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Éléktromagnétisme

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Éléktromagnétisme
Listrik · Magnétisme
Kotak ieu: tempo • sawala • édit

Éléktromagnétisme, dina fisika, nyaéta fénoména di mana hiji médan ngeprakkeun hiji gaya kana partikel nu sifatna mibanda muatan listrik, sarta sabalikna kapangaruhan ku hadirna sarta gerak partikel dimaksud.

Artikel ieu keur dikeureuyeuh, ditarjamahkeun tina basa Inggis.
Bantuanna didagoan pikeun narjamahkeun.

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.

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 héat 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 theréafter 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 reséarch throughout the scientific community in electrodynamics. They influenced French physicist André-Marie Ampére'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.

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.

This unification, which was observed by Michael Faraday, extended by James Clerk Maxwell, and partially reformulated by Oliver Heaviside, is one of the triumphs of 19th century physics. It had far-réaching 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 théoretical implications of electromagnetism led to the development of special relativity by Albert Einstein in 1905.

The electromagnetic force

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  Artikel utama: 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.

As it turns out, the electromagnetic force is the one responsible for practically all the phenomena encountered in daily life, with the exception of gravity. Roughly spéaking, 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

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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 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 néarby compass needle. However, the effect did not become widely known until 1820, when Ørsted performed a similar experiment. Ørsted's work influenced Ampére to produce a théory of electromagnetism that set the subject on a mathematical foundation.

An accurate théory 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 théory 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 théories 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 théory of kinematics that is compatible with classical electromagnetism.

In addition, relativity théory 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".

The photoelectric effect

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In another paper published in that same yéar, Albert Einstein undermined the very foundations of classical electromagnetism. His théory 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 théory of the photoelectric effect extended the insights that appéared 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 léads 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 théories were progenitors of quantum mechanics, which, when formulated in 1925, necessitated the invention of a quantum théory of electromagnetism. This théory, completed in the 1940s, is known as quantum electrodynamics (or "QED"), and is one of the most accurate théories known to physics.

Definition

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The term electrodynamics is sometimes used to refer to the combination of electromagnetism with mechanics, and déals with the effects of the electromagnetic field on the dynamic behavior of electrically charged particles.

Units

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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 abampere. The units are:

In the electromagnetic cgs system, electrical current is a fundamental quantity defined via Ampére's law and takes the permeability as a dimensionless quantity (relative perméability) whose value in a vacuum is unity. As a consequence, the square of the speed of light appéars explicitly in some of the equations interrelating quantities in this system. Citakan:SI electromagnetism units

See also

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References

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Web

Books

  • Durney, Carl H. and Johnson, Curtis C. (1969). Introduction to modern electromagnetics. McGraw-Hill. ISBN 0-07-018388-0. 
  • Rao, Nannapaneni N. (1994). Elements of engineering electromagnetics (4th ed.). Prentice Hall. ISBN 0-13-948746-8. 
  • Tipler, Paul (1998). Physics for Scientists and Engineers: Vol. 2: Light, Electricity and Magnetism (4th ed. ed.). W. H. Freeman. ISBN 1-57259-492-6. 
  • Griffiths, David J. (1998). Introduction to Electrodynamics (3rd ed. ed.). Prentice Hall. ISBN 0-13-805326-X. 
  • Jackson, John D. (1998). Classical Electrodynamics (3rd ed. ed.). Wiley. ISBN 0-471-30932-X. 
  • Rothwell, Edward J.; Cloud, Michael J. (2001). Electromagnetics. CRC Press. ISBN 0-8493-1397-X. 
  • Wangsness, Roald K.; Cloud, Michael J. (1986). Electromagnetic Fields (2nd Edition). Wiley. ISBN 0-471-81186-6. 
  • Dibner, Bern (1961). Oersted and the discovery of electromagnetism. Blaisdell Publishing Company. ISSN 99-0317066-1 ; 18. 
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