electromagnetism

Electromagnetism is the branch of physics that studies the interaction of electric charges and currents with magnetic and electric fields. Classical electromagnetism describes how electric charges and currents create electric and magnetic fields, and how changing fields interact to produce forces and propagate energy. It is described by Maxwell's equations, which unify electricity, magnetism, and light.

Maxwell's equations consist of Gauss's law for electricity, Gauss's law for magnetism, Faraday's law of induction, and the Ampère–Maxwell law. Together they imply that, in vacuum, changing electric and magnetic fields propagate as electromagnetic waves at the speed of light. This unifies electricity, magnetism and optics and explains phenomena from radio transmission to visible light.

Electromagnetic waves carry energy and momentum. The energy density is proportional to E^2 and B^2, and the energy flux is described by the Poynting vector S = E × H. The theory predicts reflection, refraction, diffraction, and interference, as well as the behavior of antennas, waveguides, and optical media.

In quantum theory, the electromagnetic field is quantized into photons, and interactions are described by quantum electrodynamics. Classical electromagnetism is a valid approximation at macroscopic scales and low field strengths.

Applications include power generation and distribution, electric machines, wireless communication, optics, and medical imaging. Electromagnetism is one of the four fundamental interactions in nature, alongside gravity, the weak, and the strong nuclear forces.