- 行业: Printing & publishing
- Number of terms: 178089
- Number of blossaries: 0
- 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.
A quantitative measure of the ability of an optical instrument to produce separable images. The images to be resolved may differ in position because they represent (1) different points on the object, as in telescopes and microscopes, or (2) images of the same object in light of two different wavelengths, as in prism and grating spectroscopes. For the former class of instruments, the resolving limit is usually quoted as the smallest angular or linear separation of two object points, and for the latter class, as the smallest difference in wavelength or wave number that will produce separate images. Since these quantities are inversely proportional to the power of the instrument to resolve, the term resolving power has generally fallen into disfavor. It is still commonly applied to spectroscopes, however, for which the term chromatic resolving power is used, signifying the ratio of the wavelength itself to the smallest wavelength interval resolved. The figure quoted as the resolving power or resolving limit of an instrument may be the theoretical value that would be obtained if all optical parts were perfect, or it may be the actual value found experimentally. Aberrations of lenses or defects in the ruling of gratings usually cause the actual resolution to fall below the theoretical value, which therefore represents the maximum that could be obtained with the given dimensions of the instrument in question. This maximum is fixed by the wave nature of light and may be calculated for given conditions by diffraction theory.
Industry:Science
A quantitative relationship between the quantity of heat produced in a conductor and an electric current flowing through it. As experimentally determined and announced by J. P. Joule, the law states that when a current of voltaic electricity is propagated along a metallic conductor, the heat evolved in a given time is proportional to the resistance of the conductor multiplied by the square of the electric intensity. Today the law would be stated as <i>H</i> = <i>RI</i><sup>2</sup>, where <i>H</i> is rate of evolution of heat in watts, the unit of heat being the joule; <i>R</i> is resistance in ohms; and <i>I</i> is current in amperes. This statement is more general than the one sometimes given that specifies that <i>R</i> be independent of <i>I</i>. Also, it is now known that the application of the law is not limited to metallic conductors.
Industry:Science
A quantity of a substance that contains the same number (known as the Avogadro number) of molecules as the number of atoms contained in exactly 12.000 g of carbon-12 (<sup>12</sup>C). This convention stems from the concept that the central principle guiding chemical calculations is the relation of quantities of reacting substances to the numbers of molecules involved.
Industry:Science
A quantity with the dimensions of energy which determines the thermionic emission of a solid at a given temperature. The thermionic electron current density <i>J</i> emitted by the surface of a hot conductor at a temperature <i>T</i> is given by the Richardson-Dushman formula, <i>J</i> = <i>AT</i><sup>2</sup><i>e</i><sup>−φ/<i>kT</i></sup>, where <i>A</i> is a constant, <i>k</i> is Boltzmann's constant (= 1.38 × 10<sup>−23</sup> joule per degree Celsius) and φ is the work function; the last may be determined from a plot of log (<i>J</i>/<i>T</i><sup>2</sup>) versus 1/<i>T</i>. For metals, φ may also be determined by measuring the photoemission as a function of the frequency of the incident electromagnetic radiation; φ is then equal to the minimum (threshold) frequency for which electron emission is observed times Planck's constant <i>h</i> (= 6.63 × 10<sup>−</sup> joule second). The work function of a solid is usually expressed in electronvolts (1 eV is the energy gained by an electron as it passes through a potential difference of 1 V, and is equal to 1.60 × 10<sup>−19</sup> J). A list of average values of work functions (in electronvolts) for metals is given in the <b>table</b>.
Industry:Science
A quantity, designated by the letter <i>h</i>, which occurs repeatedly in those fundamental formulas of physics that describe the microworld, the world of atoms and nuclei and elementary particles. Historically, <i>h</i> occurred first in 1900 in work of Max Planck on the theory of blackbody radiation, but its physical significance was at first not clear. Some physicists would claim that it is still not clear.
Industry:Science
A quantized attribute, analogous to electric charge, introduced in the classification of a subset of elementary particles—the so-called baryons—including the proton and neutron as its lightest members. As far as is known, electric charge is absolutely conserved in all physical processes. Hypercharge was introduced to formalize the observation that certain decay modes of baryons expected to proceed by means of the strong nuclear force simply were not observed.
Industry:Science
A quantum of a spin wave; an elementary excitation of a magnetic system which is usually long-range-ordered, such as a ferromagnet.
Industry:Science
A quantum of vibrational energy in a solid or other elastic medium. This vibrational energy can be transported by elastic waves. The energy content of each wave is quantized. For a wave of frequency <i>f</i>, the energy is (<i>N</i> + ½)<i>h</i><i>f</i>, where <i>N</i> is an integer and <i>h</i> is Planck's constant. Apart from the zero-point energy, ½<i>h</i><i>f</i>, there are <i>N</i> quanta of energy <i>h</i><i>f</i>. In elastic or lattice waves, these quanta are called phonons. Quantization of energy is not related to the discreteness of the lattice, and also applies to waves in a continuum.
Industry:Science
A quantum-mechanical process which permits electrons to penetrate from one side to the other through an extremely thin potential barrier to electron flow. The barrier would be a forbidden region if the electron were treated as a classical particle. A two-terminal electronic device in which such a barrier exists and primarily governs the transport characteristic (current-voltage curve) is called a tunnel junction.
Industry:Science
A quantum-mechanical theory of the motion of electrons in solids. Its name comes from the fact that it predicts certain restricted ranges, or bands, for the energies of electrons in solids.
Industry:Science
