dipole

A dipole is a system in which two equal and opposite charges are separated by a finite distance, or more generally a distribution that has two opposite properties localized in space. The dipole moment is a vector that quantifies the strength of the separation, defined as p = q d for point charges of magnitude q separated by distance d, pointing from the negative to the positive charge.

In electric contexts, the electric dipole moment characterizes how a system responds to electric fields. Many molecules possess a permanent electric dipole moment when their geometry yields an uneven charge distribution, as in water (H2O) or hydrogen chloride (HCl). In an external field, dipoles experience torque and alignment; at the macroscopic scale, ensembles of dipoles give rise to dielectric polarization. Dipole-dipole interactions affect structure, spectroscopy, and thermodynamic properties.

Magnetic dipole moment is associated with a circulating current or a small magnet and is a vector describing the strength and orientation of the magnetic field produced by the system. Magnetic dipoles couple to magnetic fields and underpin phenomena such as electron spin, NMR, and magnetic resonance.

Outside the molecular domain, the term dipole is used for structures such as a dipole antenna, a straight conductor fed at its center and typically resonant at half a wavelength. Its radiation pattern and receiving properties derive from the dipole’s length and orientation with respect to the incident or radiated field.