Fr.: pression acoustique
Same as → sound pressure.
Fr.: pression de Coulomb
The repulsive interaction due to the → Coulomb energy between two ions. If the ionic charge is Z, then the Coulomb potential energy is Z2e2/a, where a is some typical separation between the ions. The Coulomb pressure is expected to become important when the ratio ΓC = Z2e2/akT is much larger than 1. In that case, Coulomb effects dominate those of → thermal agitation and the gas settles down into a → crystal.
fešâr-e vâgeni (#)
Fr.: pression de dégénérescence
Pressure in a degenerate electron or neutron gas. → degenerate matter.
Fr.: pression dynamique
A property of a moving → fluid defined by (1/2)ρv2 in → Bernoulli's law, where ρ is → density of fluid and v is → velocity. Dynamic pressure is the difference between → total pressure and → static pressure. Also called → velocity pressure. → ram pressure.
Fr.: pression de Fermi
Same as → degeneracy pressure.
Fr.: pression hydrodynamique
The term ρgz in the → Bernoulli equation. It is not pressure in a real sense, because its value depends on the reference level selected.
fešâr-e meqnâtisi (#)
Fr.: pression magnétique
The pressure exerted by a magnetic field on the material that contains the field.
Fr.: pression négative
A kind of pressure that contrarily to ordinary pressure pushes inward. In contrast with the → Newtonian mechanics, in → general relativity there are situations in which pressure can be negative. Positive pressure gives rise to attractive gravity, whereas negative pressure creates → repulsive gravity.
Fr.: pression osmotique
The hydrostatic pressure produced on the surface of a partially permeable membrane by osmosis.
The force per unit area.
M.E., from O.Fr. pressure, from L. pressura "action of pressing," from pressus, p.p. of premere "to press, compress."
Fešâr "squeezing, constriction, compression," verb fešordan, fešârdan "to press, squeeze;" phonetic variants Lori xošâr, Aftari xešâr, Qazvini, Qomi xošâl; cf. Khotanese ssarr- "to exhilarate;" loaned in Arm. ôšarak, in Ar. afšaraj "juice."
Fr.: élargissement par pression
A broadening of spectral lines caused mainly by the stellar atmospheric density and the surface gravity of the star. The line strength of a spectral line depends on the number of atoms in the star's atmosphere capable of absorbing the wavelength in question. For a given temperature, the more atoms there are, the stronger and broader the spectral line appears. Denser stars with higher surface gravity will exhibit greater pressure broadening of spectral lines.
Fr.: gradient de pression
The pressure difference between two adjacent regions of a fluid that results in a force being exerted from the high pressure region toward the low pressure region.
pressure gradient force
niru-ye zine-ye fešâr
Fr.: force du gradient de pression
A force resulting from → pressure gradient that is directed from high to low pressure.
Fr.: ionisation par pression
A physical state of dense matter in which the electrostatic field of one atom should influence a neighboring atom and hence disturb atomic levels. In extreme case, such as white dwarfs, electron clouds practically rub and electrons are ionized off the parent atoms.
pressure scale height
bolandi-ye marpel-e fešâr
Fr.: hauteur d'échelle de pression
A basic ingredient of the → mixing length theory that scales with the → mixing length. It is defined by the relation: HP = -dr/dln P = -Pdr/dP , where r is the height and P the pressure. See also → scale height.
Fr.: pression de radiation
The → momentum carried by → photons to a surface exposed to → electromagnetic radiation. Stellar radiation pressure on big and massive objects is insignificant, but it has considerable effects on → gas and → dust particles. Radiation pressure is particularly important for → massive stars. See, for example, → Eddington limit, → radiation-driven wind , and → radiation-driven implosion. The → solar radiation pressure is also at the origin of various physical phenomena, e.g. → gas tails in → comets and → Poynting-Robertson effect.
Fr.: pression dynamique
The pressure exerted on a body moving through a → fluid medium. For example, a → meteor traveling through the Earth's atmosphere produces a → shock wave generated by the extremely rapid → compression of air in front of the → meteoroid. It is primarily this ram pressure (rather than → friction) that heats the air which in turn heats the meteoroid as it flows around the meteoroid. The ram pressure increases with → velocity according to the relation P = (1/2)ρv2, where ρ is the density of the medium and v the relative velocity between the body and the medium. Similarly, → ram pressure stripping produces → jellyfish galaxies. Same as → dynamic pressure.
ram pressure stripping
loxtkard bâ fešâr-e qucvâr
Fr.: balayage par la pression dynamique
A process proposed to explain the observed absence of gas-rich galaxies in → galaxy clusters whereby a galaxy loses its gas when it falls into a cluster. There is a tremendous amount of hot (~ 107 K) and tenuous (~ 10-4 cm-3) gas (several 1013 → solar masses) in the → intracluster medium (ICM). Ram pressure stripping was first proposed by Gunn & Gott (1972) who noted that galaxies falling into clusters feel an ICM wind. If this wind can overcome the → gravitational attraction between the stellar and gas disks, then the gas disk will be blown away. The mapping of the gas content of spiral galaxies in the → Virgo cluster showed that the → neutral hydrogen (H I) disks of cluster spiral galaxies are disturbed and considerably reduced. Their molecular gas, more bound to the galaxy, is less perturbed, but still may be swept out in case of very strong ram pressure. These observational results indicate that the gas removal due to the rapid motion of the galaxy within the intracluster medium is responsible for the H I deficiency and the disturbed gas disks of the cluster spirals (e.g., J. A. Hester, 2006, ApJ 647:910).
solar radiation pressure
fešâr-e tâbeš xoršid (#)
Fr.: pression du rayonnement solaire
Fr.: pression de son
The periodic fluctuation above and below atmospheric pressure created by an oscillating body which provides the → sound power. Instantaneous sound pressure is the peak value of air pressure.