Electromagnetism is a branch of physics involving
the study of the electromagnetic force, a
type of physical interaction that occurs between
electrically charged particles. The electromagnetic
force usually exhibits electromagnetic fields
such as electric fields, magnetic fields,
and light, and is one of the four fundamental
interactions (commonly called forces) in nature.
The other three fundamental interactions are
the strong interaction, the weak interaction,
and gravitation. At high energy the weak force
and electromagnetic force are unified as a
single electroweak force.
Electromagnetic phenomena are defined in terms
of the electromagnetic force, sometimes called
the Lorentz force, which includes both electricity
and magnetism as different manifestations
of the same phenomenon. 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 atoms and molecules in matter,
and is a manifestation of the electromagnetic
force. Electrons are bound by the electromagnetic
force to atomic nuclei, and their orbital
shapes and their influence on nearby atoms
with their electrons is described by quantum
mechanics. The electromagnetic force governs
all chemical processes, which arise from interactions
between the electrons of neighboring atoms.
There are numerous mathematical descriptions
of the electromagnetic field. In classical
electrodynamics, electric fields are described
as electric potential and electric current.
In Faraday'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,
particularly the establishment of the speed
of light based on properties of the "medium"
of propagation (permeability and permittivity),
led to the development of special relativity
by Albert Einstein in 1905.
== History of the theory ==
Originally, electricity and magnetism were
considered to be two separate forces. This
view changed, however, with the publication
of James Clerk Maxwell's 1873 A Treatise on
Electricity and Magnetism in which the interactions
of positive and negative charges were shown
to be mediated 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 (or states of polarization
at individual points) attract or repel one
another in a manner similar to positive and
negative charges and always exist as pairs:
every north pole is yoked to a south pole.
An electric current inside a wire creates
a corresponding circumferential magnetic field
outside the wire. Its direction (clockwise
or counter-clockwise) depends on the direction
of the current in the wire.
A current is induced in a loop of wire when
it is moved toward or away from a magnetic
field, or a magnet is moved towards or away
from it; the direction of current depends
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 away 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
(oersted) 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 has
had far-reaching consequences, one of which
was the understanding of the nature of light.
Unlike what was proposed by the electromagnetic
theory of that time, light and other electromagnetic
waves are at present seen as taking the form
of quantized, self-propagating oscillatory
electromagnetic field disturbances 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 relationship between electricity and magnetism.
In 1802, Gian Domenico Romagnosi, an Italian
legal scholar, deflected a magnetic needle
using a Voltaic pile. The factual setup of
the experiment is not completely clear, so
if current flew across the needle or not.
An account of the discovery was published
in 1802 in an Italian newspaper, but it was
largely overlooked by the contemporary scientific
community, because Romagnosi seemingly did
not belong to this community.
An earlier (1735), and often neglected, connection
between electricity and magnetism was reported
by a Dr. Cookson. The account stated, "A tradesman
at Wakefield in Yorkshire, having put up a
great number of knives and forks in a large
box ... and having placed the box in the corner
of a large room, there happened a sudden storm
of thunder, lightning, &c. ... The owner emptying
the box on a counter where some nails lay,
the persons who took up the knives, that lay
on the nails, observed that the knives took
up the nails. On this the whole number was
tried, and found
to do the same, and that, to such a degree
as to take up large nails, packing
needles, and other iron things of considerable
weight ..." E. T. Whittaker suggested in 1910
that
this particular event was responsible for
lightning to be "credited with the power of
magnetizing steel; and it was doubtless this
which led Franklin in 1751 to attempt to magnetize
a sewing-needle by means of the discharge
of Leyden jars."
== 
Fundamental forces ==
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.
(In particle physics though, the electroweak
interaction is the unified description of
two of the four known fundamental interactions
of nature: electromagnetism and the weak interaction);
the strong nuclear force, which binds quarks
to form nucleons, and binds nucleons to form
nuclei
the gravitational force.All other forces (e.g.,
friction, contact forces) are derived from
these four fundamental forces (including momentum
which is carried by the movement of particles).The
electromagnetic force is responsible for practically
all 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 between the electrically charged atomic
nuclei and electrons of the atoms. Electromagnetic
forces also explain how these particles carry
momentum by their movement. This includes
the forces we experience in "pushing" or "pulling"
ordinary material objects, which result from
the intermolecular forces that act between
the individual molecules in our bodies and
those in the objects. The electromagnetic
force is also involved in all forms of chemical
phenomena.
A necessary part of understanding the intra-atomic
and intermolecular forces is the effective
force generated by the momentum of the electrons'
movement, such that as electrons move between
interacting atoms they carry 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 ==
In 1600, 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.
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 during the period between 1820
and 1873 when it culminated in the publication
of a treatise by James Clerk Maxwell, which
unified the preceding developments into a
single theory and discovered the electromagnetic
nature of light. In classical electromagnetism,
the behavior of the electromagnetic field
is described by 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 that is 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 (electromagnetism and classical
mechanics) 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 replaced classical
kinematics with a new theory of kinematics
compatible with classical electromagnetism.
(For more information, see History of special
relativity.)
In addition, relativity theory implies that
in moving frames of reference, a magnetic
field transforms to a field with a nonzero
electric component and conversely, a moving
electric field transforms to a nonzero magnetic
component, thus firmly showing that the phenomena
are two sides of the same coin. Hence the
term "electromagnetism". (For more information,
see Classical electromagnetism and special
relativity and Covariant formulation of classical
electromagnetism.)
== Extension to nonlinear phenomena ==
The Maxwell equations are linear, in that
a change in the sources (the charges and currents)
results in a proportional change of the fields.
Nonlinear dynamics can occur when electromagnetic
fields couple to matter that follows nonlinear
dynamical laws. This is studied, for example,
in the subject of magnetohydrodynamics, which
combines Maxwell theory with the Navier–Stokes
equations.
== 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 (relative permeability)
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
(such as Maxwell's equations) 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.
== See also
