- 行业: Printing & publishing
- Number of terms: 178089
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McGraw Hill Financial, Inc. is an American publicly traded corporation headquartered in Rockefeller Center in New York City. Its primary areas of business are financial, publishing, and business services.
An entity that can be loosely described as a quantum of energy of electromagnetic radiation. According to classical electromagnetic theory, an electromagnetic wave can transfer arbitrarily small amounts of energy to matter. According to the quantum theory of radiation, however, the energy is transferred in discrete amounts. The energy of a photon is the product of Planck's constant and the frequency of the electromagnetic field. In addition to energy, the photon possesses momentum and also possesses angular momentum corresponding to a spin of unity. The interaction of radiation with matter involves the absorption, scattering, and emission of photons. Consequently, the energy interchange is inherently quantized.
Industry:Science
An entity that comprises the majority of the energy of the universe and is responsible for the accelerating cosmic expansion. Its name derives from the inference that it is nonluminous. The nature of dark energy is speculative. Leading theories propose that dark energy is a static, cosmological constant consisting of quantum zero-point energy, and a dynamical condensate of a new, low-mass particle. Alternatively, it has been proposed that the dark energy phenomena are due to a change in the form of gravitation on cosmological length scales. Confirmation of any one of these ideas would have a profound impact on physics. Dark energy appears to be distinct from dark matter, nonluminous particles which make up the majority of the mass of galaxies and galaxy clusters. Determining the nature of dark energy is widely regarded as one of the most important problems in physics and astronomy.
Industry:Science
An enyme that was first identified and named by Alexander Fleming, who recognized its bacteriolytic properties. It has been designated muramidase, since it is known to facilitate the hydrolysis of a β-1-4-glycosidic bond between <i>N</i>-acetylglucosamine and <i>N</i>-acetylmuramic acid in bacterial cell walls; it also hydrolyzes similar glycosidic bonds in fragments of chitin. In recognition of this enzyme function, the International Enzyme Commission assigned the official chemical name of <i>N</i>-acetylmuramide glycanohydrolyase, coded as 3.2.1.17.
Industry:Science
An enzyme that is a plasminogen activator. Urokinase cleaves the plasma protein plasminogen, forming the active enzyme plasmin, which subsequently degrades fibrin. Thus urokinase is an essential component in the fibrinolytic clot-dissolving system in the human body. Other plasminogen activators include tissue plasminogen activator and streptokinase.
Industry:Science
An enzyme which breaks down (hydrolyzes) starch, the reserve carbohydrate in plants, and glycogen, the reserve carbohydrate in animals, into reducing fermentable sugars, mainly maltose, and reducing nonfermentable or slowly fermentable dextrins. Amylases are classified as saccharifying (β-amylase) and as dextrinizing (α-amylases). The α- and β-amylases are specific for the α- and β-glucosidic bonds which connect the monosaccharide units into large aggregates, the polysaccharides. The α-amylases are found in all types of organs and tissues, whereas β-amylase is found almost exclusively in higher plants.
Industry:Science
An enzyme, specifically an endodeoxyribonuclease, that recognizes a short specific sequence within a deoxyribonucleic acid (DNA) molecule and then catalyzes double-strand cleavage of that molecule. Restriction enzymes have been found in bacteria, where they serve to protect against the deleterious effects of foreign DNA, and in viruses that infect <i>Chlorella</i>-like organisms, the only eukaryotes from which those enzymes have yet been isolated.
Industry:Science
An equation which is used to study the nonequilibrium behavior of a collection of particles. In a state of equilibrium a gas of particles has uniform composition and constant temperature and density. If the gas is subjected to a temperature difference or disturbed by externally applied electric, magnetic, or mechanical forces, it will be set in motion and the temperature, density, and composition may become functions of position and time; in other words, the gas moves out of equilibrium. The Boltzmann equation applies to a quantity known as the distribution function, which describes this nonequilibriium state mathematically and specifies how quickly and in what manner the state of the gas changes when the disturbing forces are varied.
Industry:Science
An equilibrium in a chemical reaction in which at least one ionic species is produced, consumed, or changed from one medium to another.
Industry:Science
An essentially ionic approach to chemical bonding which is often used with coordination compounds. These compounds consist of a central transition-metal ion that is surrounded by a regular array of coordinated atoms or ligands. Accordingly, the ligands are assumed to be sources of negative charge which perturb the energy levels of the central metal ion. In this respect the ligands subject the metal ion to an electric field which is analogous to the electric or crystal field produced by the regular distribution of nearest neighbors within an ionic crystalline lattice. For example, the crystal field produced by the Cl ion ligand in octahedral TiCl<sub>6</sub><sup>3-</sup> is considered to be similar to that produced by the octahedral array of the six Cl ions about each Na ion in NaCl. The Na ion with its rare-gas configuration has an electronic charge distribution which is spherically symmetric both within and without the crystal field. The paramagnetic Ti(III) ion, which possesses one 3<i>d</i> electron (<i>d</i><sup>1</sup>), has a spherically symmetric charge distribution only in the absence of the crystal field produced by the ligands. The presence of the ligands destroys the spherical symmetry and produces a more complex set of energy levels within the central metal ion. The crystal field theory allows the energy levels to be calculated and related to experimental observation.
Industry:Science
An evacuated electron-beam tube in which an initial velocity modulation imparted to electrons in the beam results subsequently in density modulation of the beam. A klystron is used either as an amplifier in the microwave region or as an oscillator. For use as an amplifier, a klystron receives microwave energy at an input cavity through which the electron beam passes. The microwave energy modulates the velocities of electrons in the beam, which then enters a drift space. Here the faster electrons overtake the slower to form bunches. In this manner, the uniform current density of the initial beam is converted to an alternating current. The bunched beam with its significant component of alternating current then passes through an output cavity to which the beam transfers its ac energy.
Industry:Science
