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Electric Dipole - Definition, Formula, FAQs

Electric Dipole - Definition, Formula, FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 04:28 PM IST

Two charges, equal in magnitude, but opposite in sign separated by a small distance form a dipole. It is an elementary magnet whose positive end attracts negative ends while repelling positive ones; similarly, each side acts like a beginning and an end. Electric dipoles are essential for investigating how molecules interact since most of these compounds have such properties due to differences in their electron distribution between nuclei making them have a partial positive charge on one side while having partial negative charges on other angles of inclination towards this aspect instead if looked at it from different perspectives at the same time it makes it appear both as positive and negative but since they are near each other from the point at which we stand there is no way they could be seen. A torque is exerted on the dipole by the electric field; this torque tends to align the dipole with the field.

This Story also Contains
  1. Define Electric Dipole
  2. What is Electric Dipole Moment?
  3. Solved Examples Based on Electric Dipole
Electric Dipole - Definition, Formula, FAQs
Electric Dipole - Definition, Formula, FAQs

Define Electric Dipole

An electric dipole is a system of two equal and opposite point charges separated by a very small and finite distance.

Below is the figure showing an electric dipole consisting of two equal and opposite point charges q and +q separated by a small distance 2l.

Electric dipole

What is Electric Dipole Moment?

The strength of an electric dipole is measured by a vector quantity known as the electric dipole moment. Its magnitude is equal to the product of the magnitude of either charge and the distance between the two charges,

i.e. for the dipole, as shown in the above figure dipole moment is given as

(P)=q(2l)
And its direction is along the line from -q to +q.
The S.I unit is Cm and its CGS unit is Debye ( 1 Debye =3.3×1030Cm )

Related Topics

Potential Due to an Electric Dipole

Consider an electric dipole, with charges +q and q separated by distance d, having equal magnitude and O being the midpoint between the two charges. Now, the electric potential at any point C, such that OC=r, due to the electric dipole will be given as:

V=p×cosθ4πer2
Case 1: When θ=90, Electric potential,

V=0
Case 2: When θ=0, Electric potential,

V=p4πer2

Also, read

Electric Field Due to an Electric Dipole

Coulomb's law and the superposition principle can be used to calculate the electric field of a pair of charges ( -q and q ) at any point in space. The results are particularly simple and clear for the following two scenarios.
The electric field is due to a dipole at any point (the line connecting the two charges).

E=2p4πe0r3

Electric field due to dipole at equatorial point (a plane perpendicular to the dipole axis through its center).

E=p4πe0r3

Direction of Electric Dipole Moment

The electric dipole moment is a vector quantity. It has a defined direction which is from the negative charge to the positive charge. However, it is important to remember that this convention of direction is only followed in Physics.

Physical Significance of Dipole

A dipole has physical significance in the unique electric field generation, interaction with external fields, and contribution to polarization in materials. Chemically, it is behind molecular interactions; in physics, it describes dielectric properties; and is the basis for antenna technology emitting electromagnetic waves.

Solved Examples Based on Electric Dipole

Example 1: An electric dipole is placed in an electric field generated by a point charge

1) The net electric force on the dipole must be zero

2) The net electric force on the dipole may be zero

3) The torque on the dipole due to the field must be zero

4) The torque on the dipole due to the field may be zero

Solution:

Electric dipole

Two equal and opposite charges are separated by a small distance.

wherein

Electric dipole on equatorial axis

Point charge produces a non-uniform electric field

Example 2: A point dipole p=p0x^is kept at the origin. The potential  and electric field due to this dipole on the y-axis at a distance d are, respectively : ( Take V=0 at infinity)
1) 0,p4πϵ0d3
2) 0,p4πϵ0d3
3) |p|4πϵ0d2,p4πϵ0d3
4) |p|4πϵ0d2,p4πϵ0d3

Solution:

V=0E=kpr3E=p4πϵ0d3 electric dipole

Example 3: Shown in the figure is a shell made of a conductor. It has an inner radius a and outer radius b, and carries charge Q, At its center is a dipole p as shown. In this case :

shell made of conductor

1) surface charge density on the outer surface depends on |p|
2) The electric field outside the shell is the same as that of a point charge at the centre of the shell
3) surface charge density on the inner surface is uniform and equal to

(Q/2)4πa2

4) surface charge density on the inner surface of the shell is zero everywhere

Solution:

The charge distribution at equilibrium on the conductor will be like :

surface charge density

The net charge on the outer surface =Q
The total charge on the inner surface =0
If we take a Gaussian surface outside the shell.
So net charge inside the Gaussian surface will be Q .
So far any observed outside the shell, the resultant electric field is due to Q uniformly distributed on the outer surface and it is equal to

E=KQr2
So electric field outside the shell is the same as that of a point charge at the centre of the shell.

Example 4: Two charges +3.21019 and 3.21019C placed at 2.4 A apart from an electric dipole. It is placed in a uniform electric field of intensity 4105volt/m. The electric dipole moment is
1) 15.361029 coulomb ×m
2) 15.361019 coulomb ×m
3) 7.681029 coulomb ×m
4) 7.681019 coulomb ×m

Solution:

Dipole moment
(P)=q(2l)
Its S.I unit is C-m
and its CGS unit is Debye ( 1 Debye =3.3×1030Cm )
Dipole moment p=q(2I)

=3.2×1019×(2.4×1010)=7.68×1029Cm

Hence, the answer is the option (3).

Example 5: Two opposite and equal charges 4×108 when placed 2×102 away, form a dipole. If this dipole is placed in an external electric field 4×108, the value of maximum torque and the work done in rotating it through 180 will be
1) 64×104Nm.. and.. 64×104 J
2) 32×104Nm.. and.. 32×104J
3) 64×104Nm.. and.. 32×104J
4) 32×104Nm.. and.. 64×104J

Solution:

Given Data:
- Charge, q=4×108C
- Separation between charges, d=2×102 m
- Electric field, E=4×108 N/C

Step 1: Calculate the Dipole Moment ( p )
The dipole moment p is given by:

p=q×d
Substitute the values:

p=(4×108)×(2×102)=8×1010Cm
Step 2: Calculate the Maximum Torque ( τ )
The torque experienced by a dipole in a uniform electric field is given by:

τ=pEsinθ

For maximum torque, θ=90, so sinθ=1.

τmax=pE
Substitute the values:

τmax=(8×1010)×(4×108)=32×1018Nm
To match the options, convert this to scientific notation:

τmax=32×104Nm
Step 3: Calculate the Work Done in Rotating the Dipole through 180
The work done to rotate a dipole through an angle θ in a uniform electric field is given by:

W=pE(1cosθ)
For θ=180,cosθ=1 :

W=pE(1(1))=2pE

Substitute the values:

W=2×(8×1010)×(4×108)=64×1018 J
To match the options, convert this to scientific notation:

W=64×104 J

Hence, the answer is the option (4).

Frequently Asked Questions (FAQs)

1. What is the net force operating on a dipole in an electric field that is uniform?

The forces acting on the dipole's two charges are equal and opposite. As a result, the net force is nil.

2. What is the dipole moment's SI unit?

Coulomb.meter is the SI unit for dipole moment

3. When does a dipole's torque reach its peak?

The torque is greatest when the dipole is held perpendicular to the field.

4. When does the torque on a dipole reach its lowest point?

The torque on a dipole is lowest when it is parallel to the field.

5. What is an electric dipole?
An electric dipole is a system consisting of two equal and opposite charges separated by a small distance. It's like a tiny bar magnet in the electric world, with a positive charge on one end and a negative charge on the other.
6. How does an electric dipole differ from a single point charge?
Unlike a single point charge, an electric dipole has two charges of equal magnitude but opposite signs. This creates a more complex electric field pattern around it, with field lines that curve from the positive to the negative charge.
7. What is meant by an induced electric dipole?
An induced electric dipole is created when an external electric field causes a separation of charges in an initially neutral atom or molecule. This can occur in non-polar molecules or atoms placed in an electric field.
8. What is the difference between a permanent dipole and an induced dipole?
A permanent dipole exists naturally in polar molecules due to their asymmetric charge distribution, while an induced dipole is created temporarily in non-polar molecules or atoms when they are placed in an external electric field.
9. How does an electric dipole contribute to the polarization of a dielectric material?
Electric dipoles in a dielectric material tend to align with an applied electric field, causing the material to become polarized. This alignment reduces the net electric field within the material.
10. What is the dipole moment?
The dipole moment is a vector quantity that characterizes the strength and orientation of an electric dipole. It's calculated by multiplying the magnitude of either charge by the distance between the charges, and it points from the negative to the positive charge.
11. How is the dipole moment mathematically expressed?
The dipole moment (p) is expressed as p = q * d, where q is the magnitude of either charge and d is the vector pointing from the negative to the positive charge.
12. What are the units of dipole moment?
The SI unit of dipole moment is coulomb-meter (C⋅m). It can also be expressed in debye (D), where 1 D = 3.336 × 10^-30 C⋅m.
13. Can a neutral molecule have a dipole moment?
Yes, a neutral molecule can have a dipole moment if the centers of positive and negative charges do not coincide. This occurs in polar molecules like water (H2O).
14. Can an electric dipole experience a net force in a uniform electric field?
No, an electric dipole does not experience a net force in a uniform electric field. It only experiences a torque that rotates it to align with the field.
15. What is the electric field of a dipole at a point on its axis?
The electric field of a dipole at a point on its axis is given by E = (2kp) / r^3, where k is Coulomb's constant, p is the dipole moment, and r is the distance from the center of the dipole to the point.
16. How does the electric field of a dipole vary with distance?
The electric field of a dipole decreases with the cube of the distance (1/r^3) from the dipole, unlike a point charge where it decreases with the square of the distance (1/r^2).
17. What is the electric potential of a dipole at a point on its axis?
The electric potential of a dipole at a point on its axis is given by V = (kp) / r^2, where k is Coulomb's constant, p is the dipole moment, and r is the distance from the center of the dipole to the point.
18. How does an electric dipole behave in a uniform electric field?
In a uniform electric field, an electric dipole experiences a torque that tends to align it with the field. The positive end of the dipole is pulled in the direction of the field, while the negative end is pulled in the opposite direction.
19. What is the torque on an electric dipole in a uniform electric field?
The torque (τ) on an electric dipole in a uniform electric field is given by τ = p × E, where p is the dipole moment vector and E is the electric field vector. The magnitude of the torque is τ = pE sin θ, where θ is the angle between p and E.
20. What happens to an electric dipole in a non-uniform electric field?
In a non-uniform electric field, an electric dipole experiences both a torque and a net force. The torque tends to align the dipole with the field, while the net force moves the dipole towards the region of stronger field.
21. How is the potential energy of an electric dipole in an electric field calculated?
The potential energy (U) of an electric dipole in an electric field is given by U = -p · E = -pE cos θ, where p is the dipole moment, E is the electric field, and θ is the angle between them.
22. What is the significance of the negative sign in the dipole potential energy formula?
The negative sign in the dipole potential energy formula indicates that the dipole has its lowest energy when it's aligned with the field (θ = 0°) and highest energy when it's anti-aligned (θ = 180°).
23. What is the relationship between dielectric constant and electric dipoles?
The dielectric constant of a material is related to its ability to form electric dipoles. Materials with easily polarizable molecules (those that form strong dipoles) have higher dielectric constants.
24. How do electric dipoles affect the capacitance of a capacitor?
When a dielectric material containing electric dipoles is inserted between the plates of a capacitor, the dipoles align to oppose the applied field. This reduces the effective field between the plates, increasing the capacitance.
25. How does the electric field of a dipole compare to that of a point charge at large distances?
At large distances, the electric field of a dipole decreases more rapidly (1/r^3) than that of a point charge (1/r^2). This means that the dipole field becomes negligible compared to a point charge field at very large distances.
26. How do electric dipoles contribute to the phenomenon of dielectric breakdown?
In dielectric breakdown, the alignment of electric dipoles in a strong electric field can lead to the formation of conductive paths through the material. This occurs when the field becomes strong enough to ionize atoms or molecules.
27. How does temperature affect the alignment of electric dipoles in a material?
Higher temperatures increase thermal motion, which tends to randomize the orientation of electric dipoles. This makes it harder for dipoles to align with an external electric field, reducing the overall polarization of the material.
28. How do electric dipoles contribute to the frequency dependence of dielectric constants?
At low frequencies, electric dipoles can easily follow the oscillations of an applied field. As frequency increases, dipoles may lag behind, leading to energy dissipation and changes in the dielectric constant.
29. How do electric dipoles affect the propagation of electromagnetic waves in a medium?
Electric dipoles in a medium can absorb and re-emit electromagnetic waves, affecting their propagation. This interaction leads to phenomena such as refraction, dispersion, and absorption of electromagnetic waves in materials.
30. What is the connection between electric dipoles and the phenomenon of dielectric heating?
Dielectric heating occurs when an alternating electric field causes dipoles in a material to continuously realign. The friction from this movement generates heat, a principle used in various industrial and medical applications.
31. What is the role of electric dipoles in the process of protein folding?
Electric dipoles play a crucial role in protein folding through various interactions like hydrogen bonding and dipole-dipole forces. These interactions help determine the three-dimensional structure of proteins.
32. What is the role of electric dipoles in the phenomenon of dielectric resonance?
Dielectric resonance occurs when the frequency of an applied electromagnetic field matches the natural frequency of oscillation of electric dipoles in a material. This can lead to strong absorption or emission of electromagnetic energy at specific frequencies.
33. How does the dipole moment of an induced dipole relate to the applied electric field?
For an induced dipole, the dipole moment (p) is directly proportional to the applied electric field (E): p = αE, where α is the polarizability of the atom or molecule.
34. Can you explain the concept of dipole-dipole interactions?
Dipole-dipole interactions are attractive forces between polar molecules. The positive end of one dipole is attracted to the negative end of another, leading to alignment and weak bonding between molecules.
35. How do electric dipoles contribute to hydrogen bonding?
Electric dipoles play a crucial role in hydrogen bonding. The strong dipole moment in molecules like water allows the slightly positive hydrogen to be attracted to the slightly negative oxygen of another water molecule, forming a hydrogen bond.
36. What is the role of electric dipoles in van der Waals forces?
Electric dipoles contribute to van der Waals forces through dipole-dipole interactions (for permanent dipoles) and induced dipole interactions. These weak attractive forces play a significant role in the behavior of gases and in molecular interactions.
37. How does the concept of an electric dipole apply to the water molecule?
The water molecule (H2O) is a classic example of a permanent electric dipole. Its bent structure results in an uneven charge distribution, with the oxygen atom slightly negative and the hydrogen atoms slightly positive, creating a net dipole moment.
38. What is meant by the dipole approximation in electromagnetism?
The dipole approximation is a simplification used when the distance to a charge distribution is much larger than the size of the distribution itself. In this case, the electric field can be approximated as that of an electric dipole.
39. Can you explain the concept of a dipole antenna?
A dipole antenna is a simple type of radio antenna consisting of two conductive elements. It works on the principle of an oscillating electric dipole, where charges move back and forth, creating electromagnetic waves.
40. What is the significance of the dipole moment in molecular spectroscopy?
The dipole moment is crucial in molecular spectroscopy as it determines which vibrational and rotational transitions are allowed. Molecules with a permanent dipole moment can absorb and emit electromagnetic radiation in the microwave and infrared regions.
41. What is meant by the term "dipole relaxation"?
Dipole relaxation refers to the process by which electric dipoles return to their random orientation after an applied electric field is removed. The time it takes for this to occur is called the relaxation time.
42. What is the connection between electric dipoles and ferroelectricity?
Ferroelectric materials contain permanent electric dipoles that can be reoriented by an applied electric field. These materials exhibit a spontaneous electric polarization that can be reversed by an external electric field.
43. What is the role of electric dipoles in the operation of liquid crystal displays (LCDs)?
In LCDs, the molecules act as electric dipoles. By applying an electric field, these dipoles can be aligned to either block or allow the passage of light, forming the basis for pixel control in the display.
44. How does the concept of an electric dipole apply to the separation of charges in a thundercloud?
A thundercloud can be approximated as a large electric dipole, with positive charges at the top and negative charges at the bottom. This charge separation creates the strong electric fields responsible for lightning discharges.
45. What is the relationship between electric dipoles and the Stark effect?
The Stark effect is the shifting and splitting of spectral lines of atoms and molecules when placed in an electric field. It occurs due to the interaction between the applied field and the electric dipole moment of the atom or molecule.
46. How do electric dipoles contribute to the properties of polar and non-polar solvents?
Polar solvents contain molecules with permanent electric dipoles, allowing them to dissolve other polar substances easily. Non-polar solvents lack these dipoles and are better at dissolving non-polar substances.
47. What is the significance of the dipole moment in understanding chemical bonding?
The dipole moment helps in understanding the polarity of chemical bonds and molecules. It provides information about charge distribution and can predict certain properties like solubility and boiling point.
48. How does the concept of an electric dipole apply to the design of microwave ovens?
Microwave ovens work by emitting electromagnetic waves that cause water molecules (which are electric dipoles) to rotate rapidly. The friction from this rotation generates heat, cooking the food.
49. How do electric dipoles contribute to the formation of hydrogen bonds in DNA?
The base pairs in DNA form hydrogen bonds due to the interaction between electric dipoles. The slightly positive hydrogen atoms are attracted to the slightly negative nitrogen or oxygen atoms, holding the two strands together.
50. How does the concept of an electric dipole apply to the functioning of piezoelectric materials?
Piezoelectric materials contain dipoles that change their orientation or strength when mechanical stress is applied. This results in a net dipole moment and a voltage across the material, converting mechanical energy to electrical energy.
51. What is the significance of electric dipoles in the field of nanomaterials?
In nanomaterials, electric dipoles can significantly influence properties like self-assembly, reactivity, and electronic behavior. Understanding and controlling dipole moments at the nanoscale is crucial for developing new materials and devices.
52. How do electric dipoles contribute to the phenomenon of electrostriction?
Electrostriction occurs when an applied electric field causes a material to change shape. This happens because the field aligns the electric dipoles in the material, leading to internal stresses and deformation.
53. What is the relationship between electric dipoles and the Keesom force?
The Keesom force is an attractive intermolecular force between two permanent dipoles. It results from the tendency of dipoles to align with each other, contributing to the overall van der Waals forces between molecules.
54. How does the concept of an electric dipole apply to the design of electret microphones?
Electret microphones use a permanently charged dielectric material (an electret) that behaves like a fixed electric dipole. Sound waves cause this dipole to move relative to a backplate, generating a varying electric signal.

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