- 行业: Printing & publishing
- Number of terms: 178089
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- Company Profile:
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.
The precise measurement of time and time interval is a key enabler of modern technology in which a profusion of electronic devices and communications equipments depend on the synchronization of time and frequency in order to operate properly over distances of thousands of miles or more. Cell phones, mobile phones, the Internet, and satellite navigation, among other ubiquitous consumer services, all depend on clocks and oscillators that control their operation. Universal access by these and related services to a standardized reference time has benefited from the establishment in 1970 of a time scale called Coordinated Universal Time (UTC), and by the means to maintain that time scale both nationally and internationally.
Industry:Science
The negatively charged constituent of ordinary matter, responsible for the chemical properties of matter, and the carrier of electricity. The electron is the lightest known particle that possesses an electric charge. Its rest mass <i>m</i><sub><i>e</i></sub> is 9.1 × 10<sup>−31</sup> kg (2.0 × 10<sup>−30</sup> lb), about 1/1836 of the mass of the proton or neutron, which are, respectively, the positively charged and neutral constituents of ordinary matter. The rest mass of an electron can also be expressed as 0.511 MeV/<i>c</i><sup>2</sup>, where <i>c</i> is the speed of light. Electrons were discovered in 1895 by J. J. Thomson in the form of cathode rays. The electron was the first elementary particle to be identified.
Industry:Science
The number of elementary entities in one mole of a substance. A mole is defined as an amount of a substance that contains as many elementary entities as there are atoms in exactly 12 g of <sup>12</sup>C; the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles. Experiments give 6.0221367 × 10<sup>23</sup> as the value of Avogadro's number. In most calculations the coefficient is rounded off to 6.02. Thus, a mole of <sup>12</sup>C atoms has 6.02 × 10<sup>23</sup> carbon atoms, a mole of water molecules contains 6.02 × 10<sup>23</sup> H<sub>2</sub>O molecules, a mole of electrons contains 6.02 × 10<sup>23</sup> electrons, and so forth.
Industry:Science
The means by which radio signals are transported through space from a transmitting antenna to a receiving antenna. Radio signals are electromagnetic waves which travel with the velocity of light and can be reflected, refracted, diffracted, scattered, and absorbed. Unlike visible light, radio frequencies cover many octaves, from about 10 kHz to 60,000 MHz (wavelengths from 30,000 m to 0.5 cm). Since frequency is an important parameter, radio propagation characteristics vary over a wide range. At the higher radio frequencies the similarity with visible light is evident. At the lower frequencies the radio waves follow the surface of the Earth by a mechanism that in geometrical optics is unimportant and relatively unknown.
Industry:Science
The limited capacity of all normal human and other animal cells to reproduce and function. The gradual decline in normal physiological function of the cells is referred to as aging or senescence. The aging process ends with the death of individual cells and then, generally, the whole animal. Aging occurs in all animals, except those that do not reach a fixed body size such as some tortoises and sharks, sturgeon, and several other kinds of fishes. These animals die as the result of accidents or disease, but losses in normal physiological function do not seem to occur. Examples of cells that do not age are those composing the germ plasm (sex cells) and many kinds of cancer cells. These cells are presumed to be immortal.
Industry:Science
The temporary blocking from view of one celestial body by another. The occulting body is the one closer to the observer, and can be a planet, moon, ring system, or other body, usually in the solar system. The occulted body is smaller in apparent, projected size and is usually a distant star, although it can also be a spacecraft radio signal, as in the case of the <i>Voyager</i> spacecraft radio occultations at the outer planets, or another solar system body. Examples of occultations are that of a star by a planet, a lunar occultation of a star, and an occultation by Pluto of its satellite charon. Although a solar eclipse is not usually thought of in these terms, this event is actually an occultation of the Sun by the Moon.
Industry:Science
The oscillations of atoms in a solid about their equilibrium positions. In a crystal, these positions form a regular lattice. Because the atoms are bound not to their average positions but to the neighboring atoms, vibrations of neighbors are not independent of each other. In a regular lattice with harmonic forces between atoms, the normal modes of vibrations are lattice waves. These are progressive waves, and at low frequencies they are the elastic waves in the corresponding anisotropic continuum. The spectrum of lattice waves ranges from these low frequencies to frequencies of the order of 10<sup>13</sup> Hz, and sometimes even higher. The wavelengths at these highest frequencies are of the order of interatomic spacings.
Industry:Science
The process whereby the intensity of a beam of electromagnetic radiation is attenuated in passing through a material medium by conversion of the energy of the radiation to an equivalent amount of energy which appears within the medium; the radiant energy is converted into heat or some other form of molecular energy. A perfectly transparent medium permits the passage of a beam of radiation without any change in intensity other than that caused by the spread or convergence of the beam, and the total radiant energy emergent from such a medium equals that which entered it, whereas the emergent energy from an absorbing medium is less than that which enters, and, in the case of highly opaque media, is reduced practically to zero.
Industry:Science
The resultant vertical force exerted on a body by a static fluid in which it is submerged or floating. The buoyant force <i>F</i><sub><i>B</i></sub> acts vertically upward, in opposition to the gravitational force that causes it. Its magnitude is equal to the weight of fluid displaced, and its line of action is through the centroid of the displaced volume, which is known as the center of buoyancy. With <i>V</i> the displaced volume of fluid and γ the specific weight of fluid (weight per unit volume), the buoyant force equation becomes <i>F</i><sub><i>B</i></sub> = γ<i>V</i>. The magnitude of the buoyant force must also be given by the difference of vertical components of fluid force on the lower and upper sides of the body.
Industry:Science
The Runge vector describes certain unchanging features of a nonrelativistic two-body interaction for which the potential energy is inversely proportional to the distance <i>r</i> between the bodies or, alternatively, in which each body exerts a force on the other that is directed along the line between them and proportional to <i>r</i><sup>−2</sup>. Two basic interactions in nature are of this type: the gravitational interaction between two masses (called the classical Kepler problem), and the Coulomb interaction between like or unlike charges (as in the hydrogen atom). Both at the classical level and the quantum-mechanical level, the existence of a Runge vector is a reflection of the symmetry inherent in the interaction.
Industry:Science
