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dimensionless quantity candâ-ye bivâmun Fr.: quantité sans dimension A quantity without an associated → physical dimension. Dimensionless quantities are defined as the ratio of two quantities with the same dimension. The magnitude of such quantities is independent of the system of units used. A dimensionless quantity is not always a ratio; for instance, the number of people in a room is a dimensionless quantity. Examples include the → Alfven Mach number, → Ekman number, → Froude number, → Mach number, → Prandtl number, → Rayleigh number, → Reynolds number, → Richardson number, → Rossby number, → Toomre parameter. See also → large number. → dimension |
disciplinarity hâvešânigi, hâvešânmandi Fr.: disciplinarité The quality or state of being → disciplinary. → disciplinary; → -ity. |
discontinuity nâpeyvastegi (#) Fr.: discontinuité A break in sequence or continuity of anything.
→ Balmer discontinuity M.L. discontinuitas, from discontinuus, from → dis- + continuus, → continuous. Nâpeyvastegi, noun from nâpeyvasté "discontinuous," from nâ- "non, un-," → a-, + peyvasté, → continuous. |
disk instability nâpâydâri-ye gerdé, ~ disk Fr.: instabilité de disque 1) General:
The process by which an → accretion disk
cools, causing it to fragment into self-gravitating
→ clumps. → disk; → instability. |
disk instability model (DIM) model-e nâpâydâri-ye gerdé, ~ ~ disk Fr.: modèle d'instabilité de disque A model describing → dwarf novae and → Soft X-ray Transient (SXT)s. Accordingly, these objects are triggered by an → accretion disk instability due to an abrupt change in opacities (→ opacity) at → temperatures at which hydrogen is partially ionized. All versions of the DIM have this ingredient. They differ in assumptions about → viscosity, and about what happens at the inner and outer disk radii. Basically, during → quiescence, material accumulates in the accretion disk until a critical point is reached. The disk then becomes unstable and is dumped onto the → compact object, releasing a burst of → X-rays. However, the greater duration of SXT bursts (months) and the time interval between bursts (decades) cannot be accounted for by the standard disk instability model used for dwarf novae, and additional factors such as X-ray illumination and irradiation of the accretion disk are required for the model to match the observed properties of SXTs (J-P Lasota and J-M Hameury, 1995). → disk; → instability; → model. |
distributivity vâbâžandegi Fr.: distributivité The state or quality of being distributive. Noun of → distributive. |
diversity gunâguni (#) Fr.: diversité The state or fact of being diverse; difference; unlikeness. |
drift velocity tond-ye delek Fr.: vitesse de dérive The average velocity of a charged particle in a plasma in response to an applied electric field. |
duality dogânegi (#) Fr.: dualité The quality or character of being twofold, as the → wave-particle duality. M.E dualitie, from L.L. dualitas. Dogânegi, from dogânag + -i. |
ductility rešâyandi Fr.: ductilité The property of a metal that allows it to be elongated into wire or threads without fracture. For example, → copper and → silver are highly ductile metals. |
durability pâyešmandi, pâyandegi Fr.: durabilité The ability to withstand damage or decay. |
dynamic viscosity vošksâni-y tavânik Fr.: viscosité dynamique Same as → viscosity and → absolute viscosity. |
eccentricity osmarkazi Fr.: eccentricité The amount by which the orbit deviates from circularity: e = c/a, where c is the distance from the center to a focus and a the semi-major axis. If e = 0, the orbit is a circle. If e < 1, the orbit is an ellipse, if e > 1 it is a hyperbola, and if e = 1 it is a parabola. The eccentricity is one of the six → orbital elements that define a → Keplerian orbit. |
Eddington luminosity tâbandegi-ye Eddington Fr.: luminosité d'Eddington Same as → Eddington limit. → Eddington limit; → luminosity. |
effective gravity gerâni-ye oskarmand Fr.: gravité effective In a → rotating star, the sum of the → gravity and the → centrifugal acceleration. The effective gravity is a function of the rotation velocity (Ω) and the → colatitude (θ). At the pole (θ = 0°) and the equator (θ = 90°) the effective gravity is radial. See also → total gravity. |
Einsteinian relativity bâzânigi-ye Einsteini Fr.: relativité einsteinienne The laws of physics are the same in all → inertial reference frames and are invariant under the → Lorentz transformation. The → speed of light is a → physical constant, i.e. it is the same for all observers in uniform motion. Einsteinian relativity is prompted by the → Newton-Maxwell incompatibility. See also: → Galilean relativity, → Newtonian relativity. → Einstein; → relativity. |
elasticity kešâyandi (#) Fr.: élasticité The ability of a body which has been → deformed by an applied → force to return to its original shape when the force is removed. Up to a certain point the material obeys → Hooke's law. See also → ductility, → plasticity. |
electric intensity dartanuyi-e barqi (#) Fr.: intensité électrique The strength of an electric field at any point as measured by the force exerted upon a unit positive charge placed at that point. |
electrical conductivity hâznadegi-e barqi Fr.: conductivité électrique A measure of a material's ability to conduct an electrical current. It is the reciprocal of the → resistivity. Conductivity is expressed by σ = ne^{2}l/(2mv), where n is the number of electrons per cm^{3} volume of the → conductor, e is the → electron charge, l is the → mean free path, m is the → electron mass, and v is the arithmetic mean velocity of thermal motion of electrons at a given temperature. → electrical; → conductivity. |
electricity barq (#) Fr.: électricité 1) The physical phenomena arising from the behavior of → electrons
and → protons that is caused by the → attraction
of particles with opposite → charges and the
→ repulsion of particles with the same charge. From L. electrum "amber," from Gk. elektron "amber" + -ity a suffix used to form abstract nouns expressing state or condition. Barq, Pers. term, used also in Ar. and Hebrew (barak "lightening"); variants in
Pers.: varq, barx, balk, belak, bala;
Lârestâni belak; Tabari, Lahijâni, Semnâni, Sorxeyi, Sangesari belk;
Gilaki val; Lori beleyz; Kurd. bilese;
Tokharian AB pâlk; Mid/Mod.Pers. bir "lightening,"
Mid.Pers. brâh "brilliance, splendour," br'z- "to shine, beam,"
Mod.Pers. barâz "beauty, grace, elegance;"
Av. brāz- "to shine, beam; splendour," brazāiti "shines;" cf.
Skt. bhrāj- "to shine, beam, sparkle," bhrajate "shines;"
Gk. phlegein "to burn;" L. fulgere "to shine," fulmen "lightning,"
flagrare "to blaze, burn;" O.H.G. beraht "bright;" O.E. beorht
"bright;" E. → bright;
PIE base *bherəg-; *bhrēg- "to shine; white." |
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