''
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 can be produced by stationary
electric charges, and gives rise to the electric
force, which causes
static electricity and drives the flow of
electric current in
electrical conductors. The magnetic field can be produced by the
motion of electric charges, such as an electric current flowing along a wire, and gives 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. This subject deals with the effects of the electromagnetic field on the mechanical behavior of electrically charged particles.
The force that the electromagnetic field exerts on electrically charged particles, called the
electromagnetic force, is one of the four fundamental forces. The other fundamental forces are the
strong nuclear force (which holds
atomic nuclei together), the weak nuclear force (which causes certain forms of
radioactive decay), and the
gravitational force. All other forces are ultimately derived 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. Roughly speaking, all the forces involved in interactions between
atoms can be traced to the electromagnetic force acting on the electrically charged
protons and
electrons inside the atoms. This includes the forces we experience in "pushing" or "pulling" ordinary material objects, which come from the
intermolecular forces between the individual
molecules in our bodies and those in the objects. It also includes all forms of
chemical phenomena, which arise from interactions between
electron orbitals.
Furthermore,
light is actually a kind of travelling disturbance in the electromagnetic field (i.e. electromagnetic waves.) Therefore, all
optical phenomena are actually electromagnetic phenomena.
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, but it is compatible with
special relativity. 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. In 1905,
Albert Einstein solved the problem with the introduction of
special relativity, which replaces classical kinematics with a new theory of kinematics that is compatible with classical electromagnetism. In this theory, magnetism turns out to be the effect that relativity has on simple electrostatics and does not need a special set of equations (like Maxwell's equations in a classical Universe).
Remarkably, in another paper published in that same year, Einstein undermined the very foundations of classical electromagnetism. His theory of the
photoelectric effect posited that light could exist in discrete particle-like quantities, which later came to be known as
photons. Einstein's theory of the photoelectric effect extended the insights that appeared in the solution of the
ultraviolet catastrophe presented by
Max Planck in 1900. In his work, Planck showed that hot objects emit
electromagnetic radiation in discrete packets, which leads to a finite total
energy emitted as black body radiation. Both of these results were in direct contradiction with the classical view of light as a continuous wave. Planck's and Einstein's theories were progenitors of
quantum mechanics, which, when formulated in 1925, necessitated the invention of a quantum theory of electromagnetism. This theory, completed in the
1940s, is known as
quantum electrodynamics (or "QED"), and is one of the most accurate theories known to physics.
SI electricity units
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