Dipole

A dipole is a fundamental concept in electromagnetism and molecular physics that refers to a system consisting of two equal and opposite poles or charges separated by a finite distance. Dipoles may be electric or magnetic, and both types exhibit characteristic field patterns and measurable dipole moments. The term itself derives from Greek, reflecting the two-pole structure that defines the phenomenon. Dipoles are essential to understanding atomic behaviour, molecular structure, and the interactions that govern physical and chemical systems.

Electric and Magnetic Dipoles

Electric dipoles arise from the spatial separation of positive and negative charges. A simple electric dipole consists of two charges of equal magnitude and opposite sign separated by a small distance. The electric dipole moment is a vector quantity that points from the negative charge to the positive charge, and its magnitude equals the product of the charge strength and the separation between them. The precise definition of this moment relies upon the dipole limit, in which separation tends to zero while charge magnitude tends to infinity in such a way that the product remains constant.
A permanent electric dipole is known as an electret. Electrets maintain a stable electric dipole moment without the need for an external field.
Magnetic dipoles, by contrast, are produced by circulating electric currents. A current loop forms a magnetic dipole whose moment is equal to the product of the current and the loop’s area, with its direction determined by the right-hand grip rule. Bar magnets are classic examples of permanent magnetic dipoles, their magnetism arising from the intrinsic magnetic moments of electrons. These intrinsic moments stem not from literal current loops but from fundamental quantum-mechanical properties such as spin.
Permanent magnets possess two poles, termed north and south, which should not be confused with hypothetical magnetic monopoles. Although a freely suspended magnet’s north-seeking pole points towards the Earth’s geographic north, Earth’s own geomagnetic north pole is physically a magnetic south pole, demonstrating that the planet itself behaves as a large dipole with reversed labeling relative to compass conventions.

Classification and Theoretical Dipoles

A physical dipole refers to two distinct, oppositely charged or magnetised sources, but in physics the dipole concept extends further. The field of a dipole at large distances depends predominantly on its dipole moment. A point dipole is an idealised limit in which the spatial separation of the poles tends to zero while the moment remains fixed. The resulting field forms the leading term in the multipole expansion of the system’s potential, decaying with distance more slowly than higher-order terms such as quadrupoles.
Magnetic monopoles have not been observed, yet magnetic dipoles exist in quantum systems, particularly in the spin of elementary particles such as electrons. A theoretical magnetic point dipole possesses a field mathematically identical to that of an electric point dipole, allowing classical formulas to approximate the behaviour of very small current loops.
Any system of charges or currents possesses an associated dipole moment representing the best far-field approximation of its behaviour. For electric systems, the monopole term dominates when charge is present; where total charge is zero, the dipole term becomes the leading contribution.

Molecular Dipoles and Their Properties

Molecules often exhibit electric dipole moments due to uneven distributions of electron density. In polar molecules such as hydrogen fluoride, electrons are shared unequally, producing a permanent dipole. The magnitude of molecular dipoles is traditionally measured in the debye, a non-SI unit named after Peter Debye, who pioneered the study of molecular dipole behaviour.
There are three principal types of molecular dipoles:

• Permanent dipoles, arising from differences in electronegativity between bonded atoms.
• Instantaneous dipoles, produced momentarily by random fluctuations in electron position. These form the basis of London dispersion forces.
• Induced dipoles, generated when an external electric field distorts the electron distribution of a molecule. This can be caused by nearby ions, polar molecules, or macroscopic fields such as those in a charged capacitor.

The magnitude of an induced dipole depends on both the strength of the external field and the polarizability of the molecule. Empirical dipole moments can be determined from measurements of dielectric constants, and characteristic gas-phase values range widely depending on structure and bonding.
The overall molecular dipole moment can be approximated by the vector sum of individual bond dipole moments. This makes dipole measurement a useful tool for deducing molecular geometry. For instance, the zero dipole moment of carbon dioxide indicates that its two C–O bond moments cancel in a linear arrangement. Water, being bent, has a non-zero dipole. In ozone, even though the O–O bonds connect identical atoms, the bent geometry and resonance structures produce a net moment with local charge asymmetry.
Geometric effects are also evident in cis–trans isomerism. For example, cis-1,2-dichloroethene has a measurable dipole because the polar C–Cl bonds lie on the same side of the double bond. In the trans isomer, symmetry causes the individual dipole moments to cancel. Similarly, boron trifluoride contains polar B–F bonds yet has zero overall dipole because its trigonal planar geometry distributes the bond moments symmetrically about the central atom.