Fr.: densité de charge
The → electric charge per unit volume in space, or per unit area on a surface, or per unit length of a line. They are respectively called volume- (ρ), surface- (σ), or line (λ) charge density.
Fr.: densité de colonne
Density of the interstellar matter lying between an object and the Earth in a cylinder with a unity base.
Fr.: densité critique
1) Cosmology: The average density of matter in the Universe
that would be needed to eventually halt the
→ cosmic expansion.
In a spatially → flat Universe,
the critical density is expressed by
ρc = (3c2/8πG)Ht2,
where c is the → speed of light,
G is the → gravitational constant, and
Ht the → Hubble parameter.
The critical density is currently 9.3 × 10-30g cm-3,
about 6 hydrogen atoms per cubic meter
(for H0 = 70 km s-1 Mpc-1).
Fr.: densité de courant
The electric current per unit of cross-sectional area perpendicular to the direction of current flow. It is a vector quantity and represented by symbol J. Electric current density is usually expressed in amperes per square meter.
The amount of any quantity per unit volume. The mass density is the
mass per unit volume. The energy density is the energy per unit
volume; particle density is the number of particles per unit volume.
Noun form of → dense.
Fr.: fluctuations de densité
In the early Universe, localized enhancements in the density of either matter alone or matter and radiation. According to models, very small initial fluctuations (less than 1 percent) can lead to subsequent formation of galaxies.
Fr.: paramètre de densité
One of the four terms that describe an arranged version of the
→ Friedmann equations. They are all time dependent.
Fr.: profile de densité
mowj-e cagâli (#)
Fr.: onde de densité
A wave phenomenon in which the density fluctuations of a physical quantity propagates in a compressible medium. For example, the → spiral arms of a → galaxy are believed to be due to a density wave which results from the natural instability of the → galactic disk caused by its own gravitational force. A common example of a density wave concerns traffic flow. A slow-moving vehicle on a narrow two-lane road causes a high density of cars to pile up behind it. As it moves down the highway the "traffic density wave" moves slowly too. But the density wave of cars does not keep the same cars in it. Instead, the first cars leave the density wave when they pass the slow vehicle and continue on at a more normal speed and new ones are added as they approach the density wave from behind. Moreover, the speed with which the density wave moves is lower than the average speed of the traffic and that the density wave can persist well after its original cause is gone. See → density wave theory.
density wave theory
negare-ye mowj-e cagâli
Fr.: théorie des ondes de densité
One possible explanation for → spiral arms,
first put forward by B. Lindblad in about 1925 and developed later by
C.C. Lin and F. H. Shu. According to this theory, spiral arms are not material
structures, but regions of somewhat enhanced density, created by
→ density waves. Density waves are perturbations amplified by
the self-gravity of
the → galactic disk. The perturbation results from natural
non-asymmetry in the disk and enhanced by environmental processes, such as galaxy encounters.
Density waves rotate around the → galactic center and periodically
compress the disk material upon their passage. If the spiral arms were
rigid structures rotating like a pinwheel,
the → differential rotation
of the galaxy would wind up the arms completely in a relatively
short time (with respect to the age of the galaxy), → winding problem.
Inside the region defined by the → corotation radius,
density waves rotate more slowly than the galaxy's stars and gas; outside that
region they rotate faster.
density-bounded H II region
nâhiye-ye H II-ye cagâli karânmand
Fr.: bornée par la densité
An → H II region which lacks enough matter to absorb all → Lyman continuum photons of the → exciting star(s). In such an H II region a part of the ionizing photons escape into the → interstellar medium. See also → ionization-bounded H II region.
cagâli-ye elektroni (#)
Fr.: densité électronique
Fr.: densité d'énergie
The amount of energy in the form of radiation per unit volume, expressed in ergs cm-3. In particular, the energy density of blackbody radiation at temperature T is aT4, where the radiation constant a = 7.56 × 10-15 erg cm-3 (K)-4.
Fr.: densité de flux
Flux of radiation that falls on a detector per unit surface area of the detector per unit bandwidth of the radiation per unit time.
Fr.: densité lagrangienne
A quantity, denoted Ld, describing a continuous system in the
→ Lagrangian formalism, and defined as the
→ Lagrangian per unit volume.
It is related to the Lagrangian L by:
magnetic flux density
cagâli-ye šâr-e meqnâtisi (#)
Fr.: densité du flux magnétique
A vector quantity measuring the strength and direction of the magnetic field. It is the → magnetic flux per unit area of a magnetic field at right angles to the magnetic force. Magnetic flux density is expressed in → teslas. Also called → magnetic induction.
Fr.: densité massique
The mass per unit area of the ring material, integrated through the thickness of the ring. Sometimes called → surface density (Ellis et al., 2007, Planetary Ring Systems, Springer).
maximum density of water
cagâli-ye bišine-ye âb
Fr.: densité maximale de l'eau
The density of pure water occurring at 3.98 °C, which is 1.0000 g cm-3, or 1000 kg m-3. Water when cooled down contracts normally until the temperature is 3.98 °C, after which it expands. Because the maximum density of water occurs at about 4 °C, water becomes increasingly lighter at 3 °C, 2 °C, 1 °C, and 0 °C (→ freezing point). The density of liquid water at 0 °C is greater than the density of frozen water at the same temperature. Thus water is heavier as a liquid than as a solid, and this is why ice floats on water. When a mass of water cools below 4 °C, the density decreases and allows water to rise to the surface, where freezing occurs. The layer of ice formed on the surface does not sink and it acts as a thermal isolator, thus protecting the biological environment beneath it. This property of water liquid is very unusual; molecules pack more closely than in the crystal structure of ice. The reason is that → hydrogen bonds between liquid water are not stable, they are continuously broken and new bonds are created. In the crystal structure of ice molecules have a fixed pattern creating empty space between molecules.
Fr.: relation morphologie-densité
An observationally determined relationship between the → morphological classification of galaxies and the → environments in which they are located. Specifically, the morphology-density relation indicates that early-type galaxies (→ ETG) are preferentially located in high density environments, whereas late-type galaxies (→ LTG) are preferentially found in low density environments. Hence, spiral galaxies are rare in the high densities of clusters and are common in the lower density group environments. Early-type galaxies, on the other hand, are common in clusters and are rarely found in isolation.
neutral density filter
pâlâye-ye cagâli-ye natâr
Fr.: filtre neutre
A filter having a flat response over the range of wavelengths of interest. Also called neutral filter or gray filter.