Electromagnetism

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 is carried by electromagnetic fields composed of

electric field

s and

magnetic field

s, and it is responsible for

electromagnetic radiation

such as

light

. It is one of the four fundamental interactions (commonly called forces) in

nature

, together with 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. The electromagnetic attraction between atomic nuclei and their orbital

electron

s holds

atom

s together. Electromagnetic forces are responsible for the chemical bonds between atoms which create

molecule

s, and

intermolecular force

s. The electromagnetic force governs all chemical processes, which arise from interactions between the electrons of neighboring atoms. Electromagnetism is very widely used in modern technology, and electromagnetic theory is the basis of

electric power engineering

and electronics including digital technology.

There are numerous mathematical descriptions of the electromagnetic field. Most prominently, 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 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 ' or ' 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.

In April 1820,

Hans Christian Ørsted

observed that an electrical current in a wire caused a nearby compass needle to move. 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

photon

s. 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 flowed 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 strong nuclear force, which binds

quark

s to form nucleons, and binds nucleons to form nuclei.
* 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

gravitational force

.
All other forces (e.g.,

friction

, contact forces) are derived from these four fundamental forces and they are known as non-fundamental forces.

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

atom

s can be explained by the electromagnetic force acting between the electrically charged atomic nuclei and

electron

s 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 force

s that act between the individual

molecule

s 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

Gian 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

.Purcell: p. 278: Chapter 6.1, "Definition of the Magnetic Field." Lorentz force and force equation.

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:

*

ampere

(electric current)
*

coulomb

(electric charge)
* farad (capacitance)
* henry (inductance)
*

ohm

(resistance)
* siemens (conductance)
* tesla (magnetic flux density)
*

volt

(electric potential)
*

watt

(power)
* weber (magnetic flux)

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

* Abraham–Lorentz force
* Aeromagnetic surveys
* Computational electromagnetics
* Double-slit experiment
*

Electromagnet

*

Electromagnetic induction

* Electromagnetic wave equation
* Electromagnetic scattering
* Electromechanics
*

Geophysics

* Introduction to electromagnetism
* Magnetostatics
* Magnetoquasistatic field
*

Optics

* Relativistic electromagnetism
* Wheeler–Feynman absorber theory

References

Further reading

Web sources

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General references

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External links

Magnetic Field Strength Converter


– from Eric Weisstein's World of Physics
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