Electromagnetism, or the electromagnetic force
is one of the four fundamental interactions
in nature, the other three being the strong
interaction, the weak interaction, and gravitation.
This force is described by electromagnetic
fields, and has innumerable physical instances
including the interaction of electrically
charged particles and the interaction of uncharged
magnetic force fields with electrical conductors.
The word electromagnetism is a compound form
of two Greek terms, ἢλεκτρον, ēlektron,
"amber", and μαγνήτης, magnetic, from
"magnítis líthos", which means "magnesian
stone", a type of iron ore.
The science of electromagnetic phenomena is
defined in terms of the electromagnetic force,
sometimes called the Lorentz force, which
includes both electricity and magnetism as
elements of one phenomenon.
During the quark epoch, the electroweak force
split into the electromagnetic and weak force.
The electromagnetic force plays a major role
in determining the internal properties of
most objects encountered in daily life.
Ordinary matter takes its form as a result
of intermolecular forces between individual
molecules in matter.
Electrons are bound by electromagnetic wave
mechanics into orbitals around atomic nuclei
to form atoms, which are the building blocks
of molecules.
This governs the processes involved in chemistry,
which arise from interactions between the
electrons of neighboring atoms, which are
in turn determined by the interaction between
electromagnetic force and the momentum of
the electrons.
There are numerous mathematical descriptions
of the electromagnetic field.
In classical electrodynamics, electric fields
are described as electric potential and electric
current in Ohm's law, magnetic fields are
associated with electromagnetic induction
and magnetism, and Maxwell's equations describe
how electric and magnetic fields are generated
and altered by each other and by charges and
currents.
The theoretical implications of electromagnetism,
in particular the establishment of the speed
of light based on properties of the "medium"
of propagation, led to the development of
special relativity by Albert Einstein in 1905.
History of the theory
Originally electricity and magnetism were
thought of as two separate forces.
This view changed, however, with the publication
of James Clerk Maxwell's 1873 Treatise on
Electricity and Magnetism in which the interactions
of positive and negative charges were shown
to be regulated by one force.
There are four main effects resulting from
these interactions, all of which have been
clearly demonstrated by experiments:
Electric charges attract or repel one another
with a force inversely proportional to the
square of the distance between them: unlike
charges attract, like ones repel.
Magnetic poles attract or repel one another
in a similar way and always come in pairs:
every north pole is yoked to a south pole.
An electric current in a wire creates a circular
magnetic field around the wire, its direction
depending on that of the current.
A current is induced in a loop of wire when
it is moved towards or away from a magnetic
field, or a magnet is moved towards or away
from it, the direction of current depending
on that of the movement.
While preparing for an evening lecture on
21 April 1820, Hans Christian Ørsted made
a surprising observation.
As he was setting up his materials, he noticed
a compass needle deflected from magnetic north
when the electric current from the battery
he was using was switched on and off.
This deflection convinced him that magnetic
fields radiate from all sides of a wire carrying
an electric current, just as light and heat
do, and that it confirmed a direct relationship
between electricity and magnetism.
At the time of discovery, Ørsted did not
suggest any satisfactory explanation of the
phenomenon, nor did he try to represent the
phenomenon in a mathematical framework.
However, three months later he began more
intensive investigations.
Soon thereafter he published his findings,
proving that an electric current produces
a magnetic field as it flows through a wire.
The CGS unit of magnetic induction is named
in honor of his contributions to the field
of electromagnetism.
His findings resulted in intensive research
throughout the scientific community in electrodynamics.
They influenced French physicist André-Marie
Ampère's developments of a single mathematical
form to represent the magnetic forces between
current-carrying conductors.
Ørsted's discovery also represented a major
step toward a unified concept of energy.
This unification, which was observed by Michael
Faraday, extended by James Clerk Maxwell,
and partially reformulated by Oliver Heaviside
and Heinrich Hertz, is one of the key accomplishments
of 19th century mathematical physics.
It had far-reaching consequences, one of which
was the understanding of the nature of light.
Unlike what was proposed in Electromagnetism,
light and other electromagnetic waves are
at the present seen as taking the form of
quantized, self-propagating oscillatory electromagnetic
field disturbances which have been called
photons.
Different frequencies of oscillation give
rise to the different forms of electromagnetic
radiation, from radio waves at the lowest
frequencies, to visible light at intermediate
frequencies, to gamma rays at the highest
frequencies.
Ørsted was not the only person to examine
the relation between electricity and magnetism.
In 1802 Gian Domenico Romagnosi, an Italian
legal scholar, deflected a magnetic needle
by electrostatic charges.
Actually, no galvanic current existed in the
setup and hence no electromagnetism was present.
An account of the discovery was published
in 1802 in an Italian newspaper, but it was
largely overlooked by the contemporary scientific
community.
Overview
The electromagnetic force is one of the four
known fundamental forces.
The other fundamental forces are:
the weak nuclear force, which binds to all
known particles in the Standard Model, and
causes certain forms of radioactive decay.;
the strong nuclear force, which binds quarks
to form nucleons, and binds nucleons to form
nuclei
the gravitational force.
All other forces are ultimately derived from
these fundamental forces and momentum carried
by the movement of particles.
The electromagnetic force is the one responsible
for practically all the phenomena one encounters
in daily life above the nuclear scale, with
the exception of gravity.
Roughly speaking, all the forces involved
in interactions between atoms can be explained
by the electromagnetic force acting on the
electrically charged atomic nuclei and electrons
inside and around the atoms, together with
how these particles carry momentum by their
movement.
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.
A necessary part of understanding the intra-atomic
to intermolecular forces is the effective
force generated by the momentum of the electrons'
movement, and that electrons move between
interacting atoms, carrying momentum with
them.
As a collection of electrons becomes more
confined, their minimum momentum necessarily
increases due to the Pauli exclusion principle.
The behaviour of matter at the molecular scale
including its density is determined by the
balance between the electromagnetic force
and the force generated by the exchange of
momentum carried by the electrons themselves.
Classical electrodynamics
The scientist William Gilbert proposed, in
his De Magnete, that electricity and magnetism,
while both capable of causing attraction and
repulsion of objects, were distinct effects.
Mariners had noticed that lightning strikes
had the ability to disturb a compass needle,
but the link between lightning and electricity
was not confirmed until Benjamin Franklin's
proposed experiments in 1752.
One of the first to discover and publish a
link between man-made electric current and
magnetism was Romagnosi, who in 1802 noticed
that connecting a wire across a voltaic pile
deflected a nearby compass needle.
However, the effect did not become widely
known until 1820, when Ørsted performed a
similar experiment.
Ørsted's work influenced Ampère to produce
a theory of electromagnetism that set the
subject on a mathematical foundation.
A 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 in a vacuum is a universal constant,
dependent only on the electrical permittivity
and magnetic permeability of free space.
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 experimental efforts failed
to detect the presence of the aether.
After important contributions of Hendrik Lorentz
and Henri Poincaré, 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 addition, relativity theory shows that
in moving frames of reference a magnetic field
transforms to a field with a nonzero electric
component and vice versa; thus firmly showing
that they are two sides of the same coin,
and thus the term "electromagnetism".
(For more information, see Classical electromagnetism
and special relativity and Covariant formulation
of classical electromagnetism.
Photoelectric effect
In another paper published in that same year,
Albert Einstein undermined the very foundations
of classical electromagnetism.
In his theory of the photoelectric effect
and inspired by the idea of Max Planck's "quanta",
he posited that light could exist in discrete
particle-like quantities as well, 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-1950s,
is known as quantum electrodynamics, and,
in situations where perturbation theory is
applicable, is one of the most accurate theories
known to physics.
Quantities and units
Electromagnetic units are part of a system
of electrical units based primarily upon the
magnetic properties of electric currents,
the fundamental SI unit being the ampere.
The units are:
In the electromagnetic cgs system, electric
current is a fundamental quantity defined
via Ampère's law and takes the permeability
as a dimensionless quantity whose value in
a vacuum is unity.
As a consequence, the square of the speed
of light appears explicitly in some of the
equations interrelating quantities in this
system.
Formulas for physical laws of electromagnetism
need to be adjusted depending on what system
of units one uses.
This is because there is no one-to-one correspondence
between electromagnetic units in SI and those
in CGS, as is the case for mechanical units.
Furthermore, within CGS, there are several
plausible choices of electromagnetic units,
leading to different unit "sub-systems", including
Gaussian, "ESU", "EMU", and Heaviside–Lorentz.
Among these choices, Gaussian units are the
most common today, and in fact the phrase
"CGS units" is often used to refer specifically
to CGS-Gaussian units.
Electromagnetic phenomena
With the exception of gravitation, electromagnetic
phenomena as described by quantum electrodynamics
account for almost all physical phenomena
observable to the unaided human senses, including
light and other electromagnetic radiation,
all of chemistry, most of mechanics, and,
of course, magnetism and electricity.
Magnetic monopoles are not strictly electromagnetic
phenomena, since in standard electromagnetism,
magnetic fields are generated not by true
"magnetic charge" but by currents.
There are, however, condensed matter analogs
of magnetic monopoles in exotic materials
created in the laboratory.
Electromagnetic induction
Electromagnetic Induction is the Induction
of an electromotive force in a circuit by
varying the magnetic flux linked with the
circuit.
The phenomenon was first investigated in 1830-31
by Joseph Henry and Michael Faraday, who discovered
that when the magnetic field around an electromagnet
was increased and decreased, an electric current
should be detected by nearby conductor.
A current can also be induced by constantly
moving a permanent magnet in and out of a
coil of wire, or by constantly moving a conductor
near a stationary permanent magnet.
The induced electromotive force is proportional
to the rate of change of the magnetic flux
cutting across the circuit.

Electromagnetic surveys
The conductive properties of rocks near the
Earth's surface can be mapped by ground, borehole,
and airborne electromagnetic methods.
The resulting geophysical data are useful
for geological mapping, mineral exploration,
petroleum exploration, geotechnical investigations,
and unexploded ordnance detection.
See also
Notes
Further reading
Web sources
Lecture notes
Textbooks
General references
External links
Oppelt, Arnulf.
"magnetic field strength".
Retrieved 2007-06-04. 
"magnetic field strength converter".
Retrieved 2007-06-04. 
Electromagnetic Force - from Eric Weisstein's
World of Physics
Goudarzi, Sara.
"Ties That Bind Atoms Weaker Than Thought".
LiveScience.com.
Retrieved 2013-11-12. 
Quarked Electromagnetic force - A good introduction
for kids
The Deflection of a Magnetic Compass Needle
by a Current in a Wire
