Bohr's first postulate
farâvas-e naxost-e Bohr
Fr.: premier postulat de Bohr
One of the postulates used in the → Bohr model, whereby there are certain steady states of the atom in which electrons can only travel in stable orbits. In spite of their acceleration, the electrons do not radiate electromagnetic waves when they move along stationary orbits.
Fr.: postulat de Bohr
One of the three postulates advanced in the → Bohr model which led to the correct prediction of the observed line spectrum of hydrogen atom. See also → Bohr's first postulate, → Bohr's second postulate, → Bohr's third postulate,
Bohr's second postulate
farâvas-e dovom-e Bohr
Fr.: deuxième postulat de Bohr
One of the postulates used in the → Bohr model, whereby when an atom is in the steady state an electron travelling in a circular orbit should have → quantized values of the → angular momentum which comply with the condition p = n(h/2π), where p is the angular momentum of the electron, h is → Planck's constant, and n is a positive integer called → quantum number.
Bohr's third postulate
farâvas-e sevom-e Bohr
Fr.: troisième postulat de Bohr
One of the postulates used in the → Bohr model, whereby the atom emits (absorbs) a quantum of electromagnetic energy (→ photon) when the electron passes from an orbit with a greater (lesser) n value to one with a lesser (greater) value. The energy of the quantum is equal to the difference between the energies of the electron on its orbits before and after the transition or "jump": hν = ε1 - ε2, where h is the → Planck's constant and ν the frequency of the transition.
noqte-ye juš (#)
Fr.: point d'ébullition
The temperature at which a liquid changes to a gas (vapor) at normal atmospheric pressure. In other words, the temperature at which the vapor pressure of a liquid is equal to the external pressure.
M.E. boillen; O.Fr. boillir, from L. bullire "to bubble, seethe," from bulla "a bubble, knob;" → point.
Noqté, → point; juš "boiling," present stem of jušidan "to boil;" Khotanese jis- "to boil;" Av. yaēšiiant- "boiling;" cf. Skt. yas- "to boil, become hot," yasyati "boils, seethes;" Gk. zein "to bubble, boil, cook;" O.H.G. jesan "to ferment, foam;" Ger. Gischt "foam, froth," gären "to ferment;" O.E. gist; E. yeast.
Fr.: globule de Bok
A small, roughly spherical cloud of → interstellar dust and gas that appears as a dark compact globule when viewed against the background of an → H II region. Bok globules range in mass from about 1 to 1,000 or more → solar masses, and in size from about 10,000 → astronomical units to 3 → light-years. They typically have temperatures of around 10 → Kelvin. Bok globules are thought to represent a stage in the collapse of a dense fragment of → molecular clouds that are in the process of forming new stars. → elephant trunk.
In honor of Bart Jan Bok (1906-1983), the Dutch-American astronomer, who first observed these objects. In 1947, in collaboration with Edith F. Reilly, he put forward the hypothesis that these globules were undergoing → gravitational collapse to form new stars (Bok & Reilly, 1947, ApJ 105, 255); → globule.
Bolide, Fr., from L. bolis, bolidis, from Gk. bolis, bolidos "missile, flash of lightning," from ballein "to throw;" PIE *gwelH1- "to throw;" → ballistics.
Garzin "arrow;" cf. Tâleši ger "meteor" (from Proto-Iranian *garH- "to throw"), cognate with Gk. ballein, as above; → ballistics.
1) An instrument for measuring the intensity of radiant energy
in amounts as small as one millionth of an erg.
It uses the change in resistance of a thin conductor caused by
the heating effect of the radiation.
→ actinometer, → photometer, →
pyrheliometer, → pyrometer,
From Gk. bole "stroke, beam of light," from ballein "to throw" + middle suffix -o- + → -meter..
Tâvsanj, from tâv "light, brightness, heat, warmth" (from tâbidan "to radiate") + sanj, → -meter.
Of or relating to or measured by a → bolometer.
aršâyeš-e tâvsanji, ~ tâvsanjik
Fr.: correction bolométrique
tâbandegi-ye tâvsanji, ~ tâvsanjik
Fr.: luminosité bolométrique
The total rate of energy output of an object integrated over all wavelengths.
borz-e tâvsanji, ~ tâvsanjik
Fr.: magnitude bolométrique
The magnitude of an astronomical object for the entire range of its electromagnetic spectrum.
Fr.: constante de Boltzmann
Fr.: facteur de Boltzmann
The factor e-E/kT involved in the probability for atoms having an excitation energy E and temperature T, where k is Boltzmann's constant.
Fr.: constante de Boltzmann
The physical constant, noted by k, relating the mean → kinetic energy of → molecules in an → ideal gas to their → absolute temperature. It is given by the ratio of the → gas constant to → Avogadro's number. Its value is about 1.380 x 10-16erg K-1.
Named after the Austrian physicist Ludwig Boltzmann (1844-1906), who made important contributions to the theory of statistical mechanics; → constant.
Boltzmann's entropy formula
disul-e dargâšt-e Boltzmann
Fr.: formule d'entropie de Boltzmann
In → statistical thermodynamics, a probability equation relating the → entropy S of an → ideal gas to the quantity Ω, which is the number of → microstates corresponding to a given → macrostate: S = k. ln Ω. Same as → Boltzmann's relation.
Fr.: équation de Boltzmann
1) An equation that expresses the relative number (per unit volume) of → excited atoms in different states as a function of the temperature for a gas in → thermal equilibrium: Nu/Nl = (gu/gl) exp (-ΔE/kTex), where Nu and Nl are the upper level and lower level populations respectively, gu and gl the → statistical weights, ΔE = hν the energy difference between the states, k is → Boltzmann's constant, and h → Planck's constant.
Fr.: relation de Boltzmann
A relation between the → entropy of a given → state of a → thermodynamic system and the → probability of the state: S = k . ln Ω where S is the entropy of the system, k is → Boltzmann's constant, and Ω the thermodynamic probability of the state. Boltzmann's relation connects → statistical mechanics and → thermodynamics. Ω is the number of possible → microstates of the system, and it represents the → randomness of the system. The relation also describes the statistical meaning of the → second law of thermodynamics. This expression has been carved above Boltzmann's name on his tombstone in Zentralfreihof in Vienna. Same as → Boltzmann's entropy formula.
Bond, variant of band, from M.E. bende, O.E. bend, from O.Fr. bande, bende, PIE *bendh- "to bind" (cf. Goth bandi "that which binds;" Av./O.Pers. band- "to bind, fetter," banda- "band, tie" (see below); Skt. bandh- "to bind, tie, fasten," bandhah "a tying, bandage").
Band "band, tie," from Mid.Pers., O.Pers./Av. band- "to bind," banda- "band, tie," also present stem of bastan "to bind, shut," → shutter.
Fr.: albedo de Bond
The fraction of the total amount of electromagnetic radiation falling upon a non-luminous spherical body that is reflected in all directions by that body. The bond albedo takes into account all wavelengths at all → phase angles. Compare with → geometric albedo.
Named after the American astronomer George Phillips Bond (1825-1865), who proposed it; → albedo.