potential energy U of a dipole in a uniform electric field is given by:
U = −p ⋅ E = −pE cos θ
Lowest energy (most stable): when θ = 0∘
Highest energy (least stable): when θ = 180∘
Applications of Electric Dipoles
Molecular behaviour: Water molecules are dipoles.
Used in antennas and communication.
Helps in explaining polarisation in materials
Physical significance of dipoles
In most molecules, centre of positive charges and that of negative charges exists at the same place. So they have no dipole moment. For example, CO2, CH4 etc.
if an electric field is applied then they develop a dipole moment. But some molecules have permanent dipole moment because their centres of negative charges and of positive charges do not coincide. Such molecules (H2O, NH) are called polar molecules.
Key Points to Remember
An electric dipole has equal and opposite charges separated by a small distance.
The dipole moment is from negative to positive.
The electric field of a dipole decreases rapidly as we move away it depends on 1 / r3.
In a uniform field, dipoles rotate to align with the field.
Dipole concepts are widely used in physics, chemistry, and engineering.
An electric dipole consists of two equal and opposite charges separated by a small distance. It is a simple arrangement that creates an electric field and dipole moment.
The dipole moment is a vector quantity given by p = q × 2a, where q is the charge and 2a is the separation vector. It indicates the strength and direction of the dipole, pointing from the negative to the positive charge.
The SI unit of dipole moment is Coulomb-meter (C·m).
The electric field of a dipole decreases rapidly with distance:
On the axial line, E ∝ 1 / r3
On the equatorial line, E ∝ 1 / r3