The electromagnetic force is one of the four fundamental interactions and acts on particles that possess electric charge. It is described by quantum electrodynamics, or QED, at the quantum level and by classical electromagnetism at macroscopic scales. The electromagnetic interaction includes both electric and magnetic phenomena, which are understood as interconnected aspects of a single electromagnetic field. Charged particles generate electromagnetic fields and respond to electromagnetic fields produced by other charges and currents.
The photon is the massless gauge boson associated with the electromagnetic interaction. Because the photon has zero rest mass, electromagnetic effects can have a long range. At the classical level, the electric force between two point charges is described by Coulomb's law, with magnitude proportional to the product of the charges and inversely proportional to the square of their separation in vacuum. Magnetic forces arise from moving charges and currents and from the relativistic structure of electromagnetism.
Electric charge is the fundamental source property for electromagnetic interactions. There are positive and negative electric charges, and charges of the same sign repel while opposite signs attract under ordinary electrostatic conditions. Electric charge is conserved in physical interactions, meaning that the total electric charge of an isolated system remains constant. The SI unit of charge is the coulomb.
The electromagnetic force is much stronger than gravity at the scale of individual elementary charged particles, although macroscopic matter is often electrically neutral because positive and negative charges largely cancel. Residual electromagnetic effects nevertheless dominate chemistry, material structure, friction, contact forces, elasticity and much of ordinary matter. The apparent solidity of a table, for example, ultimately involves electromagnetic interactions between the electrons and nuclei in the atoms of the material, together with quantum mechanical principles that prevent matter from simply occupying the same states.
The electromagnetic field has both electric and magnetic components. A stationary charge produces an electric field, while moving charges and changing electric fields are associated with magnetic effects. Maxwell's equations describe how electric and magnetic fields are generated and how they evolve. A changing magnetic field can induce an electric field, while changing electric fields contribute to magnetic field generation. Electromagnetic waves can propagate through vacuum without requiring a material medium.
Light is an electromagnetic wave, as are radio waves, microwaves, infrared radiation, ultraviolet radiation, X rays and gamma rays, differing primarily in frequency, wavelength and photon energy. The speed of electromagnetic waves in vacuum is the constant c, approximately 299792458 metres per second. This relationship between electricity, magnetism and light was one of the major achievements of nineteenth century physics.
In quantum electrodynamics, electromagnetic interactions are described through the exchange and interaction of photons. A photon is massless, electrically neutral and has spin one. Virtual photons appear in perturbative descriptions of electromagnetic interactions between charged particles, while real photons are observable electromagnetic radiation. The virtual particle language is a mathematical component of quantum field calculations and should not be interpreted as ordinary photons travelling freely between charges in exactly the same way as detectable light.
QED has been tested with extraordinary precision, including measurements of quantities such as the electron's anomalous magnetic moment. The electromagnetic coupling is characterised by the fine structure constant, approximately 1 divided by 137 at low energies. Quantum corrections cause the effective electromagnetic coupling to vary with energy scale because of vacuum polarisation and other quantum effects.
Electromagnetism is responsible for a vast range of everyday phenomena. Chemical bonds arise from electromagnetic interactions between electrons and atomic nuclei, while the detailed stability of atoms depends on quantum mechanics and electromagnetic forces. Electric motors convert electrical energy into mechanical motion through electromagnetic forces on currents and magnetic fields. Generators convert mechanical motion into electrical energy through electromagnetic induction. Transformers use changing magnetic fields to transfer electrical energy between circuits.
Radio communication uses electromagnetic waves, while optical fibre communication uses light. Microwave ovens use electromagnetic radiation to transfer energy to matter through interactions that depend on molecular and material properties. Electrical circuits, antennas, speakers, microphones, magnetic storage technologies and countless other devices depend on electromagnetic principles.
The electromagnetic interaction has both attractive and repulsive effects and can operate over macroscopic distances. However, its apparent strength in everyday matter depends heavily on charge distribution and screening. A neutral atom has positive nuclear charge balanced by negative electron charge, so its long range electric field largely cancels. At short distances, however, electromagnetic interactions remain essential to atomic and molecular structure. Conductors can rearrange mobile charges in response to external electric fields, producing shielding and induced charge distributions. Dielectric materials can polarise, altering electric fields inside matter. Magnetic materials can also exhibit collective responses arising from electron magnetic moments and quantum interactions. These material responses make macroscopic electromagnetism more complex than simply applying Coulomb's law to isolated point charges.
Electromagnetic radiation carries energy and momentum. The energy of a photon is related to its frequency by E = h f, where h is Planck's constant and f is frequency. Higher frequency electromagnetic radiation therefore has higher energy per photon. Radio waves have low photon energies compared with ultraviolet, X rays and gamma rays, although total radiation energy also depends on the number of photons and field intensity. Electromagnetic radiation can be absorbed, emitted, scattered and reflected by matter.
Ionising radiation such as sufficiently energetic ultraviolet, X rays and gamma rays can remove electrons from atoms and molecules, while lower energy radiation generally produces nonionising effects such as excitation, heating or molecular rotation depending on frequency and material. The electromagnetic spectrum is therefore a continuous range rather than a collection of fundamentally unrelated types of radiation.
The electromagnetic force is one of the four fundamental interactions, alongside gravity, the strong interaction and the weak interaction. Unlike the strong and weak interactions, electromagnetism has an infinite classical range because the photon is massless. It is vastly stronger than gravity between elementary charged particles, but gravity dominates astronomical structures because macroscopic objects contain enormous amounts of mass and are usually nearly electrically neutral.
Electromagnetism and the weak interaction are unified within the electroweak theory at sufficiently high energies, while quantum electrodynamics provides the quantum description of electromagnetic processes. The electromagnetic force is therefore simultaneously a fundamental quantum interaction and the basis of classical electricity, magnetism, optics, chemistry and much of material science.
