electromagnetic theory of light
negare-ye barqâmeqnâti-ye nur
Fr.: théorie électromagnétique de la lumière
The theory describing light as a wave phenomenon resulting from the combination of two electric and magnetic fields vibrating transversely and mutually at right angles. → electromagnetic radiation; → electromagnetic wave; → Maxwell's equations.
Fr.: onde électromagnétique
A wave produced by oscillation or acceleration of an electric charge. → electromagnetic radiation.
1) The science dealing with the physical relations between → electricity
and → magnetism. Same as
→ electromagnetic theory.
A ferroamagnetic substance, which possesses → ferromagnetism.
Relative to or characterized by → ferromagnetism.
A property of certain substances which are enormously more magnetic than any other known substance. Ferromagnetic substances, such as the chemical elements iron, nickel, cobalt, some of the rare earths, and ceratin alloys, achieve maximum → magnetization at relatively low magnetic field strengths. Their large → magnetic permeabilityies (greater than unity) vary with the strength of the applied field. When the temperature of a ferromagnet is increased the property vanishes gradually due to randomizing effects of thermal agitation. Beyond a definite temperature for each substance ( → Curie temperature) it ceases to behave as a ferromagnet and becomes a → paramagnet. Ferromagnetism is due to the alignment of the → magnetic moments of uncompensated electrons in the crystal lattice. Under the influence of an external magnetizing field, all of the uncompensated electrons line up with their → spins in the direction of the field. In contrast with paramagnetic substances, in which spins interact only with an external magnetic field, in ferromagnets the spins interact with each others, each of them trying to align the others in its own direction. This coupling gives rise to a spontaneous alignment of the moments over macroscopic regions called domains. The domains undergo further alignment when the substance is subjected to an applied field. Ferromagnets retain their magnetisation even when the external magnetic field has been removed. See also → antiferromagnetism ; → diamagnetism; → magnetism.
force-free magnetic field
Fr.: champ magnétique sans force
The condition in a plasma when the → Lorentz force is zero, that is when the electric current flows along the magnetic field. Force-free magnetic fields are encountered in astrophysical plasmas with negligible gas pressure. The solar corona is the best available example of such fields in action in a plasma.
fossil magnetic field
meydân-e meqnâtisi-ye sangvâré, ~ ~ sangvâre-yi
Fr.: champ magnétique fossile
In a physical system, the → magnetic field belonging to an earlier magnetic process or event. A fossil magnetic field may be a vanished one or exist in relic forms. As an example, the solar magnetic field, which was present during the formation of the Sun, has disappeared over the last 4.6 billions years.
frozen magnetic field line
xatt-e meydân-e meqnâtisi-ye yax basté, ~ ~ ~ rocidé
Fr.: ligne de champ magnétique gelée
Gauss's law for magnetism
qânun-e Gauss dar meqnâtmandi
Fr.: loi de Gauss en magnétisme
The → magnetic flux through an arbitrary closed surface equals zero. Mathematically, in differential form: ∇ . B = 0 and in integral form: ΦB = ∫B.dS = 0 (closed surface integral). This is one of the four → Maxwell's equations. This law expresses the fact that there are no free magnetic poles (→ monopoles) in nature and that all the lines of force of a magnetic field are closed curves.
Of or pertaining to → geomagnetism.
žirandegi-ye zamin-meqnâtisi, ~ zamin-meqnâti
Fr.: activité géomagnétique
Fr.: excursion géomagnétique
A geophysical event, distinguished from the → magnetic reversal, in which the Earth's magnetic field departs for a relatively short time from its usual near axial configuration, without establishing a reversed direction. During the excursion the intensity and direction of the Earth's magnetic field undergo drastic changes. Palaeomagnetic measurements have revealed that since the last full reversal the Earth's magnetic field has, for brief intervals, deviated from the behavior expected during "normal" secular variation.
meydân-e zamin-meqnâtisi, ~ zamin-meqnâti
Fr.: champ géomagnétique
The magnetic field that is generated within the Earth and extends out around the Earth. The intensity of the magnetic field at the Earth's surface is about 0.32 → gauss at the equator and 0.62 gauss at the north pole.
tufân-e zamin-meqnâtisi, ~ zamin-meqnâti
Fr.: orage géomagnétique
A branch of geophysics concerned with the study of the Earth's → geomagnetic field, including its origin, spatial extent, and variations in time.
giant magnetoresistance (GMR)
meqnât-istâdegi-ye kalân, istâdegi-ye meqnâtisi-ye ~
Fr.: magnétorésistance géante
A quantum mechanical phenomenon where the resistance of certain materials drops dramatically upon application of a magnetic field in certain structures composed of alternating layers of magnetic and nonmagnetic metals. The basis of the GMR is the dependence of the electrical resistivity of electrons in a magnetic metal on the direction of the electron spin, either parallel or anti-parallel to the magnetic moment of the layers. The 2007 Nobel Prize in physics was awarded to the French physicist Albert Fert (1938-) and German physicist Peter Grünberg (1939-) for the discovery of GMR.
Same as → magnetohydrodynamics.
ideal magnetohydrodynamics (MHD)
meqnâtohidrotavânik-e ârmâni, ~ minevâr
Fr.: magnétohydrodynamique idéale
Magnetohydrodynamics of a → plasma with very large (infinite) → conductivity. In this condition, → Ohm's law reduces to E = -v × B, where E represents → electric field, B → magnetic field, and v the → fluid velocity. Ideal MHD is the simplest model to describe the dynamics of plasmas immersed in a magnetic field. It is concerned with → one-fluid magnetohydrodynamics and neglects → resistivity. This theory treats the plasma composed of many charged particles with locally neutral charge as a continuous single → fluid. Ideal MHD does not provide information on the velocity distribution and neglects the physics relating to wave-particle interactions, as does the two-fluid theory as well. It does have the advantage that the macroscopic dynamics of the → magnetized plasma can be analyzed in realistic three-dimensional geometries (K. Nishikawa & M. Wakatani, 2000, Plasma Physics, Springer). See also → non-ideal magnetohydrodynamics.
interplanetary magnetic field
meydân-e meqnâtisi-ye andarsayyârei
Fr.: champ magnétique interstellaire
The magnetic field that is carried along with the solar wind and fills the solar system space. It is wound into a spiral structure by the rotation of the Sun. At the Earth's distance from the Sun, it has a strength of about 5 x 10-5 gauss.