Digital recording is a technique for preserving audio signals and video or visual images as a series of pulses that can be stored on magnetic tapes, optical discs (compact discs), or computer diskettes. These pulses are stored in the form of a series of binary digits (that is, zeros and ones). To make the recording, an analog-to-digital converter transforms the sound signal or visual image into di…
Electricity is a natural phenomenon resulting from one of the most basic properties of matter, electrical charge. Our understanding of electrical principles has developed from a long history of experimentation. Electrical technology, essential to modern society for energy transmission and information processing, is the result of our knowledge about electrical charge at rest and electrical charge i…
Digitalis is a drug that has been used for centuries to treat heart disease. The active ingredient in the drug is glycoside, a chemical compound that contains a sugar molecule linked to another molecule. The glycoside compound can be broken down into a sugar and nonsugar compound. Though current digitalis drugs are synthetic, that is, man-made, early forms of the drug were derived from a plant. Di…
The electrocardiogram, ECG or EKG, directly measures microvoltages in the heart muscle (myocardium) occurring over specific periods of time in a cardiac, i.e., a heartbeat, otherwise known as a cardiac impulse. With each heartbeat, electrical currents called action potentials, measured in millivolts (mV), travel at predictable velocities through a conducting system in the heart. The potentials ori…
An electroencephalogram, usually abbreviated EEG, is a medical test that records electrical activity in the brain. During the test, the brain's spontaneous electrical signals are traced onto paper. The electroencephalograph is the machine that amplifies and records the electrical signals from the brain. The electroencephalogram is the paper strip the machine produces. The EEG changes with d…
Dik-diks (genus Madoqua) are small (dog-sized) African antelopes belonging to the family of Bovidae, which includes cattle, sheep, and goats, as well as antelopes, gazelles, and impalas. Like all bovids, dik-diks have even-toed hooves, horns, and a four-chambered stomach. There are five species of dik-dik—Kirk's (the largest), Günther's, Salt's, Red-bellied, and …
Electrochemical cells are devices based on the principle that when a chemical oxidation-reduction reaction takes place, electrons are being transferred from one chemical species to another. In one type of electrochemical cell called a voltaic or galvanic cell, these electrons are deliberately taken outside the cell and made to flow through an electric circuit to operate some kind of electrical dev…
Divergent evolution occurs when a group from a specific population develops into a new species. In order to adapt to various environmental conditions, the two groups develop into distinct species due to differences in the demands driven by the environmental circumstances. A good example of how divergent evolution occurs is in comparing how a human foot evolved to be very different from a monkey&#x…
Electrolysis is the process of causing a chemical reaction to occur by passing an electric current through a substance or mixture of substances, most often in liquid form. Electrolysis frequently results in the decomposition of a compound into its elements. To carry out an electrolysis, two electrodes, a positive electrode (anode) and a negative electrode (cathode), are immersed into the material …
Dinosaurs are a group of now-extinct, terrestrial reptiles in the order Dinosauria that lived from about 225 million years ago to 66 million years ago, during the Mesozoic era. Species of dinosaurs ranged from chicken-sized creatures such as the 2 lb (1 kg) predator Compsognathus to colossal, herbivorous animals known as "sauropods," which were larger than any terrestrial animals tha…
An electrolyte is a substance that will allow current to flow through the solution when dissolved in water. Electrolytes promote this current flow because they produce positive and negative ions when dissolved. The current flows through the solution in the form of positive ions (cations) moving toward the negative electrode and negative ion (anions) moving the positive electrode. Electrolytes can …
A diode is an electronic device that has two electrodes arranged in such a way that electrons can flow in only one direction. Because of this ability to control the flow of electrodes, a diode is commonly used as a rectifier, a device that connects alternating current into direct current. In general, two types of diodes exist. Older diodes were vacuum tubes containing two metal components, while n…
Chlorinated dioxins are a diverse group of organic chemicals. TCDD, or 2,3,7,8-tetrachlorodibenzo-p-dioxin, is a particular dioxin that is toxic to some species of animals in extremely small concentrations. As such, TCDD is the most environmentally controversial of the chlorinated dioxins, and the focus of this entry. …
An electromagnetic field is an area in which electric and magnetic forces are interacting. It arises from electric charges in motion. Electromagnetic fields are directly related to the strength and direction of the force that a charged particle, called the "test" charge, would be subject to under the electromagnetic force caused by another charged particle or group of particles, call…
Electromagnetic induction is the generation of an electromotive force in a closed electrical circuit. It results from a changing magnetic field as it passes through the circuit. Some of the most basic components of electrical power systems—such as generators and transformers—make use of electromagnetic induction. …
Diphtheria is a serious disease caused by the bacterium Corynebacterium diptheriae. Usually, the bacteria initially infect the throat and pharynx. During the course of the infection, a membrane-like growth appearing on the throat can obstruct breathing. Some strains of this bacterium release a toxin, a substance that acts as a poison in the body. This toxin, when released into the bloodstream, tra…
Dipole, literally, means "two poles," two electrical charges, one negative and one positive. Dipoles are common in atoms whenever electrons (-) are unevenly distributed around nuclei (+), and in molecules whenever electrons are unevenly shared between two atoms in a covalent bond. When a dipole is present, the atom or covalent bond is said to be polarized, or divided into negative an…
The electromagnetic spectrum encompasses a continuous range of frequencies or wavelengths of electromagnetic radiation, ranging from long wavelength, low energy radio waves to short wavelength, high frequency, high-energy gamma rays. The electromagnetic spectrum is traditionally divided into regions of radio waves, microwaves, infrared radiation, visible light, ultraviolet rays, x rays, and gamma …
If one quantity increases (or decreases) each time another quantity increases (or decreases), the two quantities are said to vary together. The most common form of this is direct variation in which the ratio of the two amounts is always the same. For example, speed and distance traveled vary directly for a given time. If you travel at 4 mph (6.5 kph) for three hours, you go 12 mi (19.5 km), but at…
Disease can be defined as a change in the body processes that impairs its normal ability to function. Every day the physiology of the human body demands that oxygenation, acidity, salinity, and other functions be maintained within a very narrow spectrum. A deviation from the norm can be brought about by organ failure, toxins, heredity, radiation, or invading bacteria and viruses. Normally the body…
Electromagnetism is a branch of physical science that involves all the phenomena in which electricity and magnetism interact. This field is especially important to electronics because a magnetic field is created by an electric current. The rules of electromagnetism are responsible for the way charged particles of atoms interact. Some of the rules of electrostatics, the study of electric charges at…
Like all other forms of information, photographs and images have entered the electronic age. In 1981, the Sony Corporation unveiled its filmless, electronic camera (termed a still video camera) the Mavica. Mavica is an acronym for Magnetic Video Camera; it uses a still video system to record 50 analog images on a diskette. Although they are recorded on a diskette, they are not digital images. The …
In an electric circuit, electromotive force is the work done by a source on an electrical charge. Because it is not really a force, the term is actually a misnomer; it is more commonly referred to by the initials EMF. EMF is another term for electrical potential, or the difference in charge across a battery or voltage source. For a circuit with no current flowing, the potential difference is calle…
Crestfish, also called unicornfish, are a small family (Lophotidae) of deepwater, marine bony fish in the order Lampridiformes. These rare fish have unusual boxlike heads with protruding foreheads and ribbon-shaped silvery bodies with crimson fins. The prominent dorsal fin extends from the tip of the head to beyond the tail; the first rays of this fin form a crest above the head, giving these fish…
Dissociation is the process by which a molecule separates into ions. It may also be called ionization, but because there are other ways to form ions, the term dissociation is preferred. Substances dissociate to different degrees, ranging from substances that dissociate very slightly, such as water, to those that dissociate almost completely, such as strong acids and bases. The extent to which a su…
All atoms found in nature have a positively charged nucleus about which the negatively charged electrons move. The atom is electrically neutral and thus the positive electrical charge on the nucleus has the same magnitude as the negative charge due to all the electrons. The electrons are held in the atom by the attractive force exerted on them by the positively charged nucleus. They move very rapi…
Creutzfeldt-Jakob disease is a rare encephalopathy, or brain disease, that causes a swift, progressive dementia and neuromuscular changes. It was first described by German psychiatrist Alfons Maria Jakob (1884–1931) in 1921. He gave credit to Hans Gerhard Creutzfeldt (1885–1964), also a German psychiatrist, for describing the syndrome first without realizing he had stumbled onto a re…
The term electron cloud is used to describe the area around an atomic nucleus where an electron will probably be. It is also described as the "fuzzy" orbit of an atomic electron. An electron bound to the nucleus of an atom is often thought of as orbiting the nucleus in much the same manner that a planet orbits a sun, but this is not a valid visualization. An electron is not bound by …
Crickets (order Orthoptera, family Grillidae) are found throughout the world except for the polar regions. More than 900 species have been described. Often heard, but more seldom seen, at first glance crickets are quite similar to grasshoppers and bush crickets—also known as long-horned grasshoppers or katydids—but may be distinguished from these insects by their much longer, thread-…
Distance has two different meanings. It is a number used to characterize the shortest length between two geometric figures, and it is the total length of a path. In the first case, the distance between two points is the simplest instance. In the case of parallel lines, the distance between the two lines is the length of a perpendicular segment connecting them. If two figures such as line segments,…
All species have particular requirements for their ecological habitat. These specific needs are known as critical habitat, and they must be satisfied if the species is to survive. Critical habitat can involve specific types of food, a habitat required for breeding (as is the case of species that nest in tree cavities), or some other crucial environmental requirement. Some critical habitat features…
Distillation is one of the most important processes for separating the components of a solution. The solution is heated to form a vapor of the more volatile components A typical laboratory distillation setup. Illustration by Hans & Cassidy. Courtesy of Gale Group. in the system, and the vapor is then cooled, condensed, and collected as drops of liquid. By repeating vaporization and c…
The distributive property states that the multiplication "distributes" over addition. Thus a × (b + c) = a × b + a × c and (b + c) × a = b × a + c × a for all real or complex numbers a, b, and c. The distributive property is behind the common multiplication algorithm. For example, 27 × 4 means 4 × (2 tens + 7 ones). To complete …
The crocodile order (Crocodylia) consists of several families of large, unmistakable, amphibious reptiles: the crocodiles (Crocodylidae), gavials (Gavialidae), and the alligators and caimans (Alligatoridae). Although these animals look superficially like lizards, they are different in many important respects, and are believed by biologists to be the most highly evolved of the living reptiles. Croc…
A forest in Homestead, Florida, that was destroyed by Hurricane Hugo. JLM Visuals. Reproduced by permission. In the ecological context, disturbance is regarded as an event of intense environmental stress occurring over a relatively short period of time and causing large changes in the affected ecosystem. Disturbance can result from natural causes or from the activities of humans. Disturbanc…
Diurnal cycles refer to patterns within about a 24-hour period that typically reoccur each day. Most daily cycles are caused by the rotation of Earth, which spins once around its axis about every 24 hours. The term diurnal comes from the Latin word diurnus, meaning daily. Diurnal cycles such as temperature diurnal cycles, diurnal tides, and solar diurnal cycles affect global processes. Diurnal tid…
Division is the mathematical operation that is the inverse of multiplication. If one multiplies 47 by 92 then divides by 92, the result is the original 47. In general, (ab)/b = a. Likewise, if one divides first then multiplies, the two operations nullify each other: (a/b)b = a. This latter relationship can be taken as the definition of division: a/b is a number which, when multiplied by b, yields …
Crop rotation is a method of maintaining soil fertility and structure by planting a particular parcel of agricultural land with alternating plant species. Most crop rotation schedules require that a field contain a different crop each year, and some schemes incorporate times when the field remains uncultivated, or lies fallow. Farmers rotate crops to control erosion, promote soil fertility, contai…
Electronics is a field of engineering and applied physics that grew out of the study and application of electricity. Electricity concerns the generation and transmission of power and uses metal conductors. Electronics manipulates the flow of electrons in a variety of ways and accomplishes this by using gases, materials like silicon and germanium that are semiconductors, and other devices like sola…
Genetic, genomic, or DNA fingerprinting is the term applied to a range of techniques that are used to show similarities and dissimilarities between the DNA present in different individuals. Genetic fingerprinting is an important tool in the arsenal of forensic investigators. Genetic fingerprinting allows for positive identification, not only of body remains, but also of suspects in custody. Geneti…
DNA, short for deoxyribonucleic acid, is a double-stranded, helical molecule that forms the molecular basis for heredity. For DNA replication to occur, this molecule must first unwind, or "unzip," itself to allow the information-encoding bases to become accessible. The base pairing within DNA is of a complementary nature and, consequently, when the molecule unzips, due to the action …
Deoxyribonucleic acid (DNA) synthesis is a process by which strands of nucleic acids are created. In a cell, DNA synthesis takes place in a process known as replication. Using genetic engineering and enzyme chemistry, scientists have also developed man-made methods for synthesizing DNA. The DNA molecule was discovered by Francis Crick, James Watson, and Maurice Wilkins. In 1953, Watson and Crick u…
DNA technology has revolutionized modern science. Deoxyribonucleic acid (DNA), or an organism's genetic material—inherited from one generation to the next—holds many clues that have unlocked some of the mysteries behind human behavior, disease, evolution, and aging. As technological advances lead to a better understanding of DNA, new DNA-based technologies will emerge. Recent …
The use of a vaccine constructed of a protein has traditionally been to induce the formation of an antibody to the particular protein. Antibodies are crucial to an or ganism's attempt to stop an infection caused by a microorganism. In the early 1990s, scientists observed that plasmid DNA (DNA that is present in bacteria that is not part of the main body of DNA) could affect test animals. Wo…
Dobsonflies are species of medium- to large-sized insects in the order Neuroptera, family Corydalidae. The life cycle of dobsonflies is characterized by a complete metamorphosis, with four developmental stages: egg, larva, pupa, and adult. Adult dobsonflies are usually found near freshwater, especially streams, either resting on vegetation or engaged in an awkward, fluttering flight. Sometimes adu…
Electrophoresis is a technique used for the separation of biological molecules based on their movement due to the influence of a direct electric current. The technique was pioneered in 1937 by the Swedish chemist Arne Tiselius for the separation of proteins. It has now been extended to the separation of many other different classes of biomolecules including nucleic acids, carbohydrates and amino a…
Electrostatics is the study of the behavior of electric charges that are at rest. The phenomenon of static electricity has been known for well over 2,000 years, and a variety of electrostatic devices have been created over the centuries. In the late sixteenth century, William Gilbert (1544-1603) began experimenting with static electricity, pointing out the difference between static electric attrac…
Crops are any organisms that humans utilize as a source of food, materials, or energy. Crops may be utilized for subsistence purposes, to barter for other goods, or to sell for a cash profit. They may be harvested from wild ecosystems, or they may be husbanded and managed, as occurs with domesticated species in agriculture. In general, the purpose of management is to increase the amount of crop pr…
Dogwood refers to certain species of trees and shrubs in the dogwood family (Cornaceae). The dogwoods are in the genus Cornus, which mostly occur in temperate and boreal forests of the Northern Hemisphere. Species in the dogwood family have seasonally deciduous foliage. The leaves are simple, usually untoothed, and generally have an opposite arrangement on the twig. The flowers of dogwoods develop…
If two fractions are equal, say then it is always true that the products of the numbers given by are also equal or ad = bc. This is the most common form of cross multiplication. That implies ad = bc can be shown by multiplying both sides of by the common denominator bd and canceling. Cross multiplying is a common first step in solving proportions. is equivalent to The…
The domain of a relation is the set that contains all the first elements, x, from the ordered pairs (x,y) that make up the relation. In mathematics, a relation is defined as a set of ordered pairs (x,y) for which each y depends on x in a predetermined way. If x represents an element from the set X, and y represents an element from the set Y, the Cartesian product of X and Y is the set of all possi…
In solid geometry, the cross section of a three-dimensional object is a two-dimensional figure obtained by slicing the object perpendicular to its axis and viewing it end on. Thus, a sausage has a circular cross section, a 4 × 4 fence post has a square cross section, and a football has a circular cross section when sliced one way and an elliptical cross section when sliced another way. More…
Domestic donkeys, members of the order Perissodactyla, are large single-hoofed horse-like mammals with elongated heads. Donkeys usually stand between 9.5 and 11 hands high measured at the withers, that is, 38-44 in (95-110 cm) tall. Because of the large amount of interbreeding among different donkey species, donkeys differ markedly in appearance. They can be brown, gray, black, roan (a mixture of …
The members of the crow family (Corvidae) are among the world's most intelligent birds. The family has recently undergone taxonomic expansion, brought about by evidence gathered through genetic testing, and now includes such diverse species as birds-of-paradise, orioles, and drongos. Crows and jays belong to the subfamily Corvinae. The corvids comprise 113 species in 25 genera, which includ…
The crustacea (subphylum Mandibulata, class Crustacea) are a diverse group of animals. This class includes some of the more familiar arthropods, including barnacles, copepods, crabs, prawns, lobsters, and wood lice. More than 30,000 species have been identified, the majority of which are marine-dwelling. Terrestrial species such as woodlice and pill bugs are believed to have evolved from marine sp…
Cryobiology is the scientific study of the effects of freezing and sub-freezing temperatures on biological fluids, cells, and tissues. It is an extension of cryogenics, which is the study of the properties of matter at very low temperatures. Cryobiological techniques have application in genetic research, livestock breeding, infertility treatment, and organ transplantation. A related field, cryogen…
Dopamine is a neurotransmitter (a chemical used to send signals between nerve cells) in the same family as epinephrine (adrenaline). Dopamine is one of the primary neurotransmitters and it affects motor functions (movement), emotions, learning, and behavior. It was originally identified as the brain chemical associated with pleasure. A decrease in the amount of dopamine in specific sections of the…
Cryogenics is the science of producing and studying low-temperature environments. The word cryogenics comes from the Greek word "kryos," meaning cold; combined with a shortened form of the English verb "to generate," it has come to mean the generation of temperatures well below those of normal human experience. More specifically, a low-temperature environment is termed …
The Doppler effect was named after Johann Christian Doppler (1803-1853). This Austrian physicist observed and explained the changes in pitch and frequency of sound and light waves, as well as all other types of waves, caused by the motion of moving bodies. The general rule of the Doppler effect is that the wave frequencies of moving bodies rise as they travel toward an observer and fall as they re…
A scanner is a computer accessory (peripheral) used to digitize pictures. A scanner converts a visual image to a digital signal. The signal is interpretable by computer software, which allows the image to be recorded, manipulated, and even sent electronically to another computer. Even in the early 1990s, computers were used more for "in house" functions, such as preparing documents.…
Dories are bony fish in the family Zeidae. A dory has an oval body with a back that rises so that the main part of the body is higher than the head. The body itself is relatively thin and compressed and appears oval in side view. Another distinguishing mark of the dories is a dark spot on each side of the body surrounded by a yellow ring. Dories typically are found in the middle depths of the seas…
Dormice are approximately ten species of rodents that make up the family Gliridae. Dormice typically live in trees, bushes, hedgerows, gardens, and rock piles. Dormice have a superficial resemblance to squirrels (family Sciuridae), but they are smaller and differ in many other anatomical and behavioral characters. Dormice have soft fur, and a long, bushy tail. Their forefeet have four digits, the …
New drugs undergo double-blind testing to determine whether they are effective. The test is called double-blind because neither the doctor who is administering the medication nor the patient who is taking it knows whether the patient is getting the experimental drug or a neutral substitute, called a placebo. Getting a new drug approved is a long, complex process in order to ensure the drug is safe…
The double helix refers to DNA's "spiral staircase" structure, consisting of two right-handed helical polynucleotide chains coiled around a central axis. Genes, which are specific regions of DNA, contain the instructions for synthesizing every protein. Because life cannot exist without proteins, the discovery of DNA's structure unveiled the secret of life: protein synth…
A crystal is a solid in which the particles that make up the solid take up a highly ordered, definite, geometric arrangement that is repeated in all directions within the crystal. In ancient Greece, Archimedes made a study of regular solids, and Plato and Aristotle speculated on the relationship between regular solids and the elements. In the sixteenth century, the German naturalist, Giorgius Agri…
Every cubic equation has either three real roots as in our first example or one real root and a pair of (conjugate) complex roots as in our second example. There is a formula for finding the roots of a cubic equation that is similar to the one for the quadratic equation but much more complicated. It was first used by Geronimo Cardano in 1545, even though he had obtained the formula from Niccolo Ta…
Down syndrome is the most common cause of mental retardation. It can be caused by the presence of an extra chromosome. Chromosomes contain sequences of DNA called genes, which represent the genetic information that exists within almost every cell of the body. Twenty-three distinctive pairs, or 46 total chromosomes, are located within the nucleus (central DNA-containing structure) of each cell. Whe…
Dragonflies are large flying insects in the order Odonata. Dragonflies can be as large as 3 in (7.5 cm) in length, with a wing span of up to 8 in (20 cm). The fossilized remains of a huge dragonfly-like insect that had a wingspread of more than 2 ft (70 cm) is known from the Carboniferous period, some 300 million years ago. Dragonflies are very distinctive insects, with large eyes that almost cove…
Cuckoos, coucals, anis, malkohas, and roadrunners are approximately 127 species of birds that make up the family Cuculidae. These birds are mostly tropical in distribution, but some species also breed in the temperate zones. Many species are parasitic breeders, laying their eggs in the nests of other species of birds. Species of the cuckoo family occupy a great diversity of habitats, ranging from …
Drift nets are lengthy, free-floating, 26-49 ft (8-15 m) deep nets, each as long as 55 mi (90 km). Drift nets are used to snare fish by their gills in pelagic, open-water situations. Because drift nets are not very selective of species, their use results in a large by-catch of non-target fish, sharks, turtles, seabirds, and marine mammals, which are usually jettisoned, dead, back to the ocean. Dri…
Drongos are 20 species of handsome birds that make up the family of perching birds known as Dicruridae. Drongos occur in Africa, southern and southeastern Asia, and Australasia. Their usual habitats are open forests, savannas, and some types of cultivated areas with trees. Drongos are typically black colored with a beautiful, greenish or purplish iridescence. The wings of these elegant, jay-sized …
Curare (pronounced cue-rah'-ree) is a general term for certain chemical substances found in different plants throughout the world's rainforests. These plants produce a harmless sap which for centuries the natives of the rainforests have refined into a deadly poison. The way of refining and delivering the poison from certain types of plants is similar for natives occupying equatorial …
Throughout the last century, the fruit fly Drosophila melanogaster, has been the workhorse for genetic studies in eukaryotes. These studies provided the basis of much of scientists' understanding of fundamental aspects of eukaryotic genetics. Cloned fruit fly genes have led to the identification of mammalian cognates. Discoveries have shown that the conservation between the fruit fly and ma…
Curlews are large, brownish shorebirds (family Scolopacidae) with long legs and lengthy, downward curving bills, adapted for probing into sediment and soil for their food of invertebrates. Although neither species of North American curlew is common, the most abundant ones are the long-billed curlew (Numenius americanus) and the whimbre, or Hudsonian curlew (N. phaeopus). The long-billed curlew bre…
Drought is characterized by low precipitation compared to the normal amount for the region, low humidity, high temperatures, and/or high wind velocities. When these conditions occur over an extended period of time, drought causes low water supplies that are inadequate to support the demands of plants, animals, and people. Drought is a temporary condition that occurs in moist climates. This is in c…
Currents are steady, smooth movements of water following a specific course; they proceed either in a cyclical pattern or as a continuous stream. In the Northern Hemisphere, currents generally move in a clockwise direction, while in the Southern Hemisphere they move counterclockwise. There are three basic types of ocean currents: surface currents; currents produced by long wave movements or tides; …
Elephants are large, four-legged, herbivorous mammals. They have a tough, almost hairless hide, a long flexible trunk, and two ivory tusks growing from their upper jaw. Only two species of elephant exist today, the African elephant (Loxodonta africana) and the Asian or Indian elephant (Elephas maximus), both of which are threatened or endangered. African elephants are the largest of all land anima…
Informally, one can picture a curve as either a line, a line segment, or a figure obtained from a line or a line segment by having the line or line segment bent, stretched, or contracted in any way. A plane curve, such as a circle, is one that lies in a plane; a curve in three dimensional space, such as one on a sphere or cylinder, is called a skew curve. A closed curve is a curve with no endpoint…
Elephant shrews are relatively small mammals in the family Macroscelididae, order Macroscelidea. Elephant shrews have a characteristic long, narrow snout that is broad at the base, and very sensitive and flexible but not retractile. This snout is movable in circular manner at the base, and has nostrils at the end. There are five genera with some 185 species of elephant shrews, living in continenta…
In the early twentieth century, Harvey Cushing, (1869–1939), an American neurosurgeon, described a set of symptoms that he identified as a syndrome later called Cushing syndrome or disease. The cause of the syndrome at the time was unknown, but since then, a better understanding of the underlying causes of Cushing syndrome have been elucidated. Cushing syndrome is a disorder that is caused …
Elephant snout fish belong to a diverse group of fishes that comprise the family Mormyridae. All are freshwater species that are confined to tropical parts of the African continent. Some 150 species have been described so far. The group takes its common or English name from the animals' extended snout. This adaptation is taken to the extreme in the genus Gnathonemus which has a pendulous, t…
Cuttlefish are squid-like cephalopod mollusks of the family Sepiidae, in the order Sepioidea. Cephalopod literally means "head-footed animal" and is the name given to advanced mollusks (such as cuttlefish, squid and octopus) whose heads are circled with tentacles. Cuttlefish have a relatively well-developed brain, sensitive organs of smell and hearing, highly developed eyes, and a re…
Ducks are waterfowl in the order Anseriformes, in the family Anatidae, which also includes geese and swans. Ducks occur on all continents except Antarctica, and are widespread in many types of aquatic habitats. Almost all ducks breed in freshwater habitats, especially shallow lakes, marshes, and swamps. Most species of ducks also winter in these habitats, sometimes additionally using grain fields …
Cybernetics is a term that was originated by American mathematician Norbert Wiener (1894–1964) in the late 1940s. Based on common relationships between humans and machines, cybernetics is the study and analysis of control and communication systems. As Wiener explains in his 1948 book, Cybernetics: or Control and Communication in the Animal and the Machine, any machine that is "intell…
Duckweeds are small, floating to slightly submerged species of flowering plants in the genus Lemna. The simple body is leaf-like, generally flat on top and convex below, lacks stems or leaves, is oval to tear-dropped in shape, and has one unbranched root that lacks vascular (conducting) tissue. The upper surface of the plant is covered with waxy compounds so as to shed water. Duckweeds are abundan…
Mounds are artificially constructed heaps or banks of earth built to contain sacred objects. Their basic construction is the same all over the world: a pit is dug and lined, and the sacred contents are deposited and covered with earth. Sometimes these objects are sprinkled with red ocher, a pigment used to make paint, perhaps as a way to revive the spirits thought to dwell within them. If we were …
Duikers are small African antelopes in the large family of Bovidae. This family of hoofed animals includes antelope, gazelles, cattle, sheep, and goats. Like all bovids, duikers have even-toed hooves, horns, and a four-chambered stomach structure that allows them to digest a diet of plants. Duikers are found throughout sub-Saharan Africa. These small antelopes range in size from 22 in (55 cm) to a…
Elephantiasis is an extreme symptom of human infection by a type of roundworm or nematode. It involves massive swelling of a limb or of the scrotum. The leg of an individual suffering from elephantiasis can become enlarged to two or three times normal diameter. The actual name of the disease or infection which causes elephantiasis is lymphatic filariasis. Lymphatic filariasis is an important paras…
The cycads are a relatively small phylum (Cycadophyta) in the plant kingdom Plantae. The cycads are considered to be gymnosperms, because they bear their seeds naked on modified leaves called sporophylls. In contrast, the evolutionarily more recent angiosperms (flowering plants) bear their seeds inside of ovaries. Cycads grow in tropical and subtropical regions of the world. Cycads are sometimes r…
A dune is a wind-blown pile of sand. Over time, dunes become well-sorted deposits of materials by wind Dunes form different characteristic shapes depending on the amount of sand, amount of moisture, and the strength and prevailing direction of the wind (i.e., windward to leeward). Illustration by Argosy. The Gale Group. or water that take on a characteristic shape and that retain that genera…
Along with squaring the circle and trisecting an angle, duplication of a cube is considered one of the three "unsolvable" problems of mathematical antiquity. According to tradition, the problem of duplication of the cube arose when the Greeks of Athens sought the assistance of the oracle at Delos in order to gain relief from a devasting epidemic. The oracle told them that to do so th…
A dust devil is a relatively small, rapidly rotating wind that stirs up dust, sand, leaves, and other material as it moves across the ground. Dust devils are also known as whirlwinds or, especially in Australia, willywillys. In most cases, dust devils are no more than 10 ft (3 m) in width and less than 300 ft (100 m) in height. Dust devils form most commonly on hot dry days in arid regions such as…
An elevator is an enclosed car that moves in a vertical shaft between the multi-story floors of a building carrying passengers or freight. All elevators are based on the principle of the counterweight, and modern elevators also use geared, electric motors and a system of cables and pulleys to propel them. The world's most often used means of mechanical transportation, it is also the safest.…
University of Illinois graduate student Michael Sveda first synthesized cyclamate in 1937. Some say that he discovered its sweet taste by chance when he accidentally got some on the cigarette he was smoking. The university eventually transferred patent rights to Abbott Laboratories, which brought the sweetener to market in 1950. Another reason for combining saccharin with cyclamate is that the swe…
In 1995, Philips and Sony introduced the digital video disc (DVD), which had the same dimensions as a standard compact disk (CD), but was able to store up to 4.7 gigabytes of data, such as high-definition digital video files. This is more than three times the capacity of a CD. DVD players use a higher-power laser than that used for CDs, which enables smaller pits (0.4 micrometre) and separation tr…
Cultural citizenship has been part of a broader discussion on cultural pluralism that began in the United States at the beginning of the twentieth century. Since then pluralism has undergone at least three noteworthy transformations, beginning with, during the first quarter of the twentieth century, attempts to preserve primarily European immigrant cultures vis-à-vis the state, followed by th…
The terms cyclone and anticyclone are used to describe areas of low and high atmospheric pressure, respectively. Air flowing around one or the other of these areas is said to be moving cyclonically in the first case and anticyclonically in the second. In the northern hemisphere, cyclonic winds travel in a counterclockwise direction and anticyclonic winds, in a clockwise direction. When a cyclone o…
Citizenship rests with territory at the heart of the definition of nation-state. If territory determines the geographical limits of state sovereignty, citizenship determines a state's population. Beyond these limits one finds foreign land, foreign sovereignty, and foreigners. Drawing the boundary within which some human beings are included and others excluded as foreigners, permitting some…
Cyclosporines are drugs used in the field of immunosuppressant medicine to prevent the rejection of transplanted organs. They were discovered by Jean F. Borel in 1972. The cyclosporine used for transplant surgery is called cyclosporine A (CsA) and in 1984 it was added to the group of medicines used to prevent transplant rejection. Cyclosporine A is the most common form of the Norwegian fungus Toly…
These small antelopes belong to the ruminant family Bovidae, and are grouped with the gazelles in the subfamily Antilopinae. The 13 species of dwarf antelopes are in the tribe Neotragini. Dwarf antelopes range from extremely small (3.3-4.4 lb or 1.5-2 kg) hare-sized royal antelopes and dik-diks to the medium-sized oribi and beira weighing from 30-50 lb (10-25 kg). Dwarf antelopes engage in territo…
An ellipse is a kind of oval. It is the oval formed by the intersection of a plane and a right circular cone-one of the four types of conic sections. The other three are the circle, the hyperbola, and the parabola. The ellipse is symmetrical along two lines, called axes. The major axis runs through the longest part of the ellipse and its center, and the minor axis is perpendicular to the major axi…
A cyclotron is a type of particle accelerator designed to accelerate protons and ions to high velocities and then release them so as to strike a target. Observations of such collisions yield information about the nature of atomic particles. In contrast to the enormous particle accelerators used in particle physics today, the first cyclotron, built in 1930 by U.S. physicist E. O. Lawrence (1901…
Elms are trees (occasionally shrubs) of flowering plants in the genus Ulmus. Elm leaves possess stipules, and often have a nonsymmetrical leaf, that is, one half is larger than the other so that the bottom ends do not meet where they are attached to the mid-rib. Elms flower in the spring. Their flowers lack petals, form reddish brown clusters in the tops of the trees, appear before the leaves have…
Embiids are small, cylindrical, soft-bodied insects in the order Embioptera that spin tubular galleries of silk, an ability that gives them the common name web-spinners. They have chewing mouthparts, and undergo paurometabolism, or gradual metamorphosis, exhibiting a definite egg, nymph, and adult stage. In the phylogeny, or evolutionary history of the class Insecta, embiids are thought to be most…
An embolism is the sudden blockage of a blood vessel by a blood clot that has been brought to that location by the bloodstream. The clot, called an embolus, from the Greek word meaning plug, is a blood clot that has formed inside the circulatory system and is floating in the bloodstream. It will remain on the move until it encounters a blood vessel too small for it to fit through, where it will pl…
Color scientists use the term "colorant" for the entire spectrum of coloring materials, including dyes and pigments. While both dyes and pigments are sources of color, they are different from one another. Pigments are particles of color that are insoluble in water, oils, and resins. They need a binder or to be suspended in a dispersing agent to impart or spread their color. Dyes are …
Cystic fibrosis (CF) is genetic disease characterized by defects in the transport of a molecule called chloride. Abnormalities in CF have been described in several organs and tissues, including the airways, pancreas, bile ducts, gastrointestinal tract, sweat glands, and male reproductive. Lung function is often normal at birth; however, airway obstruction and inflammation as well as bacterial colo…
An embryo is a stage directly after fertilization that signifies the early stages of growth and development of an organism. In humans, this stage ends during the third month of pregnancy, and is then called a fetus. Plants and invertebrate as well as vertebrate animals have an embryonic stage of development. For example, the embryo of the common North American leopard frog, Rana pipiens is from a …
Cytochromes are electron-transporting protein pigments concerned with cell respiration that contain an iron-containing molecule called heme, allied to that of hemoglobin. When the iron of heme accepts an electron, it changes from the oxidized ferric (Fe III) state to the reduced ferrous (Fe II) state. The oxidation of cytochromes to molecular oxygen and their subsequent reduction by oxidizable sub…
Developments in reproductive technology are occurring at a rapid rate in animal science as well as in human biology. In vitro fertilization, embryo culture, preservation of embryos by freezing (cryopreservation), and cloning technology yield embryos that are produced outside of the female reproductive system. Embryo transfer permits continued survival of embryos by insertion into the female reprod…
Cytology is the branch of biology that studies cells, the building blocks of life. The name for this science is translated from kytos, the Greek term for "cavity." Cytology's roots travel back to 1665, when British botanist Robert Hooke, examining a cross-section of cork, gave the spaces the name "cells," meaning "little rooms" or "cavities.&…
Dysentery is an infectious disease that has ravaged armies and prisoner-of-war camps throughout history. The disease still is a major problem in tropical countries with primitive sanitary facilities. Refugee camps in Africa resulting from many civil wars are major sinks of infestation for dysentery. …
Embryology is the study of the development of organisms. This is as true of plants as it is of animals. Seed formation proceeds following fertilization in higher plants. The seed consists of the embryo, the seed coat, and another part sometimes called the endosperm. While plants are extraordinarily important for survival of animal life, animal embryology is described here. The dictionary definitio…
In the context of ecology and environmental science, the word emission generally refers to a release of a A nickel smelting facility emitting pollutants that carry for hundreds of miles in Russian Siberia. Photograph by Josef Polleross. Stock Market. Reproduced by permission. substance or energy to the environment. Often, emissions refer to substances or energy that are ecological stresso…
Environmental stress refers to physical, chemical, and biological constraints on the productivity of species and on the development of ecosystems. When the exposure to environmental stressors increases or decreases in intensity, ecological responses result. Stressors can be natural environmental factors, or they may result from the activities of humans. Some environmental stressors exert a relativ…
Dams are structures designed to restrict the flow of a stream or river, thus forming a pond, lake, or reservoir behind the wall. Dams are used for flood control, for production of hydroelectric power, to store and distribute water for agriculture and human populations, and as recreation sites. …
Damselflies are the smaller and more delicate members of the insect order Odonata, which includes the dragonflies. The damselfly suborder Zygoptera is characterized by similar fore and hind wings, which are both narrow at the base. Most damselflies can be easily distinguished from their larger and heavier dragonfly relatives in the field by their fluttering flight, and when at rest by their holdin…
Dyslexia is a disorder that falls under the broad category of learning disabilities. It is often described as a neurological syndrome in which otherwise normal people have difficulty reading and writing. Frequently, dyslexia is defined by what it is not—dyslexia is not mental retardation, a psychiatric or emotional disorder, or a vision problem. Dyslexia is not caused by poverty, psychologi…
Dark matter is the term astronomers use to describe material in the Universe that is non-luminous—that is, material that does not emit or reflect light and that is therefore invisible. Everything seen when looking through a telescope is visible because it is either emitting or reflecting light; stars, nebulae, and galaxies are examples of luminous objects. However, luminous matter appears t…
Dysplasia is a combination of two Greek words; dys, which means difficult or disordered; and plassein, to form. In other words, dysplasia is the abnormal or disordered formation of certain structures. In medicine, dysplasia refers to cells that have acquired an abnormality in their form, size, or orientation with respect to each other. Dysplasia may occur as the result of any number of stimuli. Su…
Dystrophinopathies are progressive hereditary degenerative diseases (often called muscular dystrophies) of skeletal muscles due to an absence or deficiency of the protein dystrophin. Dystrophin and the associated proteins form a complex system that connects the intracellular cytoskeleton to the extracellular matrix. The normal operation of this system is critical for maintaining the integrity of t…
The number e, like the number pi, is a useful mathematical constant that is the basis of the system of natural logarithms. Its value correct to nine places is 2.718281828... The number e is used in complex equations to describe a process of growth or decay. It is therefore utilized in the biology, business, demographics, physics, and engineering fields. …
A digital computer is a programmable device that processes information by manipulating symbols according to logical rules. Digital computers come in a wide variety of types, ranging from tiny, special-purpose devices embedded in cars and other devices to the familiar desktop computer, the minicomputer, the mainframe, and the supercomputer. The fastest supercomputer, as of early 2003, can execute u…
Eagles are large, diurnal birds of prey in the subfamily Buteonidae, which also includes buzzards and other broad-winged hawks. The buteonids are in the order Falconiformes, which also includes falcons, osprey, goshawks, and vultures. Like all of these predatory birds, eagles have strong, raptorial (or grasping) talons, a large hooked beak, and extremely acute vision. Eagles are broadly distinguis…
The human ear is the anatomical structure responsible for hearing and balance. The ear consists of three parts: the outer, middle, and inner ears. The outer ear collects sounds from the environment and funnels them through the auditory system. The outer ear is composed of three parts, the pinna (or auricle), the external auditory canal (or external auditory meatus), and the tympanic membrane (or e…
Dating techniques are procedures used by scientists to determine the age of a specimen. Relative dating methods tell only if one sample is older or younger than another; absolute dating methods provide a date in years. The latter have generally been available only since 1947. Many absolute dating techniques take advantage of radioactive decay, whereby a radioactive form of an element is converted …
Earth is our home planet. Its surface is mostly water (about 70%), and it has a moderately dense nitrogen-and-oxygen atmosphere that supports life—the only known life in the Universe. Rich in iron and nickel, Earth is a dense, rocky ball orbiting the Sun with its only natural satellite, the Moon. A complete revolution of the earth around the Sun takes about one year, while a rotation on its…
Befitting a dynamic Earth, the study of Earth science embraces a multitude of subdisciplines. At the heart of Earth science is the study of geology. Literally meaning "to study the Earth," traditional geological studies of rocks, minerals, and local formations have within the last century, especially in the light of the development of plate tectonic theory, broadened to include studi…
Dichlorodiphenyl-trichloroacetic acid (or DDT) is a chlorinated hydrocarbon that has been widely used as an insecticide. DDT is virtually insoluble in water, but is freely soluble in oils and in the fat of organisms. DDT is also persistent in the environment. The combination of persistence and lipid solubility means that DDT biomagnifies, occurring in organisms in preference to the nonliving envir…
It is approximately 3,950 mi (6,370 km) from Earth's surface to its center. Geologists understand the structure and composition of the surface by direct observation and by analysis of rock samples raised by drilling projects; however, the depth of drill holes and, therefore, the depth limit of scientists' ability to directly observe Earth's interior is severely limited. Even t…
Deafness is the lack of functional sense of hearing in both ears. Loss of hearing can result from environmental or genetic causes and it can be temporary or permanent. Environmental loss of hearing results from occupational noise, noise pollution, accidents, or intake of certain drugs. Inherited loss of hearing can be caused by mutations in any of over a hundred of genes known to affect hearing, a…
Earth acts as a dipole magnet with the positive and negative magnetic poles near, but not aligned exactly with, the north and south geographic poles. Because of this difference, detailed maps commonly distinguish between true north and magnetic north. The difference, known as magnetic declination, must be taken into account when navigating with a magnetic compass. Its magnetic field also molds the…
A decimal fraction is a numeral that uses the numeration system, based on 10, to represent numbers that are not necessarily whole numbers. The numeral includes a dot, called the decimal point. The digits to the right of the decimal point extend the place-values to tenths, hundredths, thousandths, and so forth. For example, the decimal fraction 5.403 means "5 ones, 4 tenths, 0 hundredths, an…
All objects in the universe and our solar system move in space. The earth moves in two ways. It rotates like a top on its axis, an imaginary line through the north and south poles, and revolves in an orbit around the Sun. Centrifugal force results from the earth's rotation; without gravity, centrifugal force could cause objects to fly into space. Because the force of the earth's grav…
Fungus creates circular patterns on decomposing maple leaves. CORBIS/Gary Braasch. Reproduced by permission. Decomposition is the natural process by which large organic materials and molecules are broken down into simpler ones. The ultimate products of decomposition are simple molecules, such as carbon dioxide and water. Sometimes misunderstood as being undesirable, decomposition is actuall…
Deer are members of the order Artiodactyla, the even-toed ungulates. This order also includes the antelopes, bovines, and giraffes. Deer are generally slender and long-legged, and their most striking characteristic is the presence of antlers, which are often used to differentiate species. The deer family, Cervidae, includes about 45 species, which are divided among 17 genera and five subfamilies: …
Creationism in a general sense refers to the theory that God made the world on his own, by miraculous means, out of nothing. In a more specific sense, the one encountered in America today, creationism is the theory that the Bible, particularly the early chapters of Genesis, is a literally true guide to the history of the universe and to the history of life, including us humans, down here on earth.…
The deer mouse (Peromyscus maniculatus) is a small, native rodent with an almost ubiquitous distribution in North America. The deer mouse ranges from the subarctic boreal forest, through wide areas of more southern conifer and mixed-wood forests, to drier habitats as far south as some regions of Mexico. The deer mouse is highly variable in size and color over its range. Its body length ranges from…
Because creativity is a complex concept, it has multiple definitions. Of the various conceptions, however, three are currently most common. These are the product, person, and process definitions. According to the first, creativity is manifested in an identifiable outcome, such as a poem, painting, invention, or discovery. Moreover, this product must fulfill two essential conditions. First, it must…
The term crisis comes from the Greek noun krisis (choice, decision, judgment), deriving from the Greek verb krinein (to decide). The word makes an ancient debut in Greek historical writing via the legal, medical, and rhetorical terminology as the turning point in a decision, illness, or argument. Its definitive reappearance with reference to historical events, periods, or processes dates from the …
One of a family of related progressive movements in the law—others include critical legal studies, Latino critical legal studies ("Lat/Crit"), and feminist legal theory—critical race theory sprang up in the late l970s in response to a widespread perception that the powerful civil rights coalition of the 1960s and early 1970s had stalled. Conservative administrations an…
In the humanities, the term critical theory has had many meanings in different historical contexts. From the end of World War II through the 1960s, the term signified the use of critical and theoretical approaches within major disciplines of the humanities such as art history, literary studies, and more broadly, cultural studies. From the 1970s, the term entered into the rapidly evolving area of f…
An earthquake is the shaking or vibration of Earth's surface as the result of sudden movement along a fault, the movement of molten rock within the Earth, or human activities. The terms temblor and seism are often used as synonyms for earthquake. The location of an earthquake source within the Earth is known as its focus, and the point on the Earth's surface directly above the focus …
The concept of cultural capital originated in the work of Pierre Bourdieu (1979, pp. 10, 12), who defined it as high cultural knowledge that ultimately redounds to the owner's financial and social advantage. An example would be knowing how to "dress for success." This cultural knowledge can pay off. Although they naturally seek competent personnel, employers also prefer execut…
Earwigs are long-bodied insects with chewing mouthparts and many-jointed antennae in the order Dermaptera. Earwigs have small, vestigial forewings modified into a wing case, but their membranous hind-wings are large, folded, and functional, although they are not often used for flying. Earwigs hatch into nymphs which closely resemble the adults, only they are much smaller. Metamorphosis in earwigs …
As a discipline, cultural history is a bit over two centuries old, but it has an extensive prehistory going back to Renaissance scholarship, especially in areas of the history of literature and the history of philosophy. In the Renaissance, cultus or cultura was commonly associated with the cultivation of literature, philosophy, eloquence, law, arts, and sciences, whose fruits were the human virtu…
Deforestation refers to a longer-term conversion of forest to some other kind of ecosystem, such as agricultural or urbanized land. Sometimes, however, the term is used in reference to any situation in which forests are disturbed, for example by clear-cut harvesting, even if another forest subsequently regenerates on the site. Various human activities result in net losses of forest area and theref…
The term cultural revival refers to the formation of group identity around a common culture, where a claim is forwarded that the aspects of culture with which the group identifies have been recovered after losses due to colonization, forced or voluntary relocation, oppression, or modernization. Cultural revival is predominantly associated with minority populations and frequently underwrites demand…
Cultural studies is one of the more controversial intellectual formations of the 1990s and the first decade of the third millennium. It has experienced a period of rapid growth in the academy, appearing at many universities in a variety of forms and locations (although rarely as degree-granting departments). At the same time, it has been broadly attacked both from inside the university and outside…
The idea that history is composed of cycles is ancient. Many peoples (including the Egyptians, Chinese, Babylonians, Hindus, Maya, and Greeks) observed recurrences in astronomical phenomena. These early observations were often related to calendar systems and were the foundation for later written schemes of cosmic and historical cycles in various parts of the world. …
Eating disorders are psychological conditions that involve either overeating, voluntary starvation, or both. No one is sure what causes eating disorders, but researchers think that family dynamics, biochemical abnormalities, and society's preoccupation with thinness may all contribute. Eating disorders are virtually unknown in parts of the world where food is scarce and within less affluent…
The developing fetal heart accounts for a large percentage of the volume of the early thorax. About 20 days after fertilization, the heart develops from the fusion of paired endothelial tubes into a single tube. Heart growth subsequently involves the growth, expansion, and partitioning of this tube into four chambers separated by thickened septa of cardiac muscle and valves. Atrial development is …
The word cynic generally conveys negative ideas in modern languages. It describes someone who is unduly critical and suspicious, apathetic about certain issues and rebellious in response to others, selfish, and indifferent toward traditions and accepted beliefs, and unconcerned with the public welfare. The cynic is often viewed as a person who has severed all ties with his social context. To be c…
The Ebola virus is one of a number of viruses that cause a devastating disease in humans and closely related species such as monkeys, gorillas, and chimpanzees. The disease is known as a hemorrhagic fever because of the massive internal bleeding caused by the viral infection. Most strains of Ebola hemorrhagic fever progresses quickly from the initial appearance of symptoms to resolution, which is …
Presided over by the poet and essayist Tristan Tzara (1896–1963), who served as its principal spokesman, dada was the first truly international avant-garde movement. Although the term dada was invented in Zurich, the movement's origins were by no means limited to Switzerland. The dada spirit existed previously in several other countries, where it expressed itself in outrageous avant-…
Ebony (Diospyros spp., family Ebenaceae) are species of tropical hardwood trees favored for their hard and beautiful wood. Only the black or brown heartwood is used commercially. There are more than 300 species of ebony, ranging in size from shrubs to trees taller than 100 ft (30 m). The best commercial ebony comes from India, Madagascar, Nigeria, Zaire, and the Celebes Islands. Most species of eb…
Dance is broadly conceived as physical movement organized into patterns in time and space. Writings on dance grounded in the European intellectual tradition have tended to distinguish dance from other systems of organized movement (such as sport, military drills, synchronized labor, festival processions, and sometimes ritual) by identifying a dimension of conscious craft or artistry. The disciplin…
Dehydroepiandrosterone (DHEA) is one of the androgens secreted by the adrenal cortex. An androgen is a hormone that stimulates masculine characteristics and is present in both males and females. The adrenal glands are small structures located at the tops of the kidneys. The adrenal medulla is the central portion of the adrenal gland and the adrenal cortex is the outer portion. The adrenal glands p…
Echiuroid worms, or echiurans, commonly called spoon worms, are soft-bodied, unsegmented, marine animals of worldwide distribution. The approximately 125 species in the phylum Echiura occur mostly in the shallow intertidal zone of oceans. Most burrow or form tubes in sand or mud. Some live in discarded shells of sea urchins and sand dollars. Others inhabit cracks and crevices in rocks or coral fra…
Defining the features of Daoism (or Taoism) as one of the predominant trends in the history of Chinese thought involves accounting for its religious traits. As often happens outside the Western hemisphere—Buddhism may be the best-known example, but the same is true of Islam—the boundary between thought and religion in China is tenuous, unstable, and sometimes simply impossible to ide…
The idea of death—the irreversible end to life—has preoccupied, fascinated, and struck fear into human beings through the millennia. In the early twenty-first century, artists continue to sing about death, write about death, and depict it in paintings and photographs. Religious leaders are still talking about how to live a meaningful life in the face of death's inevitability. …
Islamic views of death and the afterlife encompass two broad streams: the individual and the collective or cosmological. The existence of an afterlife for individuals and final judgment of all creation are both central tenets of the faith. The Koran provides the foundation for Muslim views of death, with eschatological imagery leaping out from nearly every page. The Koran is supplemented by hadith…
In the animal kingdom, echolocation is an animal's determination of the position of an object by the interpretation of echoes of sounds produced by the animal. Echolocation is an elegant evolutionary adaptation to a low-light niche. The only animals that have come to exploit this unique sense ability are mammals—bats, dolphins, porpoises, and toothed whales. It is now believed that t…
Deism holds more meanings than one word should be asked to bear. Generally, to the point of almost being meaningless, it refers to the notion that reason plays an important role in determining religious knowledge. By this definition the pre-Socratic philosophers, Plato, Aristotle, Epicurus, the Stoics, Cicero, Lucretius, Buddha, Moses, Jesus, and Muhammad all qualify to varying degrees as Deists. …
A delta is a low-lying, almost flat landform, composed of sediments deposited where a river flows into a lake or an ocean. Deltas form when the volume of sediment deposited at a river mouth is greater than what waves, currents, and tides can erode. Deltas extend the coastline outward, forming new land along the shore. However, even as the delta is constructed, waves, currents, or tidal activity ma…
It is a coincidence of nature that the apparent size of the Sun and the Moon in the sky are about the same. Thus on those rare occasions when the orbital motion of Earth and Moon cause them to align with the sun, as seen from points on Earth, the Moon will just cover the surface of the Sun and day will suddenly become night. Those who are located in the converging lunar shadow that just reaches Ea…
Democracy, a direct translation of the Greek dēmokratia, means rule (kratos) by the people (dēmos). Both as a political idea, and as a political institution, democracy originated in the thought and practice of the ancient Greeks. They understood democracy literally: the people, deliberating and acting together in an assembly, was both sovereign and legislator. The people was not only t…
In the mid-1980s, democratic theory and politics in Africa entered a new phase as struggles for democratization spread across the continent and scholars began to vigorously debate the processes, prospects, and problems of Africa's democratic projects. This process was captured in an important collection edited by Peter Anyang Nyong'o, Popular Struggles for Democracy in Africa (1987),…
Dementia is a decline in a person's ability to think and learn. To distinguish true dementia from more limited difficulties due to localized brain damage, the strict medical definition requires that this decline affect at least two distinct spheres of mental activity; examples of such spheres include memory, verbal fluency, calculating ability, and understanding of time and location. Some d…
Demography is the study of human population and its changes due to deaths, births, marriages and divorces, and migration. The term demos denotes people in Greek—the term demography literally means the systematic study of people. In the early twenty-first century the discipline encompasses a broad array of subject matters, covering, among others, economic, social, public health, and politica…
Dengue fever is an illness caused by four closely related viruses (DEN-1, DEN-2, DEN-3, and DEN-4). Even though these viruses are closely related, they are recognized by the immune system as being different from each other. Thus, an infection with one virus does not provide immune protection against infections with the remaining three viral types. A person can have four bouts of dengue fever in hi…
Some years ago, Richard Cavendish, an eminent demonologist and student of the so-called black arts, observed that "[b]elief in the existence of evil supernatural beings" is so widespread that it "seems to be instinctive" (p. 8). Whether, as Cavendish suggests, these beliefs are in fact instinctive is still very much an open question. However, human beings do indeed app…
During the 1960s and 1970s, Latin American dependency theorists produced an important challenge to modernization and growth theories of development. Associated with a number of key intellectuals from Latin America—Andre Gunder Frank, Fernando Henrique Cardoso, and Peter Evans in Latin America, Samir Amin and Walter Rodney in Africa—the dependentistas turned modernization theory upsid…
The most general idea is that all events without exception are just effects. This idea has been associated with science since the seventeenth century, but it was put in some doubt by an interpretation of quantum theory in physics at the beginning of the twentieth century. Events are things that happen. Roughly speaking, they are such occurrences as a chair having such a property as a particular lo…
Development in biology refers to the process of growth and differentiation that is characteristic of living organisms. It describes the continuous changes during the life cycle of individual organisms from the early stage of a single cell until death. Development also refers to what is today known as the process of evolution, the transformation of species through time. Other meanings of developmen…
The density of an object is defined simply as the mass of the object divided by the volume of the object. For a concrete example, imagine you have two identical boxes. You are told that one is filled with feathers and the other is filled with cement. You can tell when you pick up the boxes, without looking inside, which is the box filled with cement and which is the box filled with feathers. The b…
The Greek noun dialogos derives from the verb dialegesthai, meaning "to enter into a conversation." The term dialectic, or the art of argumentation (dialectike techne), is derived from this verb as well, but in the case of Socratic dialectic the relevant Greek term is elegkhos (elenchus). Elenchus means a testing, and, since those tested by Socratic questioning are often shown inadeq…
Dialectical argument is a tool of systematic analysis. In the Talmudic framework, everything is in the moving, or dialectical, argument, the give–and–take of unsparing rationality, which, through their own capacity to reason, later generations are expected to reconstitute. Following the argument as set forth in the Talmud affords access to the issues, the argument, and the prevailing…
Ecological integrity is a relatively new concept that is being actively discussed by ecologists. However, a consensus has not yet emerged as to the definition of ecological integrity. Clearly, human activities result in many environmental changes that enhance some species, ecosystems, and ecological processes, while at the same time causing important damage to others. The challenge for the concept…
The African diaspora, together with the Jewish diaspora—the etymological and epistemological source of the term diaspora—enjoys pride of place in the increasingly crowded pantheon of diaspora studies. Studies of African diasporas can be divided into two broad categories. First, there are those that discuss the patterns of dispersal of African peoples around the world and the kinds of…
The Greek term diaspora, meaning "dispersion," has been used since ancient times as a means of describing the Jewish experience as well as the fact of Jewish settlement outside of the Jewish homeland to the present day. Originally, the term diaspora was used with respect to only three groups whose populations were dispersed in classical times: the Greeks, the Jews, and the Armenians.…
It is a somewhat common refrain in Latin America that countries need the mano dura (strong hand) of a military dictatorship in order to get things done. Surveys in the early twenty-first century reveal a growing disenchantment with civilian governments, with a surprisingly large minority of Latin Americans stating a preference for a dictatorial form of government over democracy. Such sentiments da…
Dentistry is the medical activity focused on treating the teeth, the gums and the oral cavity. This includes treating teeth damaged due to accidents or disease, filling teeth damaged due to tooth decay, and replacing damaged or injured teeth with replacement teeth. Major disciplines of dentistry include orthodontics, which focuses on the correction of tooth problems such as gaps between the teeth,…
The concept of diffusion was born to controversy. The initial debate over this issue took place during the late nineteenth and early twentieth centuries. …
Governments everywhere are increasingly recognizing the fact that human activities are causing serious environmental and ecological damage. To effectively deal with this environmental crisis, it is important to understand its dimensions and dynamics. What, specifically, are the damages, how are they changing over time, and what are the best means of prevention or mitigation? To develop answers to …
Discrimination should be understood as action and therefore distinguished from prejudice, which is a matter of thought, attitude, or belief. Racial discrimination then would consist of social action that produces unjust allocation of valued resources, based on racial categorizations of individuals or groups (Banton; Kairys, 1996). This is the standard definition and still serves in many cases: whe…
Ecological productivity refers to the primary fixation of solar energy by plants and the subsequent use of that fixed energy by plant-eating herbivores, animal-eating carnivores, and the detritivores that feed upon dead biomass. This complex of energy fixation and utilization is called a food web. Ecologists refer to the productivity of green plants as primary productivity. Gross primary productiv…
Diversity, as a word or concept, can apply to rocks, plants, animals, people, systems of law, and much else. In the United States, since the 1970s, its immediate reference, if the word is presented with nothing more to specify it, is to the diversity of races, ethnic groups, and language groups that make the United States possibly the most diverse country in the world. But its import extends far b…
It is midnight in the desert, and the full moon has just passed its apex. On the sandy ground, staff in hand, guitar and jug by his side, a dark-skinned man is nuzzled by a tawny-maned lion. Is the man dreaming? Are we? Or is this the dream of the artist, Henri Rousseau (1844–1910)? If, as some traditions have it, the Universe was dreamed into existence by its Creator, then it makes perfect…
Since the 1980s new generations of academics, collectors, curators, and enthusiasts have discovered the value of the study of dress as an analytical research tool through which to examine aspects of social and economic history, material culture, cultural and gender studies, art history, anthropology, and sociology. As a consequence, the study of the history of dress has been transformed from its m…
Dualism is a doctrine positing two equally powerful and antagonistic metaphysical principles, which are constitutive of the world and must explain our experience of the world. They are often conceived as dichotomies, such as good and evil, light and darkness, attraction and repulsion, or love and strife. Earlier forms of dualism can be traced in ancient Egyptian religion, with the contest between …
Ecological pyramids are graphical representations of the trophic structure of ecosystems. Ecological pyramids are organized with the productivity of plants on the bottom, that of herbivores above the plants, and carnivores above the herbivores. If the ecosystem sustains top carnivores, they are represented at the apex of the ecological pyramid of productivity. A fact of ecological energetics is th…
Dystopia is utopia's polarized mirror image. While utilizing many of the same concepts as utopia—for example, social stability created by authoritarian regimentation—dystopia reads these ideas pessimistically. Dystopia angrily challenges utopia's fundamental assumption of human perfectibility, arguing that humanity's inherent flaws negate the possibility of const…
If the locus classicus of eclecticism was Diogenes Laertius' Lives of the Eminent Philosophers, the locus modernus was the Introduction to Stoic Philosophy (1604) of Justus Lipsius (1547–1606), who argued that the method of critical choosing or "election" was superior to the dogmas of particular schools and represented the true road to truth. From the second quarter of …
Deoxyribonucleic acid (DNA), "the master molecule," is a natural polymer which encodes the genetic information required for the growth, development, and reproduction of an organism. Found in all cells, it consists of chains of units called nucleotides. Each nucleotide unit contains three components: the sugar deoxyribose, a phosphate group, and a nitrogen-containing amine or base wit…
Ecology is commonly seen as a lineal descendant of traditional natural history extending back to such classical figures as Aristotle, Theophrastus, and Pliny. Notable persons in this tradition include the Swedish botanist, Carl von Linné (Carolus Linnaeus; 1707–1778), who coined the phrase "economy of nature" in 1749. Gilbert White (1720–1793), a British cleric, ma…
A deposit is an accumulation of Earth materials, usually loose sediment or minerals, that is laid down by a natural agent. Deposits are all around you—the sand on the beach, the soil in your backyard, the rocks in a mountain stream. All of these consist of earth materials transported and laid down (that is, deposited) by a natural agent. These natural agents may include flowing water, ice, …
The term economics, from the Greek oikonomika, means a science or art of managing the household. In modern usage, it refers to the efficient allocation of scarce resources in the production, distribution, and consumption of goods and services to satisfy various desires. As a branch of knowledge, economics or economic science is the study of how to efficiently use limited resources—natural r…
Depression is a psychoneurotic disorder characterized by lingering sadness, inactivity, and difficulty in thinking and concentration. A significant increase or decrease in appetite and time spent sleeping, feelings of dejection and hopelessness, and sometimes suicidal tendencies may also be present. It is one of the most common psychiatric conditions encountered, and affects up to 25% of women and…
The word ecumenism itself became prevalent after the 1910 World Missionary Conference in Edinburgh, Scotland. Delegates from missionary organizations met to address the incongruity—and scandal, no less—of historically divided and competing Christian denominations preaching a message of peace and harmony among non-Christian peoples. The World Council of Churches, the primary organizat…
A middle region, Southeast Asia, was influenced by Buddhism and Confucianism, resulting in a unique blended educational experience. From the eighteenth century on, as Western culture came into the region, Asia's educational practices in general underwent a sea change that resulted in a previously unknown level of uniformity. Regional diversity, however, remains visible, reflecting the diff…
Ecology can be defined as the study of the relationships of organisms with their living and nonliving environment. Most ecologists are interested in questions involving the natural environment. Increasingly, however, ecologists are concerned about degradation associated with the ecological effects of humans and their activities. Ultimately, ecological knowledge will prove to be fundamental to the …
The imperial state increased its expenditures on education during the Tang (618–906) and Song (960–1280) dynasties, when it created the first examination system for selecting officials. In addition, the rise of Buddhism in medieval China created charitable institutions for the common people, which included temple schools and monasteries, where many commoners—male and female&#x…
The notion of ecosystem (or ecological system) refers to indeterminate ecological assemblages, consisting of communities of organisms and their environment. Ecosystems can vary greatly in size. Small ecosystems can be considered to occur in tidal pools, in a back yard, or in the rumen of an individual cow. Larger ecosystems might encompass lakes or stands of forest. Landscape-scale ecosystems comp…
European pre-university education began its long odyssey with Homer. The social and literary values expressed in his poetry informed Greek education, which became the basis of Roman education. The Renaissance revived ancient literary texts and educational programs, which were modified and adapted in subsequent centuries. European humanities education still embraces in part ancient Greco-Roman educ…
Depth perception is the ability to see the environment in three dimensions and to estimate the spatial distances of objects from ourself and from each other. Depth perception is vital for our survival, being necessary to effectively navigate around and function in the world. Without it we would be unable to tell how far objects are from us, and thus how far we would need to move to reach or avoid …
An ecotone is a zone of transition between distinct ecological communities or habitats. Usually, the word is used to refer to relatively sharp, local transitions, also known as edges. Because many physical and chemical changes in the environment tend to be continuous, ecological transitions are often similarly gradual. For example, climate and precipitation change steadily across continents and up…
Global education, or global studies, is an interdisciplinary approach to learning concepts and skills necessary to function in a world that is increasingly interconnected and multicultural. The curricula based on this approach are grounded in traditional academic disciplines but are taught in the context of project-and problem-based inquiries. The learner examines issues from the vantage point of …
The ideas produced, preserved, and transmitted through education in India have been as multifaceted as the many social groups who have lived in the South Asian subcontinent. Gender has been an important determinant for educational opportunities over time and across regions. When and where education was closely associated with making a living, parents assumed that women needed only domestic skills,…
A lifelong pursuit of learning is a characteristic ideal of Islamic piety. It underlies the concept of "Islamic" education. While the primary focus of this concept is the nurturing of religious belief in the individual, its scope broadened to incorporate various secular disciplines, literary and scientific, as it aimed at developing within the community fully integrated personalities…
Ecotourism refers to outdoor recreation, sightseeing, and guided natural history studies in remote or fragile natural areas, or archeological and cultural sites. Ecotourism usually involves travel to engage in activities such as trekking and hiking, diving, mountaineering, biking, and paddling, while exploring a region's natural highlights, observing native animals, and learning about the a…
The shoen system declined due to the failure of the court nobility in the capitol to retain strong ties with outlying estates, and led to bushi (warrior) rule. Primarily, the warrior and the monk supported medieval education (1192–1603). In 1177, the daigakuryo burned and was not rebuilt. Because the Kamakura regime (1192–1333) was established upon a warrior code, despite the relativ…
In mathematics, the derivative is the exact rate at which one quantity changes with respect to another. Geometrically, the derivative is the slope of a curve at a point on the curve, defined as the slope of the tangent to the curve at the same point. The process of finding the derivative is called differentiation. This process is central to the branch of mathematics called differential calculus. …
Edema is the accumulation of fluid in any given location in the body. Edema can result from trauma, as in a sprained ankle, or from a chronic condition such as heart or kidney failure. The word edema is from the Greek and means "swelling." The presence of edema can be an important diagnostic tool for the physician. A patient who is developing congestive heart failure often will devel…
What distinguishes the development of education in North America has been its cautious, halting, but nevertheless relentless movement toward universality and inclusivity, an impulse that owes much to the progressive ideals of the eighteenth-century Enlightenment and to the consequent development of liberal democracy on the continent. In the early twenty-first-century Canada and in the United State…
Desalination, also called desalting, is the removal of salt from seawater. It provides essential water for drinking and industry in desert regions or wherever the local water supply is brackish. In 1991, about 3.5 billion gallons of desalinated water were produced in about 4,000 desalination plants worldwide. Most of this water was produced through distillation. However, other methods, including r…
Eel grasses (or eelgrasses) are 18 species of herbaceous aquatic plants in the family Zosteraceae, 12 species of which are in the genus Zostera. However, some plant systematists have treated the eel grasses as a component of a much larger family, the pondweeds or Potamogetonaceae. Eel grasses have long, strap-like leaves that emerge from a thin rhizome growing in the surface sediment of the shallo…
Ideas about emotions and their function in human and animal life have been a major theme in philosophy—and more recently in psychology and the social sciences—since the time of the ancient Greeks. The history of ideas about emotion is an essential part of the history of ideas about human nature and about human continuity—and discontinuity—with other animals. Whether emo…
A desert is an arid land area where more water is lost through evaporation than is gained from precipitation. Deserts include the familiar hot, dry desert of rock and sand that is almost barren of plants, the semiarid deserts of scattered trees, scrub, and grasses, coastal deserts, and the deserts on the polar ice caps of the Antarctic and Greenland. Most desert regions are the result of large-sca…
El Niño and La Niña are disruptions of the oceanic and atmospheric systems of the equatorial Pacific Ocean that have far reaching effects on Earth's weather patterns. El Niño and La Niña do not change with the regularity of the seasons; instead, they repeat about every two to seven years. They are the extremes in an aperiodic, or irregular, cycle called the El Ni…
Desertification is the gradual degradation of productive arid or semi-arid land into biologically unproductive land. The French botanist, André Aubreville, coined the term in 1949 to describe to the transformation of productive agricultural land in northern Africa to desert-like, uncultivable fallowland. Loss of biological and ecological viability occurs when natural variations, like extend…
A determinant, signified by two straight lines ||, is a square array of numbers or symbols that has a specific value. For a square matrix, say, A, there exists an associated determinant, |A|, which has elements identical with the corresponding elements of the matrix. When matrices are not square, they do not possess corresponding determinants. For determinants of third order, that is, n = 3, or th…
Eland (Taurotragus oryx) are the largest African antelopes, weighing up to a 2,205 lb (1,000 kg) and standing 6.6 ft (2 m) at the shoulder. They belong to the family Bovidae in the order Artiodactyla, the even-toed hoofed mammals. Eland belong to the tribe Tragelaphini, a closely-related group of spiral-horned antelopes, whose members are not territorial. Both sexes posses long horns, and females …
Elapid snakes are extremely venomous snakes such as cobras, mambas, kraits, tiger snakes, and coral snakes in the family Elapidae. The elapids are about 120 species in the subfamily Elapinae. The sea snakes (subfamily Hydrophiinae) and subfamily Laticaudinae make up the other two subfamilies in the Elapidae. Elapid snakes have a wide distribution from warm temperate climates to tropical climates, …
Developmental processes are the series of biological changes associated with information transfer, growth, and differentiation during the life cycle of organisms. Information transfer is the transmission of DNA and other biological signals from parent cells to daughter cells. Growth is the increase in size due to cell expansion and cell division. Differentiation is the change of unspecialized cell…
Elasticity is the ability of a material to return to its original shape and size after being stretched, compressed, twisted or bent. Elastic deformation (change of shape or size) lasts only as long as a deforming force is applied to the object, and disappears once the force is removed. Greater forces may cause permanent changes of shape or size, called plastic deformation. In ordinary language, a …
The dew point is that temperature below which the water vapor in a body of air cannot all remain vapor. When a body of air is cooled to its dew point or below, some fraction of its water vapor shifts from gaseous to liquid phase to form fog or cloud droplets. If a smooth surface is available, vapor condenses directly onto it as drops of water (dew). The dew point of a body of air depends on its wa…
An electric arc is a high-current, low-voltage electrical discharge between electrodes in the presence of gases. In an electric arc, electrons are emitted from a heated cathode. Arcs can be formed in high, atmospheric, or low pressures, and in various gases. They have wide uses as highly luminous lamps, as furnaces for heating, cutting and welding, and as tools for spectrochemical analysis. …
Rub a balloon or styrofoam drinking cup against a wool sweater. It will then stick to a wall (at least on a dry day) or pick up small bits of paper. Why? The answer leads to the concept of electric charge. Electromagnetic forces are one of the four fundamental forces in nature. The other three are gravitational, strong nuclear, and weak nuclear forces. The electromagnetic force unifies both electr…
An electric circuit is a system of conducting elements designed to control the path of electric current for a particular purpose. Circuits consist of sources of electric energy, like generators and batteries; elements that transform, dissipate, or store this energy, such as resistors, capacitors, and inductors; and connecting wires. Circuits often include a fuse or circuit breaker to prevent a pow…
Diabetes mellitus is a group of diseases characterized by high levels of glucose in the blood resulting from defects in insulin production (insulin deficiency), insulin action (insulin resistance), or both. Insulin is a hormone produced by the pancreas. When eaten, foods are converted to a type of sugar called glucose that enters the bloodstream. Insulin is needed to move glucose into the body cel…
Diagnosis, from gnosis, the Greek word for knowledge, is the process of identifying a disease or disorder in a person by examining the person and studying the results of medical tests. The diagnosis begins when the patient is presented to the doctor with a set of symptoms or perceived abnormalities such as pain, nausea, fever, or other untoward feeling. Often the diagnosis is relatively simple, an…
An electric conductor is any material that can efficiently conduct electricity, such as a metal, ionic solution, or ionized gas. Usually, this term refers to the current-carrier component of an electric circuit system. …
Dialysis is a process by which small molecules in a solution are separated from large molecules. The principle behind the process was discovered by the Scottish chemist Thomas Graham in about 1861. Graham found that the rate at which some substances, such as inorganic salts, pass through a semipermeable membrane is up to 50 times as great as the rate at which other substances, such as proteins, do…
Diamond is a mineral with the same carbon composition as graphite, but with different structure. Diamonds are formed in the compression of coal. Photograph by Rick Gayle. Stock Market. Reproduced by permission. Diamonds are a globally traded commodity used for a variety of industrial and artistic purposes. In December 2000, the United Nations General Assembly unanimously adopted a resolutio…
Algae are a very diverse group of simple, nucleated, plant-like aquatic organisms that are primary producers. Primary producers are able to utilize photosynthesis to create organic molecules from sunlight, water, and carbon dioxide. Ecologically vital, algae account for roughly half of photosynthetic production of organic material on earth in both freshwater and marine environments. Algae exist ei…
Electric current is the result of the relative motion of net electric charge. In metals, the charges in motion are electrons. The magnitude of an electric current depends upon the quantity of charge that passes a chosen reference point during a specified time interval. Electric current is measured in amperes, with one ampere equal to a charge-flow of one coulomb per second. A current as small as a…
Dielectric materials are substances that have very low conductivity. That is, they are electrical insulators through which an electrical current flows only with the greatest of difficulty. Technically, a dielectric can be defined as a material with electrical conductivity of less than one millionth of a mho (a unit of electrical conductance) per centimeter. In theory, dielectrics can include solid…
Diesel engines are a class of internal combustion engine in which the fuel is burned internally and the combustion products are used as the working fluid. Unlike the spark-ignited (SI) engines found in the majority of today's automobiles in which the premixed fuel-air mixture is ignited by an electric spark, diesel engines are characterized by a spontaneously initiated combustion process wh…
An electric motor is a machine used to convert electrical energy to mechanical energy. Electric motors are extremely important to modern-day life, being used in many different places, e.g., vacuum cleaners, dishwashers, computer printers, fax machines, video cassette recorders, machine tools, printing presses, automobiles, subway systems, sewage treatment plants and water pumping stations. The maj…
The substance diethylstilbestrol (DES) is a synthetic, nonsteroidal estrogen, which was first made in 1938. Initially the substance was seen as a great scientific breakthrough, drawing on research that documented the importance of naturally occurring estrogen in women. Wide-scale use of DES by pregnant women to prevent miscarriage beginning in the 1940s ended in 1971 when researchers discovered th…
Diffraction is the deviation from a straight path that occurs when a wave such as light or sound passes around an obstacle or through an opening. The importance of diffraction in any particular situation depends on the relative size of the obstacle or opening and the wavelength of the wave that strikes it. The diffraction grating is an important device that makes use of the diffraction of light to…
A diffraction grating is an optical device consisting of many closely spaced parallel lines or grooves. In a transmission type of grating, light passes through the narrow transparent slits that lie between the dark lines on a glass or plastic plate. In a reflecting grating, light is reflected by the many parallel, narrow, smooth surfaces and absorbed or scattered by the lines cut in the reflecting…
Electric vehicles (EV), vehicles whose wheels are turned by electric motors rather than by a mechanical gasoline-powered drivetrain, have been long touted as saviors of the environment due to their low pollution and high fuel efficiency. However, they have yet to take over the highways and byways. Thomas Davenport is credited with building the first practical EV in 1834, which was quickly followed…
Diffusion is the movement of molecules along a concentration gradient, from an area of high concentration to one of low concentration. Diffusion proceeds until the two concentrations are equal. Diffusion occurs in both gases and liquids. Diffusion takes place not only in liquid solutions, but in gases. The odor of bread wafting through a house is an example of the diffusion of bread-smell chemical…
Conductivity is the term used to describe the ability of a material medium to permit the passage of particles or energy. Electrical conductivity refers to the movement of charged particles through matter. Thermal conductivity refers to the transmission of heat energy through matter. Together, these are the most significant examples of a broader classification of phenomena known as transport proces…
An electrical power supply is a device that provides the energy needed by electrical or electronic equipment to perform their functions. Often, that energy originates from a source with inappropriate electrical characteristics, and a power supply is needed to change the power to meet the equipment's requirements. Power supplies usually change alternating current into direct current, raise o…
The electrical resistance of a wire or circuit is a way of measuring the resistance to the flow of an electrical current. A good electrical conductor, such as a copper wire, will have a very low resistance. Good insulators, such as rubber or glass insulators, have a very high resistance. The resistance is measured in ohms, and is related to the current in the circuit and voltage across the circuit…
The digestive system is a group of organs responsible for the conversion of food into absorbable chemicals which are then used to provide energy for growth and repair. The digestive system is also known by a number of other names, including the gut, the digestive tube, the alimentary canal, the gastrointestinal (GI) tract, the intestinal tract, and the intestinal tube. The digestive system consist…
