Electromagnetism

Electromagnetism, more formally the electromagnetic interaction, is the fundamental interaction that governs the attraction and repulsion between objects possessing electric charge. Electromagnetism occurs through the electric and magnetic fields generated by a charge, which enact a Lorentz force onto other charged objects. In the simplest case of a stationary point charge $q$, no magnetic field is present, the force is central and either attractive (to equal charges) or repulsive (to opposite charges). It is then described by the electric potential

where $r$ is the distance from the charge and $\varepsilon_{0}$ is the vacuum permittivity. The Lorentz force is given by Coulomb's law

where $Q$ is another charge a distance $r$ from the source charge $q$.

The modern theory of electromagnetism is quantum electrodynamics (QED). Electromagnetism is an exchange interaction mediated by the Photon, one of the four known gauge bosons.

Electromagnetism has infinite range, meaning it can affect objects at any distance.

Coupling constant#

The electromagnetic coupling constant is known as the fine-structure constant, defined as

where

• $e$ is the elementary charge,
• $\hbar$ is the Planck constant,
• $c$ is the speed of light,
• $\varepsilon_{0}$ is the vacuum permittivity.

Notably, the fine-structure constant is universal. All electromagnetic interactions are regulated by the same coupling constant.

Exchange particle#

The quantum of the electromagnetic interaction is the Photon, a massless gauge boson with Spin 1 and no electric charge. The photon carries the electromagnetic force, whose intensity is determined the electromagnetic coupling constant. Simple examples of electromagnetic exchange are proton-electron attraction $e^{-}+p\to e^{-}+p$ and electron-positron attraction $e^{-}+e^{+}\to e^{-}+e^{+}$.

In Feynman diagrams, a photon carrying the electromagnetic interaction is represented by a wavy line.

Interaction with matter#

Electromagnetic radiation is mediated by photons and can convert all or part of its energy by interacting with atoms and atomic nuclei in three main ways:

• Photoelectric effect: the photon is absorbed by the atom to raise the energy eigenstate of an electron.
• Compton scattering: the photon is scattered by an electron through absorption and re-emission, so that only part of the energy is ultimately absorbed and the photon undergoes an angular deflection.
• Electron–positron pair production: the photon “decays” spontaneously into an electron–positron pair.