Electromagnetism
Electromagnetism is the
physics of the
electromagnetic field; a
field encompassing all of
space 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.
It is often convenient to understand the electromagnetic field in terms of two separate fields: the
electric field and the
magnetic field. A non-zero electric field is produced by the presence of
electrically charged particles, and gives rise to the electric
force; this is the force that causes
static electricity and drives the flow of electric charge (
electric current) in
electrical conductors. The magnetic field, on the other hand, can be produced by the motion of electric charges, or electric current, and gives rise to the magnetic force associated with
magnets.
The term "electromagnetism" comes from the individual component electrical and magnetic forces involved. A changing magnetic field produces an electric field (this is the phenomenon of
electromagnetic induction, which underlies the operation of
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, theoretically coherent entity — the electromagnetic field. This unification, which was completed by
James Clerk Maxwell, is one of the triumphs of
19th century physics. It had far-reaching consequences, one of which was the elucidation 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 theoretical implications of electromagnetism led to the development of
special relativity by
Albert Einstein in
1905.
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.
As it turns out, 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.
According to modern electromagnetic theory, electromagnetic forces are mediated by the transfer of
virtual photons.
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 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 experimental 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 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.
In another paper published in that same year, 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.
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.
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Introduction to Electromagnetism From the basics to advanced level science
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MIT Video Lectures - Electricity and Magnetism from Spring 2002. Taught by Professor Walter Lewin.
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Electricity and Magnetism - an online textbook (uses algebra, with optional calculus-based sections)
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Electromagnetic Field Theory - an online textbook (uses calculus)
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Classical Electromagnetism: An intermediate level course - an online intermediate level texbook downloadable as PDF file
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Science Aid: electromagnetism Electromagnetism, aimed at teens.
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Motion Mountain A modern introduction to electromagnetism and its effects in everyday life.
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Radio waves, the Hertzian Radiation: what it is and how it happens.
