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Bohr magneton magneton-e Bohr (#) Fr.: magnéton de Bohr A fundamental constant, first calculated by Bohr, for the intrinsic → spin magnetic moment of the electron. It is given by: μ_{B} = eħ/2m_{e} = 9.27 x 10^{-24} joule/tesla = 5.79 x 10^{-5} eV/tesla, representing the minimum amount of magnetism which can be caused by the revolution of an electron around an atomic nucleus. It serves as a unit for measuring the magnetic moments of atomic particles. |
Bohr model model-e Bohr Fr.: modèle de Bohr A model suggested in 1913 to explain the stability of atoms which classical electrodynamics was unable to account for. According to the classical view of the atom, the energy of an electron moving around a nucleus must continually diminish until the electron falls onto the nucleus. The Bohr model solves this paradox with the aid of three postulates (→ Bohr's first postulate, → Bohr's second postulate, → Bohr's third postulate). On the whole, an atom has stable orbits such that an electron moving in them does not radiate electromagnetic waves. An electron radiates only when making a transition from an orbit of higher energy to one with lower energy. The frequency of this radiation is related to the difference between the energies of the electron in these two orbits, as expressed by the equation hν = ε_{1} - ε_{2}, where h is → Planck's constant and ν the radiation frequency. The electron needs to gain energy to jump to a higher orbit. It gets that extra energy by absorbing a quantum of light (→ photon), which excites the jump. The electron does not remain on the higher orbit and returns to its lower energy orbit releasing the extra energy as radiation. Bohr's model answered many scientific questions in its time though the model itself is oversimplified and, in the strictest sense, incorrect. Electrons do not orbit the nucleus like a planet orbiting the Sun; rather, they behave as → standing waves. Same as → Bohr atom. |
Bohr radius šo'â'-e Bohr Fr.: rayon de Bohr The radius of the orbit of the hydrogen electron in its ground state (0.529 Å). |
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. |
Bohr's postulate farâvas-e Bohr 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 = ε_{1} - ε_{2}, where h is the → Planck's constant and ν the frequency of the transition. |
boiling point 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. |
Bok globule guyce-ye Bok 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 garzin Fr.: bolide A → meteor which is extremely bright, particularly one that breaks up during its passage through the → atmosphere. Also called → fireball. Bolide, Fr., from L. bolis, bolidis, from Gk. bolis, bolidos "missile, flash of lightning," from ballein "to throw;" PIE *g^{w}elH_{1}- "to throw;" → ballistics. Garzin "arrow;" cf. Tâleši ger "meteor" (from Proto-Iranian *garH- "to throw"), cognate with Gk. ballein, as above; → ballistics. |
bolometer tafsanj Fr.: bolomètre 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,
radiometer. From Gk. bole "stroke, beam of light," from ballein "to throw" + middle suffix -o- + → -meter.. Tafsanj, from taf "heat, warmth; light, brightness," from tâbidan, "→ radiate," + sanj, → -meter. |
bolometric tafsanji, tafsanjik Fr.: bolométrique Of or relating to or measured by a → bolometer. |
bolometric correction aršâyeš-e tafsanji, ~ tafsanjik Fr.: correction bolométrique The difference between the → visual magnitude and → bolometric magnitude. → bolometric; → correction. |
bolometric luminosity tâbandegi-ye tafsanji, ~ tafsanjik Fr.: luminosité bolométrique The total rate of energy output of an object integrated over all wavelengths. → bolometric; → luminosity. |
bolometric magnitude borz-e tafsanji, ~ tafsanjik Fr.: magnitude bolométrique The magnitude of an astronomical object for the entire range of its electromagnetic spectrum. → bolometric; → magnitude. |
Boltzmann constant pâyâ-ye Boltzmann Fr.: constante de Boltzmann |
Boltzmann factor karvand-e Boltzmannn 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. → Boltzmann's constant; → factor. |
Boltzmann's constant pâyâ-ye Boltzmann 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^{-16}erg 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. → Boltzmann's constant; → entropy; → formula. |
Boltzmann's equation hamugeš-e Boltzmann 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: N_{u}/N_{l} = (g_{u}/g_{l}) exp (-ΔE/kT_{ex}), where N_{u} and N_{l} are the upper level and lower level populations respectively, g_{u} and g_{l} the → statistical weights, ΔE = hν the energy difference between the states, k is → Boltzmann's constant, and h → Planck's constant. → Boltzmann's constant; → equation. |
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