Electromagnetism
Electromagnetism is the physics of the electromagnetic field: a field, encompassing all of space, composed of the electric field and the magnetic field. The electric field is produced by stationary electric charges, and gives rise to the electric force, the force experienced in static electricity; it is also the force that drives the flow of current in electrical conductors. The magnetic field is a field produced by the motion of electric charges, giving rise to the magnetic force one associates with magnets. The term "electromagnetism" comes from the fact that the electric and magnetic fields are closely intertwined, and, under many circumstances, it is impossible to consider the two separately. For instance, a changing magnetic field gives rise to an electric field; this is the phenomenon of electromagnetic induction, which underlies the operation of electrical generators, induction motors, and transformers.
The term electrodynamics is sometimes used to refer to the combination of electromagnetism with mechanics. It is concerned with the effects of the electromagnetic field on the mechanical behavior of electrically charged particles.
The force experienced by electrically charged particles due to the electromagnetic field is called the electromagnetic force. It is one of the four fundamental forces, the others being gravity, the strong nuclear force, and the weak nuclear force. All other forces ultimately arise from these fundamental forces. However, it turns out that the electromagnetic force is the one responsible for practically all the phenomena one encounters in daily life, with the exception of gravity. (The strong nuclear force is basically responsible for holding atomic nuclei together, while the weak nuclear force gives rise to certain forms of radioactive decay.) For instance, the forces between atoms are predominantly electromagnetic, arising from the positive electric charge of the protons in atomic nuclei and the negative electric charge of the electrons surrounding the nuclei. Among other things, the forces between atoms give rise to the rigidity of solids. The forces acting on the electrons in atoms are also electromagnetic in nature, and these give rise to the various phenomena studied in chemistry. Finally, it turns out that light can be described as a set of travelling disturbances in the electromagnetic field (i.e. electromagnetic waves), so all optical phenomena are actually electromagnetic.
An accurate theory of electromagnetism, known as classical electromagnetism, was developed by various physicists over the course of the 19th century, culminating in the work of James Clerk Maxwell, who unified the preceding developments into a single theory and discovered the electromagnetic nature of light. In classical electromagnetism, the electromagnetic field obeys a set of equations known as Maxwell's equations, and the electromagnetic force is given by the Lorentz force law.
One of the peculiarities of classical electromagnetism is that it is difficult to reconcile with classical mechanics. According to Maxwell's equations, the speed of light is a universal constant, dependent only on the electrical permittivity and magnetic permeability of the vacuum. This violates Galilean invariance, a long-standing cornerstone of classical mechanics. One way to reconcile the two theories is to assume the existence of a luminiferous aether through which the light propagates. However, subsequent experiments efforts failed to detect the presence of the aether. This eventually led Albert Einstein to develop the theory of relativity, which replaces classical kinematics with a new theory of kinematics which is compatible with electromagnetism.
Einstein's 1905 paper on the photoelectric effect was the first sign of the incompleteness of classical electromagnetism. It posited that light could exist as discrete particle-like quantities, which later came to be known as photons. The development of quantum mechanics necessitated the invention of a quantum theory of electromagnetism. This theory, which was completed in the 1940s, is known as quantum electrodynamics (QED).