Dictionary Definition
electromagnetic adj : pertaining to or exhibiting
magnetism produced by electric charge in motion; "electromagnetic
energy"
User Contributed Dictionary
English
Etymology
electromagnetism + -ic (of or pertaining to)Adjective
- pertaining to electromagnetism
Translations
pertaining to electromagnetism
- Czech: elektromagnetický
- Italian: elettromagnetico
Extensive Definition
Electromagnetism is the physics of the electromagnetic
field: a field
which exerts a force on
particles
that possess the property of electric
charge, and is in turn affected by the presence and motion of
those particles.
A changing magnetic
field produces an electric
field (this is the phenomenon of electromagnetic
induction, the basis of operation for electrical
generators, induction
motors, and transformers). Similarly, a
changing electric field generates a magnetic field. Because of this
interdependence of the electric and magnetic fields, it makes sense
to consider them as a single coherent entity - the electromagnetic
field.
The magnetic field is produced by the motion of
electric
charges, i.e., electric
current. The magnetic field causes the magnetic force
associated with magnets.
The theoretical implications of electromagnetism led to
the development of special
relativity by Albert
Einstein in 1905.
History
While preparing for an evening lecture on 21
April 1820, Hans
Christian Ørsted developed an experiment which provided
evidence that surprised him. 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 (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.
Ørsted was not the first 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. He interpreted his
observations as The Relation between electricity and magnetism.
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.
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 triumphs of 19th century physics. It had
far-reaching consequences, one of which was the understanding of
the nature of light. As it
turns out, what is thought of as "light" is actually a propagating
oscillatory
disturbance in the electromagnetic field, i.e., an electromagnetic
wave. 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.
The electromagnetic force
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.The electromagnetic force is the one responsible
for practically all the phenomena encountered in daily life, with
the exception of gravity. 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.
Classical electrodynamics
The scientist William Gilbert proposed, in his De Magnete (1600), 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.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 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. (For more information, see
History of special relativity.)
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.)
The photoelectric effect
In another paper published in that same year, Albert Einstein undermined the very foundations of classical electromagnetism. His theory of the photoelectric effect (for which he won the Nobel prize for physics) 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.Definition
The term electrodynamics is sometimes used to refer to the combination of electromagnetism with mechanics, and deals with the effects of the electromagnetic field on the dynamic behavior of electrically charged particles.Units
Electromagnetic units are part of a system of electrical units based primarily upon the magnetic properties of electric currents, the fundamental cgs unit being the ampere. The units are:In the electromagnetic cgs system, electrical
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.