equilibrium Condition in which the net force acting on a particle is zero. A body in equilibrium experiences no acceleration and, unless dis¬ turbed by an outside force, will remain in equilibrium indefinitely. A stable equilibrium is one in which small, externally induced displacements from that state produce forces that tend to oppose the displacement and return the body to equilibrium. An unstable equilibrium is one in which the least departures produce forces tending to increase the displacement. A brick lying on the floor is in stable equilibrium, while a ball bearing balanced on a knife-edge is in unstable equilibrium.
equilibrium, chemical Condition in the course of a reversible chemi¬ cal reaction in which no net change in the amounts of reactants and prod¬ ucts occurs: Products are reverting to reactants at the same rate as reactants are forming products. For practical purposes, the reaction under those conditions is completed. Expressed in terms of the law of mass action, the reaction rate to form products is equal to the reaction rate to re-form reactants. The ratio of the reaction rate constants (i.e., of the amounts of reactants and products, each raised to the proper power), defines the equilibrium constant. Changing the conditions of temperature or pressure changes the reaction’s equilibrium; a high temperature or pres¬ sure may be used to “push” a reaction that at ordinary conditions makes little product. See also H.-L. Le Chatelier.
equine Ve-.kwln, 'e-.kwInN Any member of the ungulate family Equidae, which includes the modern horses, zebras, and Asses, all in the genus Equus, as well as more than 60 species known only from fossils. Equines
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equinox ► Erdos I 629
descended from the dawn horse (see Eohippus). Wild horses, which once inhabited much of northern Eurasia, were smaller and had shorter legs than their domesticated descendants. See also Przewalski's horse.
equinox Ve-kwo-.naks, 'e-kw3-,naks\ Either of two moments in the year when the Sun is exactly above the Equator and day and night are of equal length all over Earth; also, either of two points in the sky where the eclip¬ tic and the celestial equator (see celestial sphere) intersect. The vernal equi¬ nox, when spring begins in the Northern Hemisphere, occurs about March 21, when the Sun moves north across the celestial equator. The autumnal equinox falls about September 23, as the Sun crosses the celestial equa¬ tor going south. See also solstice.
equinoxes, precession of the Motion of the points where the Sun crosses the celestial equator, caused by precession of Earth’s axis. Hippar¬ chus noticed that the stars’ positions were shifted consistently from ear¬ lier measures, indicating that Earth, not the stars, was moving. This precession, a wobbling in the orientation of Earth’s axis with a cycle of almost 26,000 years, is caused by the gravity of the Sun and the Moon acting on Earth’s equatorial bulge. The planets also have a small influ¬ ence on precession. Projecting Earth’s axis onto the celestial sphere locates the northern and southern celestial poles. Precession makes these points trace out circles on the sky and also makes the celestial equator wobble, changing its points of intersection (EQUiNOXes) with the ecliptic.
equipartition of energy See equipartition of energy equistetum See horsetail
equity Finance and accounting concept. Equity represents any of three separate but related values: the money value of a property or of an inter¬ est in a property in excess of claims or liens against it; a risk interest or ownership right in property; and the common stock of a corporation. In corporate finance, a basic equation holds that a company’s total assets minus total liabilities equals total owners’ equity.
equity Justice according to fairness, especially as distinguished from mechanical application of rules under common law. Courts of equity (also called chancery courts) arose in England in the 14th century in response to the increasingly strict rules of proof and other requirements of the courts of law. Equity provided remedies not available under the old writ system. Often these remedies involved something other than damages, such as specific performance of contractual obligations, enforcement of a trust, restitution of goods wrongfully acquired, imposition of an injunc¬ tion, or the correction and cancellation of false or misleading documents. The equity courts eventually established their own precedents, rules, and doctrines and began to rival the law courts in power. The two systems were united in 1873. Courts of equity also developed early in U.S. his¬ tory, but by the early 20th century most jurisdictions had combined them with courts of law into a single system. Modern courts apply both legal and equitable principles and offer both legal and equitable relief.
equivalence principle Fundamental principle of physics that in its weak form states that gravitational (see gravitation) and inertial (see iner¬ tia) masses are the same. Albert Einstein’s stronger version states that gravi¬ tation and acceleration are indistinguishable. It implies that the effect of gravity is removed in a suitably accelerated reference frame, such as an elevator with its cable cut, in which a person would experience free fall.
equivalence relation In mathematics, a generalization of the idea of equality between elements of a SET. All equivalence relations (e.g., that symbolized by the equals sign) obey three conditions: reflexivity (every element is in the relation to itself), symmetry (element A has the same relation to element B that B has to A), and transitivity (see transitive law). Congruence of triangles is an equivalence relation in geometry. Members of a set are said to be in the same equivalence class if they have an equivalence relation.
equivalent weight or combining weight Quantity of an element that exactly reacts with (equals the combining value of) 1 g of hydrogen, 8 g of oxygen, or a corresponding amount of any other element. An element’s equivalent weight is its atomic weight divided by its valence. In general, for oxidation-reduction, including electrolysis, the equivalent weight is the weight associated with the loss or gain of 6.02 x 10 23 elec¬ trons (Avogadro's number) or 96,500 coulombs of electric charge; this is also the molecular weight divided by the number of electrons lost or gained. The equivalent weight of a substance with several valences differs depend¬ ing on the number of electrons transferred in the given reaction. The
number of equivalent weights of any substance dissolved in one litre of solution is called the solution’s normality (N). See also stoichiometry.
Er, Lake Chinese Er Hai \'er-'hl\ or Erh Hai Lake, western Yunnan province, China. It lies in a deep basin at the foot of Mount Diancang between the upper Yangtze (Chang; there called the Jinsha) and Mekong rivers; it is about 30 mi (50 km) long and 6-10 mi (10-16 km) wide. The south of the basin has connections to eastern Yunnan and to Sichuan prov¬ ince and is on the main route southwest to Myanmar (Burma). The sur¬ rounding area was brought under Chinese control during the Yuan dynasty (late 13th century).
era Very long span of geologic time; in formal usage, a portion of geo¬ logic time of the second-greatest magnitude (eons are longer). Three eras are recognized: Paleozoic, Mesozoic, and Cenozoic. Because of the diffi¬ culties involved in establishing accurate chronologies, the Precambrian, or earliest, eras are classified independently. An era is composed of one or more geologic periods.
ERA See Equal Rights Amendment
Era of Good Feelings See Era of Good Feelings
erasable programmable read-only memory See EPROM
Erasmus \i-'raz-m3s\, Desiderius (b. Oct. 27, 1469, Rotterdam, Holland—d. July 12,1536, Basel, Switz.) Dutch priest and humanist, con¬ sidered the greatest European scholar of the 16th century. The illegitimate son of a priest and a physician’s daughter, he entered a monastery and was ordained a priest in 1492. He studied at the University of Paris and trav¬ eled throughout Europe, coming under the influence of St. Thomas More and John Colet. The book that first made him famous was the Adagia (1500, 1508), an annotated collection of Greek and Latin proverbs. He became noted for his editions of Classical authors, Church Fathers, and the New Testament as well as for his own works, including Handbook of a Christian Knight (1503) and Praise of Folly (1509). Using the philological methods pioneered by Italian humanists, he helped lay the groundwork for the historical-critical study of the past. By criticizing ecclesiastical abuses, he encouraged the growing urge for reform, which found expression both in the Protestant Reformation and in the Catholic Counter-Reformation. Though he saw much to admire in Martin Luther, he came under pressure to attack him; he took an independent stance, rejecting both Luther’s doc¬ trine of predestination and the powers claimed for the papacy.