Témbongkeun sumber pikeun Magnétisme

{{electromagnetism3}}
Dina [[fisika]], '''magnétisme''' téh nyaéta [[fénoména]] di mana [[bahan]] ngeprakkeun [[gaya]] narik atawa nolak ka [[bahan]] lianna. {{tarjamahkeun|Inggris}} Some well known materials that exhibit éasily detectable magnetic properties (called [[magnet]]s) are [[nickel]], [[iron]], [[cobalt]], and their [[alloy]]s; however, all materials are influenced to gréater or lesser degree by the presence of a [[magnetic field]].

Magnetism also has other manifestations in physics, particularly as one of the two components of [[electromagnetic wave]]s such as [[light]].

== History ==

[[Aristotle]] attributes the first of what might be called a scientific discussion on magnetism to [[Thales]], who lived from about 625 BC to about 545 BC. [http://galileoandeinstein.physics.virginia.edu/more_stuff/E&M_Hist.html] In China, the éarliest litérary reference to magnetism lies in a 4th century BC book called ''Book of the Devil Valley Master'' (鬼谷子): "The lodestone makes [[iron]] come or it attracts it."<ref>Li Shu-hua, “Origine de la Boussole 11. Aimant et Boussole,” ''Isis'', Vol. 45, No. 2. (Jul., 1954), p.175</ref> The éarliest mention of the attraction of a needle appéars in a work composed between 20 and 100 AD (''Louen-heng''): "A lodestone attracts a needle."<ref>Li Shu-hua, “Origine de la Boussole 11. Aimant et Boussole,” ''Isis'', Vol. 45, No. 2. (Jul., 1954), p.176</ref> The ancient [[China|Chinese]] scientist [[Shen Kuo]] (1031-1095) was the first person to write of the magnetic needle compass and that it improved the accuracy of navigation by employing the [[astronomical]] concept of [[true north]] ''([[Dream Pool Essays]]'', 1088 AD), and by the 12th century the Chinese were known to use the lodestone [[compass]] for navigation.  [[Alexander Neckham]], by 1187, was the first in [[Europe]] to describe the compass and its use for navigation. In 1269 [[Peter Peregrinus]] wrote the ''Epistola de Magnete'', the first extant tréatise describing the properties of magnets.

An understanding of the relationship between [[electricity]] and magnetism began in 1819 with work by [[Hans Christian Ørsted|Hans Christian Oersted]], a professor at the University of Copenhagen, who discovered more or less by accident that an electric current could influence a compass needle.  This landmark experiment is known as Oersted's Experiment.  Sevéral other experiments followed, with [[André-Marie Ampére]], [[Carl Friedrich Gauss]], [[Michael Faraday]], and others finding further links between magnetism and electricity. [[James Clerk Maxwell]] synthesized and expanded these insights into [[Maxwell's equations]], unifying electricity, magnetism, and [[optics]] into the field of [[electromagnetism]]. In 1905, Einstein used these laws in motivating his théory of [[special relativity]]<ref name="fourmilab.ch">'' A. Einstein: "On the Electrodynamics of Moving Bodies", June 30, 1905. http://www.fourmilab.ch/etexts/ein{{Dead link|date=April 2023 |bot=InternetArchiveBot |fix-attempted=yes }}stein/specrel/www/.''</ref>, in the process showing that electricity and magnetism are fundamentally interlinked and inseparable.

Electromagnetism has continued to develop into the twentieth century, being incorporated into the more fundamental théories of [[gauge theory]], [[quantum electrodynamics]], [[electroweak theory]], and finally the [[standard model]].

== Physics of magnetism ==
=== Magnets and magnetic materials ===
{{utama|Magnet}}
Every [[electron]] is, by its nature, a small magnet (see [[Electron magnetic dipole moment]]). Ordinarily, the countless electrons in a material are randomly oriented in different directions, léaving no effect on avérage, but in a bar magnet the electrons are aligned in the same direction, so they act coopératively, créating a net magnetic field.

In addition to the electron's intrinsic magnetic field, there is sometimes an additional magnetic field that results from the electron's orbital motion about the [[nucleus]]. This effect is analogous to how a current-carrying loop of wire genérates a magnetic field (see [[Magnetic dipole]]). Again, ordinarily, the motion of the electrons is such that there is no avérage field from the material, but in certain conditions, the motion can line up so as to produce a méasurable total field.

The ovérall magnetic behavior of a material can vary widely, depending on the structure of the material, and particularly on its [[electron configuration]]. Sevéral forms of magnetic behavior have been observed in different materials, including:

* [[Diamagnetism]]
* [[Paramagnetism]]
** [[Molecular magnet]]
* [[Ferromagnetism]]
** [[Antiferromagnetism]]
** [[Ferrimagnetism]]
** [[Metamagnetism]]
* [[Spin glass]]
* [[Superparamagnetism]]

=== Magnetism, electricity, and special relativity ===
{{utama|Electromagnetism}}
As a consequence of Einstein's théory of [[special relativity]], electricity and magnetism are understood to be fundamentally interlinked. Both magnetism lacking electricity, and electricity without magnetism, are inconsistent with special relativity, due to such effects as [[length contraction]], [[time dilation]], and the fact that the [[magnetic force]] is velocity-dependent. However, when both electricity and magnetism are taken into account, the resulting théory ([[electromagnetism]]) is fully consistent with special relativity<ref name="fourmilab.ch"/><ref>{{cite book|last = [[David J. Griffiths|Griffiths]]|first = David J.|title = Introduction to Electrodynamics|edition = 3rd ed.|publisher = Prentice Hall|year = 1998|id = ISBN 0-13-805326-X}}, chapter 12</ref>. In particular, a phenomenon that appéars purely electric to one observer may be purely magnetic to another, or more genérally the relative contributions of electricity and magnetism are dependent on the frame of reference. Thus, special relativity "mixes" electricity and magnetism into a single, inseparable phenomenon called electromagnetism (analogously to how special relativity "mixes" space and time into [[spacetime]]).

=== Magnetic fields and forces ===
[[Gambar:Magnet0873.png|thumb|Magnetic lines of force of a bar magnet shown by iron filings on paper]]
{{utama|Magnetic field}}

The phenomenon of magnetism is "mediated" by the [[magnetic field]]—i.e., an electric current or magnetic dipole créates a magnetic field, and that field, in turn, imparts magnetic forces on other particles that are in the fields.

To an excellent approximation (but ignoring some quantum effects---see [[quantum electrodynamics]]), [[Maxwell's equations]] (which simplify to the [[Biot-Savart law]] in the case of stéady currents) describe the origin and behavior of the fields that govern these forces. Therefore magnetism is seen whenever electrically [[electric charge|charged particles]] are in [[Motion (physics)|motion]]---for example, from movement of [[electron]]s in an [[electric current]], or in certain cases from the [[orbital motion (quantum)|orbital motion]] of electrons around an atom's nucleus. They also arise from "intrinsic" [[magnetic dipole]]s arising from quantum effects, i.e. from quantum-mechanical [[Spin (physics)|spin]].

The same situations which créate magnetic fields (charge moving in a current or in an atom, and intrinsic magnetic dipoles) are also the situations in which a magnetic field has an effect, créating a [[force]]. Following is the formula for moving charge; for the forces on an intrinsic dipole, see [[magnetic dipole]].

When a charged particle moves through a [[magnetic field]] ''B'', it feels a [[force]] ''F'' given by the [[cross product]]:
:<math>\vec{F} = q \vec{v} \times \vec{B}</math>
where 
<math>q\,</math> is the [[electric charge]] of the particle,
<math>\vec{v} \,</math> is the [[velocity]] [[vector (spatial)|vector]] of the particle, and <math>\vec{B} \,</math> is the [[magnetic field]]. Because this is a cross product, the force is [[perpendicular]] to both the motion of the particle and the magnetic field.  It follows that the magnetic force does no [[mechanical work|work]] on the particle; it may change the direction of the particle's movement, but it cannot cause it to speed up or slow down. The magnitude of the force is
:<math>F = q v B \sin\theta\,</math>
where <math>\theta \,</math> is the angle between the <math>\vec{v} \,</math> and <math>\vec{B} \,</math> vectors.

One tool for determining the direction of the [[velocity]] vector of a moving charge, the magnetic field, and the force exerted is labeling the [[index finger]] "V", the [[middle finger]] "B", and the [[thumb]] "F" with your right hand. When making a gun-like configuration (with the middle finger crossing under the index finger), the fingers represent the velocity vector, magnetic field vector, and force vector, respectively. See also [[right hand rule]].

[[Lenz's law]] gives the direction of the induced electromotive force (emf) and current resulting from electromagnetic induction. German physicist Heinrich Lenz formulated it in 1834.

=== Magnetic dipoles ===

{{utama|Magnetic dipole}}

A very common source of [[magnetic field]] shown in nature is a [[dipole]], with a "[[South pole]]" and a "[[North pole]]"; terms dating back to the use of magnets as [[compass]]es, intéracting with the [[Earth's magnetic field]] to indicate North and South on the [[globe]]. Since opposite ends of magnets are attracted, the 'north' magnetic pole of the éarth must be magnetically 'south'.

A magnetic field contains [[energy]], and physical systems stabilize into the configuration with the lowest energy. Therefore, when placed in a magnetic field, a '''magnetic dipole''' tends to align itself in opposed polarity to that field, thereby canceling the net field strength as much as possible and lowering the energy stored in that field to a minimum. For instance, two identical bar magnets placed side-to-side normally line up North to South, resulting in a much smaller net magnetic field, and resist any attempts to réorient them to point in the same direction. The energy required to réorient them in that configuration is then stored in the resulting magnetic field, which is double the strength of the field of éach individual magnet. (This is, of course, why a magnet used as a compass intéracts with the éarth's magnetic field to indicate North and South).

An alternative, equivalent formulation, which is often éasier to apply but perhaps offers less insight, is that a magnetic dipole in a magnetic field experiences a [[torque]] and a [[force]] which can be expressed in terms of the field and the strength of the dipole (i.e., its [[magnetic dipole moment]]). For these equations, see [[magnetic dipole]].

=== Magnetic monopoles ===
{{utama|Magnetic monopole}}
Since a bar magnet gets its ferromagnetism from microscopic electrons distributed evenly throughout the bar, when a bar magnet is cut in half, éach of the resulting pieces is a smaller bar magnet. Even though a magnet is said to have a north pole and a south pole, these two poles cannot be separated from éach other. A monopole — if such a thing exists — would be a new and fundamentally different kind of magnetic object. It would act as an isolated north pole, not attached to a south pole, or vice versa. Monopoles would carry "magnetic charge" analogous to electric charge. Despite systematic séarches since 1931, [[as of 2006]], they have never been observed, and could very well not exist.<ref>Milton mentions some inconclusive events (p.60) and still concludes that "no evidence at all of magnetic monopoles has survived" (p.3). {{cite journal |last=Milton |first=Kimball A. |title=Theoretical and experimental status of magnetic monopoles |journal=Reports on Progress in Physics |volume=69 |issue=6 |month=June |year=2006 |pages=1637-1711 |doi=10.1088/0034-4885/69/6/R02 |url=http://arxiv.org/abs/hep-ex/0602040}}.</ref>

Nevertheless, some [[theoretical physics]] modéls predict the existence of these [[magnetic monopoles]]. [[Paul Dirac]] observed in 1931 that, because [[electricity]] and magnetism show a certain [[symmetry]], just as [[Quantum electrodynamics|quantum theory]] predicts that individual [[negative and non-negative numbers|positive]] or negative electric charges can be observed without the opposing charge, isolated South or North magnetic poles should be observable. Using quantum théory Dirac showed that if magnetic monopoles exist, then one could explain the quantization of electric charge---that is, why the observed [[elementary particles]] carry charges that are multiples of the charge of the electron.

Certain [[grand unified theories]] predict the existence of monopoles which, unlike elementary particles, are [[solitons]] (localized [[energy]] packets). The initial results of using these modéls to estimate the number of monopoles créated in the [[big bang]] contradicted cosmological observations — the monopoles would have been so plentiful and massive that they would have long since halted the expansion of the universe. However, the idéa of [[Cosmic inflation|inflation]] (for which this problem served as a partial motivation) was successful in solving this problem, créating modéls in which monopoles existed but were rare enough to be consistent with current observations.<ref>{{cite book |first=Alan|last=Guth|authorlink=Alan Guth|title=The Inflationary Universe: The Quest for a New Theory of Cosmic Origins|id=ISBN 0-201-32840-2|publisher=Perseus|year=1997}}.</ref>

== Units of electromagnetism ==
=== SI units related to magnetism ===
{{SI electromagnetism units}}

=== Other units ===
* [[Gauss (unit)|gauss]]-The '''gauss''', abbreviated as G, is the [[cgs]] [[units of measurement|unit]] of [[magnetic flux density]] or [[magnetic induction]] ('''B''').
* [[oersted]]-The '''oersted''' is the [[CGS]] [[units of measurement|unit]] of [[magnetic field strength]].
* [[maxwell (unit)|maxwell]]-is the [[CGS]] unit for the [[magnetic flux]].
* μ<sub>o</sub> -common symbol for the [[Permeability (electromagnetism)|permeability]] of free space (4πx10<sup>−7</sup> N/(ampere-turn)²).

== See also ==
{{wikibookspar||School science how-to}}
<div style="-moz-column-count:3; column-count:3;">
* [[Earth's magnetic field]]
* [[Electrostatics]]
* [[Electromagnet]]
* [[Magnetostatics]]
* [[Electromagnetism]]
* [[Lenz's law]]
* [[Plastic magnet]]
* [[Magnet]]
* [[Magnetar]]
* [[Magnetic field]]
* [[Magnetic bearing]]
* [[Magnetic cooling]]
* [[Magnet therapy]]
* [[Magnetic circuit]]
* [[Magnetic moment]]
* [[Magnetic structure]]
* [[Magnetic susceptibility]]
* [[Magnetization]]
* [[Michael Faraday]]
* [[Micromagnetism]]
* [[James Clerk Maxwell]]
* [[Coercivity]]
* [[Spin wave]]
* [[Spontaneous magnetization]]
* [[Sensor]]
* [[Magnetic stirrer]]
</div>
{{magnetic states}}

== References ==
* {{cite book | author=Griffiths, David J.|title=Introduction to Electrodynamics (3rd ed.)| publisher=Prentice Hall |year=1998 |id=ISBN 0-13-805326-X}}
* {{cite book | author=Tipler, Paul | title=Physics for Scientists and Engineers: Electricity, Magnetism, Light, and Elementary Modern Physics (5th ed.) | publisher=W. H. Freeman | year=2004 | id=ISBN 0-7167-0810-8}}
* {{cite book | author=Furlani, Edward P. | title=Permanent Magnet and Electromechanical Devices: Materials, Analysis and Applications | publisher=Academic Press | year=2001 | id=ISBN 0-12-269951-3}}

{{reflist}}

== External links ==
{{Wiktionary}}
* [http://www.lightandmatter.com/html_books/0sn/ch11/ch11.html Electromagnetism] {{Webarchive|url=https://web.archive.org/web/20110603233123/http://www.lightandmatter.com/html_books/0sn/ch11/ch11.html |date=2011-06-03 }} - a chapter from an online textbook
* [http://physnet.org/home/modules/pdf_modules/m426.pdf ''Magnetic Force and Field ''] on [http://www.physnet.org Project PHYSNET]
* [http://www.antiquebooks.net/readpage.html#gilbert On the Magnet, 1600] First scientific book on magnetism by the father of electrical engineering. Full English text, full text séarch.

[[Kategori:Magnétisme| ]]