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plerion plerion Fr.: plérion A → supernova remnant which has a filled center rather than being a shell. The internal region is "filled" by energetic particles streaming from a rotating → pulsar. The → Crab Nebula is the archetypal plerion. Plerion, from Gk. pleres "full," akin to Pers. por "full," → poly-. |
plutonium plutoniom (#) Fr.: plutonium A → radioactive → chemical element, symbol Pu. → Atomic number 94; → mass number of most stable isotope 244; → melting point 640 °C; → boiling point 3,235 °C. It was first synthesized in 1940 by American chemists Glenn T. Seaborg, Edwin M. McMillan, Joseph W. Kennedy and Arthur C. Wahl in the → nuclear reaction: _{92}U^{238} + _{0}n^{1}→ _{93}Np^{239} + β^{-} (23.5 minutes) → _{94}Pu^{239} + β^{-} (2.36 days). The → half-life of _{94}Pu^{239} is 2.44 × 10^{4} yr. Plutonium-239 is a → fissile isotope. The name derives from the planet → Pluto. It was selected because it is the next planet in the solar system beyond the planet → Neptune and the element plutonium is the next element in the → periodic table beyond → neptunium. |
Pogson's ratio vâbar-e Pogson Fr.: rapport de Pogson The constant 2.512, which is the 5th → root of 100 (2.512^{5} = 100); the ratio between two successive stellar → magnitudes. → Pogson's relation; → ratio. |
Pogson's relation bâzâneš-e Pogson Fr.: relation de Pogson The equation that expresses the → magnitude
→ difference between
two objects in terms of the → logarithm of the
→ flux → ratio: Named after Norman Robert Pogson (1829-1891), the English astronomer, who introduced the magnitude scale in 1856; → relation. |
Poinsot's motion jonbeš-e Poinsot Fr.: mouvement à la Poinsot The motion of a torque free rotating rigid body in space, in general whose angular velocity vector precesses regularly about the constant angular momentum factor. After Louis Poinsot (1777-1859), French physicist and mathematician. He was the inventor of geometrical mechanics, showing how a system of forces acting on a rigid body could be resolved into a single force and a couple. |
point spread function (PSF) karyâ-ye gostareš-e noqté, ~ ~ pandé Fr.: fonction d'étalement du point The two-dimensional intensity distribution about the image of a point source. |
Poisson distribution vâbâžeš-e Poisson Fr.: distribution de Poisson A → probability function that characterizes → discrete → random events occurring independently of one another within some definite time or space. It may be regarded as an approximation of the → binomial distribution when the number of events becomes large and the probability of success becomes small. The Poisson distribution is expressed by: f(x) = (λ^{x}e^{-λ})/x!, where λ is the mean number of successes in the interval, e is the base of the → natural logarithm, and x is the number of successes we are interested in. Named after Siméon Denis Poisson (1781-1840), French mathematician, who developed the application of Fourier series to physical problems and made major contributions to the theory of probability and to the calculus of variations; → distribution. |
Poisson's equation hamugeš-e Poisson Fr.: équation de Poisson An equation (∇^{2}φ = 4πGρ) which relates the gravitational (or electromagnetic) potential to the mass density (or charge density). → Poisson distribution; → equation. |
polar bond band-e qotbi Fr.: lien polaire A chemical bond where the electrons are shared unequally between atoms. The atom that is more electronegative will pull the electrons closer to itself. |
polar equation hamugeš-e qotbi Fr.: équation polaire An equation for a curve written in terms of the → polar coordinates. |
polar motion jonbeš-e qotbi Fr.: mouvement du pôle The irregularly varying motion of the Earth's pole of rotation with respect to the Earth's crust. |
polarization qotbeš (#) Fr.: polarisation 1) Optics: A process or state in which the directions of the electric or magnetic fields
of an → electromagnetic radiation
change in a regular pattern. Light can be polarized by a
variety of ways, involving the following processes: reflection, transmission,
double refraction, and scattering. See also
→ unpolarized light;
→ linear polarization;
→ circular polarization;
→ elliptical polarization.
The study of the polarization of light from astronomical sources can yield
unique information in particular related to the properties of magnetic fields. Verbal noun of → polarize. |
polarization angle zâviye-ye qotbeš (#) Fr.: angle de polarisation Same as → polarizing angle and → Brewster angle. → polarization; → angle. |
polarization charge bâr-e qotbeš Fr.: Same as → bound charge. → polarization; → charge. |
polarization degree daraje-ye qotbeš (#) Fr.: degré de polarisation → polarization; → degree. |
polarization fraction barxe-ye qotbeš Fr.: fraction de polarisation The ratio expressed by P = (I_{par} - I_{per}) / (I_{par} + I_{per}), where I_{par} and I_{per} are the light intensities with the electric field vector respectively parallel and perpendicular to the incident beam. → polarization; → fraction. |
pollution âludegi (#) Fr.: pollution The introduction of harmful substances or light into the natural environment
as a consequence of human activities. → light pollution. M.E., from O.Fr., from L.L. pollutionem "defilement," from L. polluere "to soil, defile," from pol-, variant of por- "forth, forward, before" + -luere "smear," related to lutum "mud," and to lues "filth; plague, pestilence;;" cf. Gk. luma "filth, dirt, disgrace;" O.Ir. loth "mud, dirt;" Lith. lutynas "pool, puddle;" Pers. âludan, as below. &ACIRC;ludegi, from âludan, âlây-"to pollute, soil, stain;" Mid.Pers. âlutan; from prefixed Proto-Iranian *ā-rūta-, from rav- "to stain, soil;" Mid.Pers. Manichean Parthian rwd "rascal;" PIE base *leu- "dirty; to soil;" cf. L. lues, as above. |
polonium poloniom (#) Fr.: polonium A radioactive chemical element; symbol Po. Atomic number 84; mass number of most stable isotope 209; melting point 254°C; boiling point 962°C. The name derives from Poland, the native country of Marie Sklodowska Curie. It was discovered by Pierre and Marie Curie in 1898, from its radioactivity. |
polycyclic aromatic hydrocarbons (PAHs) hidrocarburhâ-ye aromâtik-e polisiklik, ~ ~ bol-carxe-yi Fr.: hydrocarbures aromatiques polycycliques 1) Chemistry: A family of → organic molecules composed
of carbon and hydrogen atoms (→ hydrocarbons) in which
→ carbon atoms appear in multiple loops (polycyclic)
with strong chemical → bonds that exist between them (aromatic).
PAHs are formed during the incomplete burning of coal, oil and gas, garbage, or other
organic substances like tobacco or charbroiled meat. As a pollutant, they are of
concern because some compounds (benzo(a)pyrene) have been identified as tending to
cause cancer. → poly-; → cyclic; → aromatic; → hydrocarbon. |
polygon candbar (#) Fr.: polygone A one-dimensional closed figure consisting of a series of points, each of which is called a → vertex, and the line segments, called → sides, joining the vertices. Polygons of three sides are called → triangles, and of four sides → quadrilaterals. |
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