Electromagnetism is a branch of physics. There is a close relationship between electricity and magnetism. Electromagnetism in a broad sense can be said to include electricity and magnetism, but electromagnetism in a narrow sense is a subject that discusses the interaction between electricity and magnetism. It mainly studies electromagnetic waves, electromagnetic fields and related charges, dynamics of charged objects, etc.

Electromagnetism or electrodynamics or classical electrodynamics. It is called classic because it does not include the content of modern quantum electrodynamics. The term electrodynamics is not used strictly, sometimes it refers to the part of electromagnetism left after removing electrostatics and magnetostatics, and refers to the part where electromagnetism and mechanics are combined. This part discusses the mechanical effect of electromagnetic field on charged particles.

The basic theory of electromagnetism was developed by many physicists in the19th century. Maxwell equations unified all these works through a set of equations, revealing that the essence of light is electromagnetic wave.

The basic equation of electromagnetism is Maxwell's equations, which will change in form under the relative motion transformation (galilean transformation) of classical mechanics, and the speed of light is different under different inertial coordinates under galilean transformation. The transformation that keeps the form of Maxwell's equations unchanged is Lorentz transformation. Under this transformation, the speed of light is constant in different inertial systems.

At the beginning of the 20th century, Michelson-Morey experiment supported the constant speed of light and became the cornerstone of Einstein's special theory of relativity. Lorentz transformation has become a more accurate inertial coordinate transformation method than galilean transformation.

Magnetostatic phenomena and electrostatic phenomena have long been noticed by human beings. In ancient China, the phenomena of magnet attracting iron, magnet guiding and friction electrification were discovered in the era of the Yellow Emperor. The systematic study of these phenomena began in16th century. 1600, British doctor william Gilbert (1544 ~ 1603) published "On Magnetism and Magnetism Saturated Earth is a Huge Magnet": (Demagnete, Magnetics Que Corporibus et de Magnetite Tellure). He summarized the previous research on magnetism, elaborated the essence of geomagnetism in detail, and recorded a large number of experiments, which made magnetism move from experience to science. He also recorded his research on electricity in the book.