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Triboelectric Effect

Triboelectric effect

The triboelectric effect is a type of contact electrification in which certain materials become electrically charged after coming into contact with another, different, material, and are then separated. The polarity and strength of the charges produced differ according to the materials, surface roughness, temperature, strain, and other properties. It is therefore not very predictable, and only broad generalizations can be made. Amber, for example, can acquire an electric charge by friction with a material like wool. This property, first recorded by Thales of Miletus, suggested the word "electricity", from the Greek word for amber, ēlektron. Other examples of materials that can acquire a significant charge when rubbed together include glass rubbed with silk, and hard rubber rubbed with fur.

Series

Materials are often listed in order of the polarity of charge separation when they are touched with another object. A material towards the bottom of the series, when touched to a material near the top of the series, will attain a more negative charge, and vice versa. The further away two materials are from each other on the series, the greater the charge transferred. Materials near to each other on the series may not exchange any charge, or may exchange the opposite of what is implied by the list. This depends more on the presence of rubbing, the presence of contaminants or oxides, or upon other properties than the type of material. Lists vary somewhat as to the exact order of some materials, since the charge also varies for nearby materials.

Effect

Although the word comes from the Greek for "rubbing", tribos, the two materials only need to come into contact and then separate for electrons to be exchanged. After coming into contact, a chemical bond is formed between some parts of the two surfaces, called adhesion, and charges move from one material to the other to equalize their electrochemical potential. This is what creates the net charge imbalance between the objects. When separated, some of the bonded atoms have a tendency to keep extra electrons, and some a tendency to give them away, though the imbalance will be partially destroyed by tunneling or electrical breakdown (usually corona discharge). In addition, some materials may exchange ions of differing mobility, or exchange charged fragments of larger molecules. The triboelectric effect is only related to friction because they both involve adhesion. However, the effect is greatly enhanced by rubbing the materials together, as they touch and separate many times. For surfaces with differing geometry, rubbing may also lead to heating of protrusions, causing pyroelectric charge separation which may add to the existing contact electrification, or which may oppose the existing polarity. Surface nano-effects are not well understood, and the atomic force microscope has made sudden progress possible in this field of physics. Because the surface of the material is now electrically charged, either negatively or positively, any contact with an uncharged conductive object or with an object having substantially different charge may cause an electrical discharge of the built-up static electricity; a spark. A person simply walking across a carpet may build up a charge of many thousands of volts, enough to cause a spark a centimeter long or more (this type of discharge is usually harmless, as the current, though very large, typically exists for far less than a millionth of a second).

Utilization

The effect is of considerable industrial importance both in terms of safety and also potential damage to manufactured goods. The spark produced is fully capable of igniting flammable vapours, for example, petrol or ether fumes. Means have to be found to discharge hospital trolleys which may carry such liquids. Even where only a small charge is produced, this can result in dust particles being attracted to the rubbed surface. In the case of textile manufacture this can lead to a permanent grimy mark where the cloth has been charged. Some electronic devices, most notably MOSFETs, can be accidentally destroyed by high-voltage static discharge. Such components are usually stored in a conductive foam for protection, and grounding oneself by touching the workbench or using a special bracelet or anklet is standard practice while handling unconnected integrated circuits. The triboelectric effect can also be used toward a positive end: it is the principle behind the charge build-up in a triboelectric-type Van de Graaff generator. In 1991, G. L. Paramo developed the Lorente generator. The Lorente generator is a triboelectric machine to aid in the construction and operation of electrostatic generators. It consists of four cylinders (with two bieing rigid dielectrics) that operate without friction (but are under a slight pressure). No injection of electrical charges originating from outside within the Lorente generator.

External articles and references

;References
- ;Patents
- -- Earle W. Ballentine -- "Triboeletric Generator"
- -- Gabriel L. Paramo -- "Rolling triboelectric generator" category:Electrical phenomena Category:Electricity

Contact electrification

In the late 18th century, scientists developed sensitive instruments for detecting 'electrification', otherwise known as electrostatic charge imbalance. The phenomenon of electrification by contact, or contact electrification, was quickly discovered. When two objects were touched together, sometimes the objects became spontaneously charged. One object developed a net negative charge, while the other developed an equal and opposite positive charge. Contact electrification phenomenon allowed the construction of so-called 'frictional' electrostatic generators such as Ramsden's or Winter's machines, but it also led directly to the development of most modern electrical technology such as batteries, fuel cells, electroplating, thermocouples, and semiconductor junction devices including radio detector diodes, photocells, LEDs, and thermoelectric cells.

Triboelectric contact

Main article: Triboelectric effect If two different insulators are touched together, such as when a piece of rubber is touched against a piece of glass, then the surface of the rubber will acquire an excess negative charge, and the glass will acquire an equal positive charge. If the surfaces are then pulled apart, a very high voltage is produced. This so-called "tribo" or "rubbing" effect is not well understood. It may be caused by electron-stealing via quantum tunneling, or by transfer of surface ions. Friction is not required, although in many situations it greatly increases the phenomenon. Certain phenomena related to frictionally generated electrostatic charges have been known since antiquity, though of course the modern theory of electricity was developed after the Scientific Revolution.

Electrolytic-metallic contact

If a piece of metal is touched against an electrolyte material, the metal will spontaneously become charged, while the electrolyte will acquire an equal and opposite charge. Upon first contact, a chemical reaction called a 'half-cell reaction' occurs on the metal surface. As metal ions are transferred to or from the electrolyte, and as the metal and electrolyte become oppositely charged, the increasing voltage at the thin insulating layer between metal and electrolyte will oppose the motion of the flowing ions, causing the chemical reaction to come to a stop. If a second piece of a different type of metal is placed in the same electrolyte bath, it will charge up and rise to a different voltage. If the first metal piece is touched against the second, the voltage on each metal piece will be forced to a different level, and the chemical reactions will run constantly. In this way the 'contact electrification' becomes continuous. At the same time, an electric current will appear, with the path forming a closed loop which lead from one metal part to the other, and leading out through the chemical reactions on the metal surface, through the electrolyte, then back into the chemical reactions on the second metal surface. In this way, contact electrification leads to the invention of the Galvanic cell or battery.

Metallic contact

If two metals having differing work functions are touched together, one steals electrons from the other, and the opposite net charges grow larger and larger; this is the Volta effect. The process is halted when the voltage between the two metals reaches a particular value (the difference in work function values; usually less than one volt.) If part of the junction between the metals is heated, and another part cooled, the voltage across the different parts of the junction will not be the same, and an electric current will appear. In this way contact electrification leads to the invention of the thermocouple. This is the Peltier-Seebeck effect.

Semiconductor contact

If metal touches a semiconductive material, or if two different semiconductors are placed into contact, one becomes charged slightly postive and the other slightly negative. It is found that if this junction between semiconductors is connected to a power supply, and if the power supply is set to a voltage slightly higher than the natural voltage appearing because of contact electrification, then for one polarity of voltage there will be a current between the two semiconductor parts, but if the polarity is reversed, the current stops. Thus contact electrification leads to the invention of the diode or rectifier and triggers the revolution in Semiconductor electronics and physics. If bright light is aimed at one part of the contact area between the two semiconductors, the voltage at that spot will rise, and an electric current will appear. When light meets contact electrification, the light energy is changed directly into electrical energy, allowing creation of solar cells. Later it was found that the same process can be reversed, and if a current is forced backwards across the contact region between the semiconductors, sometimes light will be emitted, allowing creation of the LED. Category:Electrical phenomena Category:History of physics

Polarity

The polarity of an object is, in general, its physical alignment. The term is often used to describe the positive and negative ends of batteries and magnets. In cell biology, polarity refers to cells not being point-symmetrical in their spatial organization. This is particularly evident for cells organized in single cell layers that separate organs or subcompartments in organisms. In chemistry, molecular polarity refers to the dipole-dipole intermolecular forces between the slightly positive end of one molecule to the negative end of another or the same molecule. Molecular polarity is dependant on the difference in electronegativity between atoms in a compound and the asymmetry of the compound's structure. See also: polar molecule, chemical bonding

Friction

Friction is the force that opposes the relative motion or tendancy of such motion of two surfaces in contact. The classical linear approximation of the force of friction known as Coulomb friction (named for Charles-Augustin de Coulomb) is expressed as: :

F_ = \frac\cdot F_\cdot \mu_ \,

where :

v \,

=velocity of surface :

F_f  \,

= the force of friction :

F_n \,

= the force normal to the contact surface :

\mu_f \,

= the coefficient of friction This very simple, yet incomplete, representation of friction is adequate for the analysis of many physical systems.

Coefficient of Friction

The coefficient of friction (also known as the frictional coefficient or the friction coefficient) is a scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together. The coefficient of friction depends on the materials used -- for example, ice on metal has a low coefficient of friction (they slide past each other easily), while rubber on pavement has a high coefficient of friction (they do not slide past each other easily). It is also important to discriminate between sliding (dynamic) friction and static friction. For sliding friction, the force of friction does not vary with the area of contact between the two objects. This means that sliding friction does not depend on the size of the contact area. However, for static friction where there is an element of adhesion, the contact area does matter. For a race car, wide wheels are used to increase the static friction with the road. However, once adhesion is lost, the size of the contact area is no longer relevant. The force of friction is always exerted in a direction that opposes movement. For example, a chair sliding to the right across a floor experiences the force of friction in the left direction. The coefficient of friction is an empirical measurement -- it has to be measured experimentally, and cannot be found through calculations. Rougher surfaces tend to have higher values. Most dry materials in combination give friction coefficient values from 0.3 to 0.6. It is difficult to maintain values outside this range. A value of 0.0 would mean there is no friction at all. Rubber in contact with other surfaces can yield friction coefficients from 1.0 to 2.0. A system with "interlocking teeth" between surfaces may be indistinguishable from friction, if the "teeth" are small, such as the grains on two sheets of sandpaper or even molecular sized "teeth". Saying that rougher surfaces experience more friction sounds safe enough - two pieces of coarse sandpaper will obviously be harder to move relative to each other than two pieces of fine sandpaper. However, if two pieces of flat metal are made progressively smoother, you will reach a point where the resistance to relative movement increases, due to a vacuum between the two surfaces. If you make them very flat and smooth, and remove all surface contaminants in a vacuum, the smooth flat surfaces will actually adhere to each other, making what is called a cold weld. Once you reach a certain degree of mechanical smoothness, the frictional resistance is found to depend on the nature of the molecular forces in the area of contact, so that substances of comparable "smoothness" can have significantly different coefficients of friction.

Types of Friction

Static Friction

Static friction occurs when the two objects are not moving relative to each other (like a desk on the ground). The coefficient of static friction is typically denoted as μ_s. The initial force to get an object moving is often dominated by static friction.
- Rolling friction occurs moving relative to each other and one "rolls" on the other (like a car's wheels on the ground). This is classified under static friction because the patch of the tire in contact with the ground, at any point while the tire spins, is stationary relative to the ground. The coefficient of rolling friction is typically denoted as μ_r.

Kinetic Friction

Kinetic friction occurs when two objects are moving relative to each other and rub together (like a sled on the ground). The coefficient of kinetic friction is typically denoted as μ_k, and is usually less than the coefficient of static friction. Examples of kinetic friction:
- Sliding friction is when two objects are rubbing against each other. Putting a book flat on a desk and moving it around is an example of sliding friction.
- Fluid friction is the friction between a solid object as it moves through a liquid or a gas. The drag of air on an airplane or of water on a swimmer are two examples of fluid friction. When an object is pushed along a surface with coefficient of friction μ_k and a perpendicular (normal) force acting on that object directed towards the surface of magnitude N, then the energy loss of the object is given by: :

U = N \mu_k d \,

Where d is the distance travelled by the object whilst in contact with the surface. This equation is identical to Energy Loss = Force x Distance as the frictional force is a non-conservative force. Note, this equation only applies to kinetic friction, not rolling friction. Physical deformation is associated with friction. While this can be beneficial, as in polishing, it is often a problem, as the materials are worn away, and may no longer hold the specified tolerances. The work done by friction can translate into deformation and heat that in the long run may affect the surface's specification and the coefficient of friction itself. Friction can in some cases cause solid materials to melt. Friction may occur between solids, gases and fluids or any combination thereof. See aerodynamics and hydrodynamics.

Reducing Friction

Devices

Devices, such as ball bearings can change sliding friction into the less significant rolling friction.

Techniques

One technique used by railroad engineers is to back up the train to create slack in the linkages between cars. This allows the train to pull forward and only take on the static friction of one car at a time, instead of all cars at once, thus spreading the static frictional force out over time.

Lubricants

A common way to reduce friction is by using a lubricant, such as oil, that is placed between the two surfaces, often dramatically lessening the coefficient of friction. The science of friction and lubrication is called tribology. Superlubricity, a recently-discovered effect, has been observed in graphite: it is the substantial decrease of friction between two sliding objects, approaching zero levels - a very small amount of frictional energy would be dissipated due to electronic and/or atomic vibrations. Lubricants to overcome friction need not always be thin, turbulent fluids or powdery solids such as graphite and talc; acoustic lubrication actually uses sound as a lubricant.

Lubricant technology

AF coatings (anti-friction coatings) have been successfully used for years as an element of heavy-duty lubrication. Typically used for applications where a permanent lubricating film is needed for metal-to-plastic or plastic-to-plastic lubrication, AF coating technology offers an economic solution to a wide range of engineering problems. The usage of AF coatings, such as Molykote® brand or other prominent anti-friction coating brand, is most successful when requirements for wear and corrosion protection and optimal coefficient of friction are properly met. A low, high, or even constant coefficient of friction is achievable, if the appropriate application and type of AF coating is utilized. A firm, completely dry, and non-contaminating lubricating film results once it is properly prepared and applied. The AF coating generally consists of the resin (epoxy, phenolic, and silicone) - a base material, which adheres well to the surface. Solid lubricants such as MoS₂, PTFE, polyamide, polyethylene, and graphite are set in this base material, passing on the anti-friction properties of an AF coating. Water-dilutable AF coatings, coatings low in solvents, as well as non-combustible or electrostactically sprayable AF coatings, are now being offered to help save energy and meet environmental protection regulations. Many products using AF technology offer corrosion protection in excess of normal industrial requirements, while some are unaffected by fuels, solvents, or oils. Application is typically simple: preferably by spraying, dipping, or brushing on thoroughly degreased metal surfaces. The drying and curing times are short (between three minutes for air-drying and sixty minutes for oven cured coatings).

Products of friction

According to the law of conservation of energy, no energy should be lost due to friction. The kinetic energy lost is transformed primarily into heat and/or motion of other objects and fluids. An airplane will heat and accelerate the air as it passes. A submarine will do the same to the water. In some cases, the "other object" to be accelerated may be the Earth. A sliding hockey puck will come to rest due to friction both by changing its energy into heat and accelerating the Earth in its direction of travel (by an immeasurable amount). Since heat and fluid motion quickly dissipate and the change in velocity of the Earth can't be seen, many early philosophers, such as Aristotle, concluded that moving objects lose energy without an opposing force.

References

- Category:Force ko:마찰력 ja:摩擦

Wool

:This article is about wool, the fibre produced from sheep. For alternative meanings see Wool (disambiguation). Wool (disambiguation) Wool (disambiguation) Wool is the fibre derived from the hair of animals of the Caprinae family, principally sheep and goats, but the hair of other mammals such as alpacas may also be called wool. This article deals with the wool produced from domestic sheep. Wool is the fibre produced as the outer coat of sheep. Most of the fibre from domestic sheep has two qualities that distinguish it from hair or fur: it has scales which overlap like shingles on a roof and it is crimped; in some fleeces the wool fibres have more than 20 bends per inch. Both the scaling and the crimp make it possible to spin and felt the fleece. They help the individual fibres attach to each other so that they stay together. Because of the crimp, wool fabrics have a greater bulk than other textiles and retain air, which causes the product to retain heat. Insulation also works both ways; bedouins and tuaregs use wool clothes to keep the heat out. The amount of crimp corresponds with the fineness of the wool fibres. A fine wool like merino may have up to a hundred crimps per inch, where the coarser wools like karakul may have as few as one to two crimps per inch. Hair, by contrast, has little if any scale and no crimp and little ability to bind into yarn. On sheep, the hair part of the fleece is called kemp. The relative amounts of kemp to wool vary from breed to breed, and make some fleeces more desirable for spinning, felting or carding into batts for quilts or other insulating products. Wool is generally a creamy white colour, although some breeds of sheep produce natural colors such as black, brown (also called moorit) and grey. Wool straight off a sheep contains a high level of grease (thus "greasy wool") which contains valuable lanolin. In this state it can be worked into yarn or knitted into water-resistant mittens, such as those of the Aran Island fishermen. The grease is generally removed for processing by scouring with detergent and alkali. After shearing, the wool is separated into five main categories: fleece (which makes up the vast bulk), pieces, bellies, crutchings and locks. The latter four are packaged and sold separately. The quality of fleece is determined by a technique known as wool classing, whereby a qualified woolclasser tries to group wools of similar gradings together to maximise the return for the farmer or sheep owner. The fibre diameter of wool varies from 15 micrometres (superfine merino) to 30 or more micrometres for the coarser wools. The finer diameters are generally more valuable.

History

As the raw material has been readily available since the widespread domestication of sheep and similar animals, the use of wool for clothing and other fabrics dates back to some of the earliest civilizations. Prior to invention of shears - probably in the Iron Age - they probably plucked the wool out by hand or by bronze combs. In medieval times, the wool trade was serious business. English wool exports - which bordered on European monopoly - were a significant source of income to the crown. Over the centuries, various British laws controlled the wool trade or required the use of wool even in burials. The smuggling of wool out of the country, known as owling, was at one time punishable by the cutting off of a hand. In 1699 English crown forbade its American colonies to trade wool with anyone else but England itself. In the Renaissance, Medicis of Florence built their wealth and banking system on wool trade with the aid of the Arte della Lana, the wool guild. Spain allowed export of Merino lambs only with royal permission. German wool - based on sheep of Spanish origin - begun to overtake British one only at the end of 19th century. Australia's colonial economy was based on sheep raising and Australian wool trade overtook Germans by 1845.

Production

Global wool production is approximately 1.3 million tonnes per annum of which 60% goes into apparel. Australia, China and New Zealand are leading commercial producers of wool. Most Australian wool comes from the merino breed. Breeds such as Lincoln and Romney produce coarser fibres and wool of these sheep is usually used for making carpets. In the United States, Texas, New Mexico and Colorado also have large commercial sheep flocks and their mainstay is the Rambouillet (or French Merino). There is also a thriving 'home flock' contingent of small scale farmers who raise small hobby flocks of specialty sheep for the handspinning market. These small scale farmers may raise any type of sheep they wish, so the selection of fleeces is quite wide.

Global wool clip 2004/2005

#Australia: 25% of global wool clip (475 million kg greasy, 2004/2005) #China: 18% #New Zealand: 11% #Argentina: 3% #Turkey: 2% #Iran: 2% #United Kingdom: 2% #India: 2% #Sudan: 2% #South Africa: 1% ([http://www.wool.com.au/attachments/Education/AWI_WoolFacts.pdf source])

Uses

In addition to clothing, wool has been used for carpeting, felt, and upholstery. Wool felt covers piano hammers and it is used to absorb odors and noise in heavy machinery and stereo speakers. Ancient Greeks lined their helmets with felt and Roman legionnaires used breastplates made of wool felt. Shoddy is recycled or remanufactured wool. To make shoddy, existing wool fabric is cut or torn apart and respun. As this process makes the wool fibres shorter, the remanufactured fabric is inferior to the original. The recycled wool may be mixed with raw wool, wool noil, or another fibre such as cotton to increase the average fibre length. Such yarns are typically used as weft yarns with a cotton warp. This process was invented in the Heavy Woollen District of West Yorkshire and created a micro-economy in this area for many years. Ragg is a sturdy wool fibre made into yarn and used in many rugged applications like gloves.

External links

- [http://www.sheepusa.org/ American Sheep Industry Association]
- [http://www.ncwga.org/ Natural Colored Wool Growers Association]
- [http://www.wool.com.au/attachments/Education/AWI_WoolFacts.pdf Wool Facts September 2005 edition: Australia's Wool Industry] ja:ウール

Thales

:For the French electronics and defence contractor, see Thales Group Thales Group Thales (in Greek: Θαλής) of Miletus (ca. 635 BC-543 BC), also known as Thales the Milesian, was a pre-Socratic Greek philosopher and one of the Seven Sages of Greece. Many regard him as the first philosopher in the Greek tradition as well as the father of science.

Life

Thales lived in the city of Miletus, in Ionia, now western Turkey. According to Herodotus, he was of Phoenician descent. It was said that Thales had no children but adopted his nephew as his son. The well-traveled Ionians had many dealings with Egypt and Babylon, and Thales may have studied in Egypt as a young man. In any event, Thales almost certainly had exposure to Egyptian mythology, astronomy, and mathematics, as well as to other traditions alien to the Homeric traditions of Greece. Perhaps because of this his inquiries into the nature of things took him beyond traditional mythology. Several anecdotes suggest that Thales was not solely a thinker; he was involved in business and politics. One story recounts that he bought all the olive presses in Miletus after predicting the weather and a good harvest for a particular year. Another version of this same story states that he bought the presses to demonstrate to his fellow Milesians that he could use his intelligence to enrich himself. However, looking at his way of thinking, getting rich was not his intent; merely to show people that by being a philosopher it was easy to enrich himself without it being the point of the exercise. Herodotus recorded that Thales advised the city-states of Ionia to form a federation. Thales is said to have died in his seat, while watching an athletic contest.

Theories and influence

Before Thales, the Greeks explained the origin and nature of the world through myths of anthropomorphic gods and heroes. Phenomena like lightning or earthquakes were attributed to actions of the gods. By contrast, Thales attempted to find naturalistic explanations of the world, without reference to the supernatural. He explained earthquakes by imagining that the Earth floats on water, and that earthquakes occur when the Earth is rocked by waves. Herodotus cites him as having predicted the solar eclipse of 585 BC that put an end to fighting between the Lydians and the Medes. Thales' most famous belief was his cosmological doctrine, which held that the world originated from water. Aristotle considered this belief roughly equivalent to the later ideas of Anaximenes, who held that everything in the world was composed of air. Thus it is sometimes assumed that Thales considered everything to be made from water. According to Lloyd, however, it is likely that while Thales saw water as an origin, he never pondered whether water continued to be the substance of the world. Thales had a profound influence on other Greek thinkers and therefore on Western history. Some believe Anaximander was a pupil of Thales. Early sources report that one of Anaximander's more famous pupils, Pythagoras, visited Thales as a young man, and that Thales advised him to travel to Egypt to further his philosophical and mathematical studies. Many philosophers followed Thales' lead in searching for explanations in nature rather than in the supernatural; others returned to supernatural explanations, but couched them in the language of philosophy rather than myth or religion. When you specifically look at the influence Thales had in the pre-Socrates era, he was one of the first thinkers who thought more in the way of logos than mythos. The difference between these two more profound ways of seeing the world is that mythos is concentrated around the stories of holy origin, while logos is concentrated around the argumentation. When the mythical man wants to explain the world the way he sees it, he explains it based on gods and powers. The mythical thought does not differ between things and persons and furthermore it does not differ between nature and culture. The way a logos thinker would present the view on the world is radically different than the mythical thinker. In its concrete form, logos is a way of thinking not only about individualism, but also the abstract. Furthermore, it focuses on sensible and continuous argumentation. This lays the foundation of philosophy and it's way of explaining the world in terms of abstract argumentation, and not in the way of gods and mythical stories. Thales is credited with first popularizing geometry in ancient Greek culture, mainly that of spatial relationships. He is the first one who separated trigonometry as an independent group from Mathematics, to be one of the four basic "elements" of geometry. The other three elements of geometry are about long, square and cube of an object.

Sources

Most of our sources for information on the Miletian philosophers (Thales, Anaximander, and Anaximenes) are the works of much later writers. The primary source for Thales' philosophy is Aristotle, who credited him with the first inquiry into the causes of things. Thales may or may not have written books. It is certain, however, that Aristotle did not have access to any work of Thales, and was writing from secondary sources of his own. While Thales' historical importance is unquestioned, this introduces a good deal of uncertainty into our understanding of him.

Interpretations

Nietzsche, in his Philosophy in the Tragic Age of the Greeks, § 3, wrote: "Greek philosophy seems to begin with an absurd notion, with the proposition that water is the primal origin and the womb of all things. Is it really necessary for us to take serious notice of this proposition? It is, and for three reasons. First, because it tells us something about the primal origin of all things; second, because it does so in language devoid of image or fable, and finally, because contained in it, if only embryonically, is the thought, 'all things are one.'"

Trivia

- In the A&E television rendition of Nero Wolfe, one of the antagonists, a mathematician, uses the name "Milton Thales" as a pseudonym, a reference to Thales of Miletus.

References

G.E.R. Lloyd, Early Greek Science: Thales to Aristotle

External links

- [http://www.iep.utm.edu/t/thales.htm Thales of Miletus from The Internet Encyclopedia of Philosophy]
- [http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Thales.html Thales of Miletus] from the MacTutor History of Mathematics archive
- [http://www.livius.org Livius], [http://www.livius.org/th/thales/thales.html Thales of Miletus] by Jona Lendering Category:635 BC births Category:543 BC deaths Category:Ancient Greek mathematicians Category:Ancient philosophers Category:Presocratic philosophers ko:탈레스 ja:タレス

Electricity

Electricity is a general term applied to phenomena involving a fundamental property of matter called an electric charge. This article will introduce and explain some of the basic principles of electricity.

Related concepts

being radiated as light as the air of Earth's atmosphere is shifted from gas to plasma and back. ]] In casual usage, the term electricity is applied to several related concepts that are better identified by more precise terms.
- Electric charge: a fundamental conserved property of some subatomic particles, which determines their electromagnetic interactions. Electrically charged matter is influenced by, and produces, electromagnetic fields.
- Electric field is an effect produced by an electric charge that exerts a force on charged objects in its vicinity.
- Electric potential the potential energy per unit charge associated with a static (time-invariant) electric field.
- Electric current: a movement or flow of electrically charged particles.
- Electrical energy: energy made available by the flow of electric charge through a conductor or from the forces between charged particles.
- Electric power: The rate at which electric energy is converted into another form, such as light, heat, or mechanical energy (or converted from another form into electric energy).

History

Ancient

According to Thales of Miletus, writing circa 600 BCE, a form of electricity was known to the Ancient Greeks who found that rubbing fur on various substances, such as amber, would cause a particular attraction between the two. The Greeks noted that the amber buttons could attract light objects such as hair and that if they rubbed the amber for long enough they could even get a spark to jump. The origin of the word "electricity" is from the Greek word ēlektron, a word the ancient Greeks used for both "amber" and "electrum," and derives from an old root, ēlek- = "shine." The same word was used for both amber and electrum, probably because of the pale yellow color of some varieties of electrum (see electrum). An object found in Iraq in 1938, dated to about 250 BCE and called the Baghdad Battery, resembles a galvanic cell and is believed by some to have been used for electroplating. Additionally, some egyptologists associate the ancient goddess Hathor with artificial light (see Hathor temple). But, remaining unproven are the conjectures that these and other similar ancient artifacts had electrical function and that their associated ancient technology contributed to the development of modern electrical knowledge.

Modern

In 1600 the English scientist William Gilbert returned to the subject in De Magnete, and coined the modern Latin word electricus from ηλεκτρον (elektron), the Greek word for "amber", which soon gave rise to the English words electric and electricity. He was followed in 1660 by Otto von Guericke, who is regarded as having invented an early electrostatic generator. Other European pioneers were Robert Boyle, who in 1675 stated that electric attraction and repulsion can act across a vacuum; Stephen Gray, who in 1729 classified materials as conductors and insulators; and C. F. Du Fay, who first identified the two types of electricity that would later be called positive and negative. The Leyden jar, a type of capacitor for electrical energy in large quantities, was invented at Leiden University by Pieter van Musschenbroek in 1745. William Watson, experimenting with the Leyden jar, discovered in 1747 that a discharge of static electricity was equivalent to an electric current. In June, 1752, Benjamin Franklin promoted his investigations of electricity and theories through the famous, though extremely dangerous, experiment of flying a kite during a thunderstorm. Following these experiments he invented a lightning rod and established the link between lightning and electricity. If Franklin did fly a kite in a storm, he did not do it the way it is often described (as it would have been dramatic but fatal). It was either Franklin (more frequently) or Ebenezer Kinnersley of Philadelphia (less frequently) who created the convention of positive and negative electricity. Franklin's observations aided later scientists such as Michael Faraday, Luigi Galvani, Alessandro Volta, André-Marie Ampère, and Georg Simon Ohm whose work provided the basis for modern electrical technology. The work of Faraday, Volta, Ampere, and Ohm is honored by society, in that fundamental units of electrical measurement are named after them. Volta worked with chemicals and discovered that chemical reactions could be used to create positively charged anodes and negatively charged cathodes. When a conductor was attached between these, the difference in the electrical potential (also known as voltage) drives a current between them through the conductor. The potential difference between two points is measured in units of volts in recognition of Volta's work. The invention of the electric telegraph showed that commercial and practical use could be made of electrical phenomena. By the end of the 19th century electrical engineering became a distinct profession, separate from the physicist or inventor. The late 19th and early 20th century produced such giants of electrical engineering as Nikola Tesla, inventor of the polyphase induction motor; Samuel Morse, inventor of the telegraph; Antonio Meucci, an inventor of the telephone; Thomas Edison inventor of the phonograph and a practical incandescent light bulb; George Westinghouse, inventor of the electric locomotive; Charles Steinmetz, theoretician of alternating current; Alexander Graham Bell, another inventor of the telephone and founder of a sucessful telephone business. The rapid advance of electrical technology in the latter 19th and early 20th centuries lead to commercial rivalry such as the so-called War of the Currents), between Edison's direct-current system or Westinghouse's alternating-current method. Often concurrent research in widely scattered locations lead to multiple claims to the invention of a device or system.

Electric charge

Electric charge is a property of certain subatomic particles (e.g., electrons and protons) which interacts with electromagnetic fields and causes attractive and repulsive forces between them. Electric charge gives rise to one of the four fundamental forces of nature, and is a conserved property of matter that can be quantified. In this sense, the phrase "quantity of electricity" is used interchangeably with the phrases "charge of electricity" and "quantity of charge." There are two types of charge: we call one kind of charge positive and the other negative. Through experimentation, we find that like-charged objects repel and opposite-charged objects attract one another. The magnitude of the force of attraction or repulsion is given by Coulomb's law.

Electric field

The concept of electric field was introduced by Michael Faraday. The electrical field force acts between two charges, in the same way that the gravitational field force acts between two masses. However, electric field is a little bit different. Gravitational force depends on mass, whereas electric force depends on the electric charge on both objects. A positive charge exerts away from the object and a negative charge pulls towards the object equally in all directions; thus it is symetric. The most common experience with electric charge in everyday life is that of static cling - when two particular types of materials are rubbed together, they tend to stick together, at least for a while.

Electric potential

The electric potential difference between two points is defined as the work done per unit charge (against electrical forces) in moving a positive point charge slowly between two points. If one of the points is taken to be a reference point with zero potential, then the electric potential at any point can be defined in terms of the work done per unit charge in moving a positive point charge from that reference point to the point at which the potential is to be determined. For isolated charges, the reference point is usually taken to be infinity. The potential is measured in volts. (1 volt = 1 joule/coulomb) The electric potential is analogous to temperature: there is a different temperature at every point in space, and the temperature gradients indicates the direction of heat flows. Similarly, there is an electric potential at every point in space, and its gradient in the the electric field indicates where charges move.

Electric current

The electric charge which occurs naturally within conductors can be forced to flow, while the charges within insulators are locked in place and cannot be moved. Devices that use charge flow principles in materials are called electronic devices. A flow of electric charge is called an electric current. A direct current (DC) is a unidirectional flow; alternating current (AC) is a flow whose time average is zero, but whose energy capability (RMS level) is not zero. With AC the electric current repeatedly changes direction. Electric current is measured in Amperes Ohm's Law is an important relationship describing the behaviour of electric currents: See also: electrical conduction For historical reasons, electric current is said to flow from the most positive part of a circuit to the most negative part. The electric current thus defined is called conventional current. It is now known that, depending on the conditions, an electric current can consist of a flow of charged particles in either direction, or even in both directions at once. The positive-to-negative convention is widely used to simplify this situation. If another definition is used - for example, "electron current" - it should be explicitly stated.

Electrical energy

Electrical energy, is the flow of electrons or ions. When electrons are flowing through a wire or through hundreds of feet of air in the case of lightning it is because they are being forced to do so by an electrical field. A force is exerted on the electrons and they move. Work is done on the charged particles. A force is pushing them through a distance. More properly, they are moving from outer orbitals of one atom to another, being pushed by the electromotive force. While the electrons are in motion they contain kinetic energy. Consquently, atomic level electricity is a form of kinetic energy.

Electric power

Electric power is the capacity of the circuit for performing work in a particular amount of time. When a charge moves in a conductor, work is done by that charge. Devices can be made which convert this work into heat (Electric arc furnaces), light (light bulbs and Fluorescent lamps), or motion, i.e. kinetic energy (electric motors). The unit for all forms of power is the watt (symbol: W). In practice, however, this is generally reserved for the real power component. Apparent power is conventionally expressed in volt-amperes (VA) since it is the simple multiple of rms voltage and current. The unit for reactive power is given the special name "VAR", which stands for volt-amperes-reactive.

SI electricity units

External links

- [http://amasci.com/miscon/whatis.html What is electricity?]
- [http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=electricity Merriam-Webster: Electricity]
- [http://www.bibliomania.com/2/9/72/119/21387/1.html Tyndall: Faraday as Discovery: Identity of Electricities]
- [http://www.eia.doe.gov/fuelelectric.html US Energy Department Statistics]
- [http://www.mouthshut.com/readreview/38842-1.html How to save on your electricity bills]
- [http://users.pandora.be/worldstandards/electricity.htm Electricity around the world]
- [http://www.tufts.edu/as/wright_center/fellows/bob_morse_04/ A Comprehensive Collection of Franklin’s Electrical Works: The Electrical Writings of Benjamin Franklin], Created and Collected by Robert A. Morse (2004)
- [http://www.telesensoryview.com/steverosecom/Articles/UnderstandingBasicElectri.html Understanding Electricity and some Electronics in 10 minutes](Steve Rose, Maui)
- [http://amasci.com/miscon/eleca.html Electricity Misconceptions]
- ko:전기 ja:電気 simple:Electricity

Silk

:For other uses of the word, see Silk (disambiguation). Silk (disambiguation) Silk is a natural protein fiber that can be woven into textiles. It is obtained from the cocoon of the silkworm larva, in the process known as sericulture, which kills the larvae. The shimmering appearance for which it is prized comes from the fibres' triangular prism-like structure, which allows silk cloth to refract incoming light at different angles.

Early history

Silk was first developed in early China, possibly as early as 6000 BC and definitely by 3000 BC. Legend gives credit to a Chinese Empress Xi Ling-Shi. Though first reserved for the Emperors of China, its use spread gradually through Chinese culture both geographically and socially. From there, silken garments began to reach regions throughout Asia. Silk rapidly became a popular luxury fabric in the many areas accessible to Chinese merchants, because of its texture and lustre. Because of the high demand for the fabric, silk was one of the staples of international trade prior to industrialization.

Silk trade

Perhaps the first evidence of the silk trade is that of an Egyptian mummy of 1070 BC. In subsequent centuries, the silk trade reached as far as the Indian subcontinent, the Middle East, Europe, and North Africa. This trade was so extensive that the major set of trade routes between Europe and Asia has become known as the Silk Road.

Secret

The Emperors of China strove to keep the knowledge of sericulture secret from other nations, in order to maintain the Chinese monopoly on its production. This effort at secrecy had mixed success. Sericulture reached Korea around 200 BC with Chinese settlers, about the first half of the 1st century AD in Khotan, and by 300 AD the practice had been established in India. Although the Roman Empire knew of and traded in silk, the secret was only to reach Europe around AD 550, via the Byzantine Empire. Legend has it that the monks working for the emperor Justinian were the first to bring silkworm eggs to Constantinople in hollow canes. The Byzantines were equally secretive, and for many centuries the weaving and trading of silk fabric was a strict imperial monopoly; all top-quality looms and weavers were located inside the Palace complex in Constantinople and the cloth produced was used in imperial robes or in diplomacy, as gifts to foreign dignitaries. The remainder was sold at exorbitant prices.

Wild Silks

"Wild silks" are produced by a number of undomesticated silkworms. Aside from differences in colours and textures, they all differ in one major respect from the domesticated varieties. The cocoons, which are gathered in the wild, have usually already been chewed through by the pupa or caterpillar ("silkworm") before the cocoons are gathered and thus the single thread which makes up the cocoon has been cut into shorter lengths. A variety of wild silks have been known and used in China, India and Europe from early times, although the scale of production has always been far smaller than that of cultivated silks. Wild silks are produced by caterpillars other than the mulberry silkworm (Bombyx mori). The term "wild" implies that these silkworms are not capable of being domesticated and artificially cultivated like the mulberry worms. Commercially reared silkworms are killed before the pupae emerge by dipping them in boiling water or they are killed with a needle, thus allowing the whole cocoon to be unravelled as one continuous thread. This allows a much stronger cloth to be woven from the silk. Wild silks also tend to be more difficult to dye than silk from the cultivated silkworm. There is ample evidence that small quantities of wild silk were already being produced in the Mediterranean and Middle East by the time the superior, and stronger, cultivated silk from China began to be imported. The beautiful and expensive golden-coloured "wild" silk called "Muga" is produced only in the Brahmaputra Valley — mainly Assam and adjoining parts of Burma. This silk has always been highly prized — not only for its beautiful natural golden sheen, which actually improves with aging and washing — but for the fact that it is the strongest natural fibre known. Garments made of it outlast those made of ordinary silk — commonly lasting fifty years or more. In addition, it absorbs moisture better than ordinary silk and is, therefore, more comfortable to wear. Nowadays, it is mainly sought after for the highest-quality saris given as presents to brides in India.

Europe

Venetian merchants traded extensively in silk and encouraged silk growers to settle in Italy. By the 13th century Italian silk was a significant source of trade. Italian silk was so popular in Europe that Francis I of France invited Italian silkmakers to France to create a French silk industry, especially in Lyon. The French Revolution interrupted production before Napoleon took power.

America

James I of England introduced silk growing to the American colonies around 1619, ostensibly to discourage tobacco planting. Only the Shakers in Kentucky adopted the practice. In the 1800s a new attempt at a silk industry began with European-born workers in Paterson, New Jersey, and the city became a US silk centre, although Japanese imports were still more important.

World War

World War II interrupted the silk trade from Japan. Silk prices increased dramatically and US industry begun to look for substitutes, which led to the use of synthetics like nylon. Synthetic silks have also been made from lyocell, a type of cellulose fibre, and are often difficult to distinguish from real silk.

Islam

In Islamic law, there is a prohibition upon Muslim men from wearing silk (as well as gold). While the command is given in the Quran without justification, many jurists believe the reasoning behind the prohibition lies in avoiding clothing for men that can be feminine or extravagant and luxurious. http://www.islamonline.net/fatwa/english/FatwaDisplay.asp?hFatwaID=61261

Animal rights

Silk has recently come under fire from animal rights activists who maintain that the common practice of boiling silkworms alive in their cocoons is cruel.

Other uses

In addition to clothing manufacture and other handicrafts, silk is also used for items like parachutes, bicycle tires, comforter filling and artillery gunpowder bags. Early bulletproof vests were also made from silk in the era of blackpowder weapons until roughly World War I. Silk undergoes a special manufacturing process to make it adequate for its use in surgery as non-absorbable sutures. Chinese doctors have also used it to make prosthetic arteries. Silk cloth is also used as a material to write on.

Stazione Bacologica Sperimentale

Stazione Bacologica Sperimentale is an Institute for Silkmoth Research in Italy. The oldest centre for such studies it was founded in 1871.

References

- Good, Irene. 1995. “On the question of silk in pre-Han Eurasia” Antiquity Vol. 69, Number 266, December 1995, pp. 959-968
- Hill, John E. 2004. The Western Regions according to the Hou Hanshu. Draft annotated English translation.[http://depts.washington.edu/uwch/silkroad/texts/hhshu/hou_han_shu.html]
- Hill, John E. 2004. The Peoples of the West from the Weilue 魏略 by Yu Huan 魚豢: A Third Century Chinese Account Composed between 239 and 265 CE. Draft annotated English translation. Appendix E. [http://depts.washington.edu/uwch/silkroad/texts/weilue/weilue.html]
- Kuhn, Dieter. 1995. “Silk Weaving in Ancient China: From Geometric Figures to Patterns of Pictorial Likeness.” Science 12 (1995): pp. 77-114.
- Liu, Xinru. 1996. Silk and Religion: An Exploration of Material Life and the Thought of People, AD 600-1200. Oxford University Press.
- Sung, Ying-Hsing. 1637. Chinese Technology in the Seventeenth Century - T'ien-kung K'ai-wu. Translated and annotated by E-tu Zen Sun and Shiou-chuan Sun. Pennsylvania State University Press, 1966. Reprint: Dover, 1997. Chap. 2. Clothing materials.

External links

- Silkworm rearing (with photo journals): http://www.wormspit.com
- [http://www.isracast.com/tech_news/271204_tech.htm Scientists have successfully created artificial silk using genetically engineered spider proteins] Category:Textiles Category:Fibers Category:Silk ja:絹 simple:Silk

Fur

:For alternative meanings, see Fur (disambiguation). Fur (disambiguation) The term fur refers to the body hair of non-human mammals also known as the pelage (like the term plumage in birds). Fur comes from the coats of animals; the animal's coat may consist of short ground hair, long guard hair, and, in some cases, medium awn hair. Not all mammals have fur; animals without fur may be referred to as "naked", as in The Naked Ape and naked mole rat. Fur usually consists of two main layers:
- Ground hair or underfur — the bottom layer consisting of wool hairs when tend to be shorter, flattened, curly and denser than the top layer.
- Guard hair — the top layer consisting of longer straight shafts of hair that stick out through the underfur. This is usually what's visible in most mammals and contains most of the pigmentation. Despite the best efforts of the animal protest industry to shroud the wearing of animal fur in controversy using hyperbole, graphic images and celebrities eager to heighten their own profiles and that of their products fur continues to be seen as a natural and relatively environmentally friendly fashion commodity.

Fur clothing

Fur has served as an important source of clothing for humans, especially in colder climates since time immemorial. Modern cultures continue to wear fur and fur trim as dictated by fashion trends and it is still considered by many as a luxury item, despite the best efforts of the animal lobby's campaigns which peaked in the early eighties with the participation of numerous celebrities, many of whom have since been seen in fur clothing. Animal furs used in garments and trim may be dyed bright colors or to mimic exotic animal patterns, or shorn down to imitate the feel of a soft velvet fabric. The term "a fur" is often used to refer to a fur coat, wrap, or shawl. Common animal sources for fur clothing and fur trimmed accessories include:
- fox
- rabbit
- mink
- beaver
- ermine
- otter
- sable
- seal
- Import and sale of seal products is currently banned in the US.
- cat
- Import, export and sales banned in the US in 2000 (Dog and Cat Protection Act of 2000 [http://www.cbp.gov/xp/cgov/import/commercial_enforcement/prohibition_products_with_dog_cat_fur.xml])
- dog
- Import, export and sales banned in the US in 2000 (Dog and Cat Protection Act of 2000 [http://www.cbp.gov/xp/cgov/import/commercial_enforcement/prohibition_products_with_dog_cat_fur.xml])
- coyote
- chinchilla The manufacturing of fur clothing involves obtaining animal pelts where the hair is left on the animal's processed skin. In contrast, leather made from sheepskin or other animal hides involves removing the fur from the skin and using only the skin. The use of wool involves shearing the animal's hair from the living animal, so that the wool can be regrown. Fake fur or "faux fur" designates any synthetic material that mimics the appearance and feel of real fur, without the use of animal products. The chemical treatment of fur to increase its felting quality is known as carroting, as the process tends to turn the tips of the fur a yellowish-red "carrot like" colour. Today, the animal protest industry continues to expend considerable energy and resources to create controvery over the methods of fur farming and trapping used to produce fur clothing but record prices for fur pelts and the ubiquity of fur clothing and trim in the fashion media demonstrates fur's tenaciously duarble appeal. To date the widespread use of dehaired animal skins in use such as leather or even sheepskins has curiously failed to attract the same investment of resources and time from the animal lobby and their fund raising campaigns.

Fur fetishes

The soft, warm texture of fur appeals to many people; for some, the attraction becomes a fur fetishism, a fetishistic attraction to people wearing fur, or in certain cases, to the fur garments themselves. Category:Mammals ja:毛皮

Adhesion

]] Adhesion is the molecular attraction exerted between bodies in contact. It is of particular interest to engineers who wish to stick things together and to biologists to understand the workings of cells.

Mechanisms of Adhesion

Five mechanisms have been proposed to explain why one material sticks to another:

Mechanical Adhesion

Two materials may be mechanically interlocked. Sewing forms a large scale mechanical bond, velcro forms one on a medium scale, and some textile adhesives form one at a small scale.

Chemical Adhesion

Two materials may form a compound at the join. The strongest joins are where atoms of the two materials swap (ionic bonding) or share (covalent bonding) outer electrons. A weaker bond is formed if oxygen, nitrogen or fluorine atoms of the two materials share a hydrogen nucleus (hydrogen bonding).

Dispersive Adhesion

Also known as Adsorption. Two materials may be held together by van der Waals forces. A van der Waals force is the attraction between two molecules that have positively and negatively charged ends. This positive and negative polarity may be a permanent property of a molecule (Keesom forces) or universally occurs in molecules, as the random movement of electrons within the molecules may result in a temporary concentration of electrons at one end (London forces).

Electrostatic Adhesion

Some conducting materials may pass electrons to form a difference in electrical charge at the join. This results in a structure similar to a capacitor and creates an attractive electrostatic force between the materials. The electrons are passed if one conducting material binds its electrons less strongly than the other does.

Diffusive Adhesion

Some materials may merge at the joint by diffusion. This may occur when the molecules of both materials are mobile and soluble in each other. This would be particularly effective with polymer chains where one end of the molecule diffuses into the other material. It is also the mechanism involved in sintering. When metal or ceramic powders are pressed together and heated, atoms diffuse from one particle to the next. This joins the particles into one. The driving force for this diffusion is typically the reduction in surface energy, though it could also be a reduction in the chemical potential.

What Makes an Adhesive Bond Strong?

The strength of the adhesion between two materials depends on which of the above mechanisms occur between the two materials, and the surface area over which the two materials contact. Materials that wet against each other tend to have a larger contact area than those that don't. Wetting depends on the surface energy of the materials.

References

- John Comyn, Adhesion Science, Royal Society of Chemistry Paperbacks, 1997
- A.J. Kinloch, Adhesion and Adhesives: Science and Technology, Chapman and Hall, 1987 Category:Materials science Category:Chemical properties

Quantum tunneling

Quantum tunneling is the quantum-mechanical effect of transitioning through a classically-forbidden energy state. It can be generalized to other types of classically-forbidden transitions as well. Consider rolling a basketball up a hill. If the ball is not given enough push, then the ball will not make it to the other side of the hill. In this case the ball does not have enough energy to roll over the hill. But in quantum mechanics, objects do not behave like classical objects, such as balls, do. In quantum mechanics objects frequently exhibit wavelike behavior and can be localized into a "particle" via measurement or interaction with the environment. The implication is that in the analogous quantum situation of a quantum particle moving against a potential hill, some of the wave can extend all the way through to the other side of the potential hill. Having some of the wave on the other side of the hill means that the quantum particle can be localized to the other side of the hill. This type of transition is not analagous to classical motion; it is called tunneling as if the particle were digging through the potential hill. As this is a quantum and non-classical effect, it can generally only be seen in microscopic phenomena where the wave nature of particles is more pronounced.

History

In the early 1900's radioactive materials were known to have characteristic exponential decay rates or half lives and the radiation emissions were known to have certain characteristic energies. By 1928 George Gamow solved the theory of the alpha-decay of a nucleus via tunneling. Classically, the particle is confined to the nucleus because of a very strong potential. Classically, it takes an enormous amount of energy to pull apart the nucleus. In Quantum Mechanics, however, there is a probability the particle can tunnel through the potential and escape. Gamow solved a model potential for the nucleus and derived a relationship between the half life of the particle and the energy of the emission. Alpha-decay via tunneling was also solved concurrently by Ronald Gurney and Edward Condon. Shortly thereafter both groups had also considered that particles could also tunnel into the nucleus. After attending a seminar by Gamow, Max Born recognized the generality of quantum mechanical tunneling. He realised that tunneling phenomena was not restricted to nuclear physics, but was a general result of Quantum Mechanics that applies to many different systems. Today the theory of tunneling is even applied to the early cosmology of the universe. Quantum tunneling was later applied to other situations such as the cold emission of electrons, and perhaps most importantly semiconductor and superconductor physics. In 1970s, in the works of R.R. Dogonadze, M.V. Volkenshtein, Z.D. Urushadze and others were formulated the first Quantum-mechanical (physical) Model of Enzyme Catalysis. These works supported a theory that enzyme catalysis use quantum-mechanical effect such as tunneling.

Semiclassical calculation

Let us consider the time-independent Schrödinger equation for one particle, in one dimension, under the influence of a hill potential

V(x)

. :

-\frac \frac \Psi(x) + V(x) \Psi(x) = E \Psi(x)

\frac \Psi(x) = \frac \left( V(x) - E \right) \Psi(x)

Now let us recast the wave function

\Psi(x)

as the exponential of a function. :

\Psi(x) = e^

\Phi

(x) + \Phi'(x)^2 = \frac \left( V(x) - E \right) Now let us separate $\Phi'(x)$ into real and imaginary parts. : $\Phi'(x) = A(x) + \imath B(x)$ : $A'(x) + A(x)^2 - B(x)^2 = \frac \left( V(x) - E \right)$ : $B'(x) - 2 A(x) B(x) = 0$ Next we want to take the semiclassical approximation to solve this. That means we expand each function as a power series in $\hbar$ . From the equations we can already see that the power series must start with at least an order of $\hbar^$ to satisfy the real part of the equation. But as we want a good classical limit, we also want to start with as high a power of plank's constant as possible. : $A(x) = \frac \sum_^\infty \hbar^i A_i(x)$ : $B(x) = \frac \sum_^\infty \hbar^i B_i(x)$ The constraints on the lowest order terms are as follows. : $A_0(x)^2 - B_0(x)^2 = 2m \left( V(x) - E \right)$ : $A_0(x) B_0(x) = 0$ If the amplitude varries slowly as compared to the phase, we set $A_0(x) = 0$ and get : $B_0(x) = \pm \sqrt$ Which is obviously only valid when you have more energy than potential - classical motion. After the same proceedure on the next order of the expansion we get : $\Psi(x) \approx C \frac$ On the other hand, if the phase varries slowly as compared to the amplitude, we set $B_0(x) = 0$ and get : $A_0(x) = \pm \sqrt$ Which is obviously only valid when you have more potential than energy - tunneling motion. Grinding out the next order of the expansion yields : $\Psi(x) \approx \frac$ It is apparent from the denominator, that both these approximate solutions are bad near the classical turning point $E = V(x)$ . What we have are the approximate solutions away from the potential hill and beneath the potential hill. Away from the potential hill, the particle acts similarly to a free wave - the phase is oscillating. Beneath the potential hill, the particle undergoes exponential changes in amplitude. In a specific tunneling problem, we might already suspect that the transition amplitude be proportional to $e^$ and thus the tunneling be exponentially dampened by large deviations from classically permitable motion. But to be complete we must find the approximate solutions everywhere and match coefficients to make a global approximate solution. We have yet to approximate the solution near the classical turning points $E=V(x)$ . Let us label a classical turning point $x_1$ . Now because we are near $E=V(x_1)$ , we can easily expand $\frac\left(V(x)-E\right)$ in a power series. : $\frac\left(V(x)-E\right) = U_1 (x - x_1) + U_2 (x - x_1)^2 + \cdots$ Let us only approximate to linear order $\frac\left(V(x)-E\right) = U_1 (x - x_1)$ : $\frac \Psi(x) = U_1 (x - x_1) \Psi(x)$ This differential equation looks deceptively simple. It takes some trickery to transform this into a Bessel equation. The solution is as follows. : $\Psi(x) = \sqrt \left( C_ J_\left(\frac\sqrt(x - x_1)^\right) + C_ J_\left(\frac\sqrt(x - x_1)^\right) \right)$ Hopefully this solution should connect the far away and beneath solutions. Given the 2 coefficients on one side of the classical turning point, we should be able to determine the 2 coefficients on the other side of the classical turning point by using this local solution to connect them. We should be able to find a relationship between $C,\theta$ and $C_,C_$ . Fortunately the Bessel function solutions will asymptote into sine, cosine and exponential functions in the proper limits. The relationship can be found as follows. : $C_ = \frac C \cos$ : $C_ = - C \sin$ Now we can easily construct global solutions and solve tunneling problems. The transmission coefficient, $\left| \frac \right|^2$ , for a particle tunneling through a single potential barrier is found to be : $T = \frac$ Where $x_1,x_2$ are the 2 classical turning points for the potential barrier. If we take the classical limit of all other physical parameters much larger than plank's constant, abbreviated as $\hbar \rightarrow 0$ , we see that the transmission coefficient correctly goes to zero. This classical limit would have failed in the unphysical, but much simpler to solve, situation of a square potential.
Tunneling trivia
One of the major applications is in electron-tunneling microscopes (see scanning tunneling microscope) used to see objects that are too small to see using conventional microscopes. Electron tunneling microscopes overcome the limiting effects of conventional microscopes (optical aberrations, wavelength limitations) by scanning the surface of an object with tunneling electrons. Tunneling is a source of major current leakage in Very-large-scale integration (VLSI) electronics. This results in the substantial power drain and heating effects that plague high-speed and mobile technology. See also: Tunnel diode
References

-
-
-
- R.R. Dogonadze and Z.D. Urushadze, Semi-classical Method of Calculation of Rates of Chemical Reactions Proceeding in Polar Liquids.- J.Electroanal.Chem., 32, 1971, pp. 235-245
- M.V. Volkenshtein, R.R. Dogonadze, A.K. Madumarov, Z.D. Urushadze and Yu.I. Kharkats, Theory of Enzyme Catalysis.-J. Molekuliarnaya Biologia, Moscow, 6, 1972, pp. 431-439 (in Russian, English summary)
External link

- http://www.physicspost.com/imageview.php?imageId=184
- [http://phys.educ.ksu.edu/vqm/html/qtunneling.html Quantum Tunneling Simulation at Visual Quantum Mechanics]
- [http://www.geocities.com/euscigeo/TheoryEnzymeCatalysisURUSHADZE.doc "About the History of the Quantum-Mechanical Theory of Enzyme Catalysis" by Prof. Dr. Zurab D. Urushadze.- Issued by the Georgian National Section of EUROSCIENCE, 2005] Category:Quantum mechanics ja:トンネル効果

Corona discharge

In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor, which occurs when the potential gradient exceeds a certain value, in situations where sparking (also known as arcing) is not favoured.

Introduction

A corona is a process by which a current, perhaps sustained, develops from an electrode with a high potential gradient in a neutral fluid, usually air, by ionising that fluid so as to create a plasma around the electrode. The ions generated eventually pass the charge to a lower potential (usually the reference point of the generator which is normally earth) When the potential gradient is large enough at a point in the fluid, the fluid at that point ionizes and it becomes conductive. If a charged object has a sharp point, the air around that point will be at a higher gradient than elsewhere, and can become conductive while other points in the air do not. When the air becomes conductive, it effectively increases the size of the conductor. If the new conductive region is less sharp, the ionization may not extend past this local region. Outside of this region of ionization and conductivity, the charged particles slowly find their way to an oppositely charged object and are neutralized. If the geometry and gradient are such that the ionized region continues to grow instead of stopping at a certain radius, a completely conductive path is formed, and a momentary (or continuous) spark (or arc) occurs. Corona discharge usually involves two asymmetric electrodes, one highly curved (such as the tip of a needle, or a narrow wire) and one of low curvature (such as a plate, or the ground). The high curvature ensures a high potential gradient around one electrode, for the generation of a plasma. Coronas may be positive, or negative. This is determined by the polarity of the voltage on the highly-curved electrode. If the curved electrode is positive with respect to the flat electrode we say we have a positive corona, if negative we say we have a negative corona. (See below for more details.) The physics of positive and negative coronas are strikingly different. This asymmetry is a result of the great difference in mass between electrons and positively charged ions, with only the electron having the ability to undergo a significant degree of ionising inelastic collision at common temperatures and pressures. An important reason for considering coronas is the production of ozone around conductors undergoing corona processes. A negative corona generates much more ozone than the corresponding positive corona.

Applications of corona discharge

Corona discharge has a number of commercial and industrial applications.
- Manufacture of ozone
- Scrubbing particles from air in air-conditioning systems
- Removal of unwanted volatile organics, such as chemical pesticides, solvents, chemical weapons agents, from the atmosphere
- Surface Treatment of polymer films to improve compatibility with adhesives or printing inks.
- Photocopying
- Air ionisers perhaps benefiting health
- Kirlian photography is believed, by some, to be of use in visualising auras.
- EHD thrusters, Lifters, and ionic wind
- Nitrogen laser Coronas can be used to generate charged surfaces, which is an effect used in electrostatic copying (photocopying). They can also be used to remove particulate matter from air streams by first charging the air, and then passing the charged stream through a comb of alternating polarity, to deposit the charged particles on the oppositely charged plates. The free-radicals and ions generated in corona reactions can be used to scrub the air of certain noxious products, through chemical reactions, and can be used to produce ozone.

Problems caused by corona discharges

Coronas can generate audible and radio-frequency noise, particularly in electric power transmission lines. They also represent a power loss and can indicate equipment degradation. Their action on atmospheric particulates, and their ozone and NOx production can also be disadvantageous to human health where power lines run through built-up areas. Therefore, power transmission equipment is designed to minimise the formation of corona discharge. Corona discharge is generally undesirable in:
- Electric power transmission, owing to loss of power in corona processes, and noise
- Inside electrical components such as transformers, capacitors, electric motors and generators. Corona progressively damages the insulation inside these devices, leading to premature failure.
- Situations where high voltages are in use, but ozone production is to be minimised

Mechanism of corona discharge

Corona discharge of both the positive and negative variety have certain mechanisms in common. # A neutral atom in the medium, in a region of strong field (high potential gradient, near the curved electrode) is ionized by an exogenous environmental event (for example, as the result of a photon interaction), to create a positive ion and an electron. # The strong field then operates on these charged particles, separating them, and preventing their recombination, and also accelerating them, imparting each of them with kinetic energy. # As a result of the energisation of the electrons (which have a much higher charge/mass ratio and so are accelerated more acutely), further electron/positive-ion pairs are created by collision with neutral atoms. These then undergo the same separating process creating an electron avalanche. # In processes which differ between positive and negative coronas, the energy of these plasma processes is converted into further initial electron dissociations to seed further avalanches. # An ion species created in this series of avalanches (which differs between positive and negative coronas) is attracted to the uncurved electrode, completing the circuit, and sustaining the current flow.

Electron avalanches

Both positive and negative coronas rely on a process known as the electron avalanche. A corona begins with a rare natural 'background' ionisation event of a neutral air molecule, perhaps as the result of photo-excitation or background radiation. This ionisation creates a positive ion, and a free electron. If this event occurs in an area with a high potential gradient, the positive ion will be strongly attracted toward, or repelled away from, the curved electrode (depending on the polarity of the corona), whereas the electron will be attracted in the opposite direction. This will, occasionally, prevent the recombination of electron and positive ion. These high-energy electrons, accelerated by the field, (whichever their direction of travel) often collide with neutral atoms inelastically, potentially ionizing those atoms. In a chain-reaction — or 'electron avalanche' — those additional electrons are also separated from their positive ions by the strong potential gradient, causing a large cloud of electrons and positive ions to be momentarily generated by just a single initial event. A number of mechanisms can sustain this process, creating avalanche after avalanche, to create a constant corona current. A secondary source of corona electrons is required as the electrons are always accelerated by the field in one direction, meaning that avalanches always proceed linearly toward or away from an electrode. The dominant mechanism for the creation of secondary electrons depends on the polarity of the corona. In each case, the energy emitted as photons by the initial avalanche is used to ionise a molecule creating another accelerable electron. What differs is the source of this electron.

Electrical properties

The current carried by the corona is determined by integrating the current density over the surface of the conductor. The power loss is determined by multiplying the current and the voltage.

Positive coronas

Properties

A positive corona is manifested as a uniform plasma across the length of a conductor. It can often be seen glowing blue/white, though much of the emissions are in the ultraviolet. The uniformity of the plasma owes itself to the homogeneous source of secondary avalanche electrons described in the mechanism section, below. With the same geometry and voltages, it appears a little smaller than the corresponding negative corona, owing to the lack of a non-ionising plasma region between the inner and outer regions. There are many fewer free electrons in a positive corona, when compared to a negative corona, except very close to the curved electrode: perhaps a thousandth of the electron density, and a hundredth of the total number of electrons. However, the electrons in a positive corona are concentrated close to the surface of the curved conductor, in a region of high-potential gradient (and therefore the electrons have a high energy), whereas in a negative corona many of the electrons are in the outer, lower-field areas. Therefore, if electrons are to be used in an application which requires a high activation energy, positive coronas may support a greater reaction constants than corresponding negative coronas; though the number of electrons may be lower, the number of a very high energy may be higher. Coronas are efficient producers of ozone in air. A positive corona generates much less ozone than the corresponding negative corona, as the reactions which produce ozone are relatively low-energy. Therefore, the greater number of electrons of a negative corona leads to an increased production. Beyond the plasma, in the unipolar region, the flow is of low-energy positive ions toward the flat electrode.

Mechanism

As with a negative corona, a positive corona is initiated by an exogenous ionisation event in a region of high potential gradient. The electrons resulting from the ionisation are attracted toward the curved electrode, and the positive ions repelled from it. By undergoing inelastic collisions closer and closer to the curved electrode, further molecules are ionized in an electron avalanche. In a positive corona, secondary electrons, for further avalanches, are generated predominantly in the fluid itself, in the region outside the plasma or avalanche region. They are created by ionization caused by the photons emitted from that plasma in the various de-excitation processes occurring within the plasma after electron collisions, the thermal energy liberated in those collisions creating photons which are radiated into the gas. The electrons resulting from the ionisation of a neutral gas molecule are then electrically attracted back toward the curved electrode, attracted into the plasma, and so begins the process of creating further avalanches inside the plasma. As can be seen, the positive corona is divided into two regions, concentric around the sharp electrode. The inner region contains ionising electrons, and positive ions, acting as a plasma, the electrons avalanche in this region, creating many further ion/electron pairs. The outer region consists almost entirely of the slowly migrating massive positive ions, moving toward the uncurved electrode along with, close to the interface of this region, secondary electrons, liberated by photons leaving the plasma, being re-accelerated into the plasma. The inner region is known as the plasma region, the outer as the unipolar region.

Negative coronas

Properties

A negative corona is manifested in a non-uniform corona, varying according to the surface features and irregularities of the curved conductor. It often appears as tufts of corona at sharp edges, the number of tufts altering with the strength of the field. The form of negative coronas is a result of its source of secondary avalanche electrons (see below). It appears a little larger than the corresponding positive corona, as electrons are allowed to drift out of the ionising region, and so the plasma continues some distance beyond it. The total number of electrons, and electron density is much greater than in the corresponding positive corona. However, they are of a predominantly lower energy, owing to being in a region of lower potential-gradient. Therefore, whilst for many reactions the increased electron density will increase the reaction rate, the lower energy of the electrons will mean that reactions which require a higher electron energy may take place at a lower rate.

Mechanism

Negative coronas are more complex than positive coronas in construction. As with positive coronas, the establishing of a corona begins with an exogenous ionisation event generating a primary electron, followed by an electron avalanche. Electrons ionised from the neutral gas are not useful in sustaining the negative corona process by generating secondary electrons for further avalanches, as the general movement of electrons in a negative corona is outward from the curved electrode. For negative corona, instead, the dominant process generating secondary electrons is the photoelectric effect, from the surface of the electrode itself. The work-function of the electrons (the energy required to liberate the electrons from the surface) is considerably lower than the ionisation energy of air at standard temperatures and pressures, making it a more liberal source of secondary electrons under these conditions. Again, the source of energy for the electron-liberation is a high-energy photon from an atom within the plasma body relaxing after excitation from an earlier collision. The use of ionised neutral gas as a source of ionisation is further diminished in a negative corona by the high-concentration of positive ions clustering around the curved electrode. Under other conditions, the collision of the positive species with the curved electrode can also cause electron liberation. The difference, then, between positive and negative coronas, in the matter of the generation of secondary electron avalanches, is that in a positive corona they are generated by the gas surrounding the plasma region, the new secondary electrons travelling inward, whereas in a negative corona they are generated by the curved electrode itself, the new secondary electrons travelling outward. A further feature of the structure of negative coronas is that as the electrons drift outwards, they encounter neutral molecules and, with electronegative molecules (such as Oxygen and Water vapour), combine to produce negative ions. These negative ions are then attracted to the positive uncurved electrode, completing the 'circuit'. A negative corona can be divided into three radial areas, around the sharp electrode. In the inner area, high-energy electrons inelastically collide with neutral atoms and cause avalanches, whilst outer electrons (usually of a lower energy) combine with neutral atoms to produce negative ions. In the intermediate region, electrons combine to form negative ions, but typically have insufficient energy to cause avalanche ionisation, but remain part of a plasma owing to the different polarities of the species present, and the ability to partake in characteristic plasma reactions. In the outer region, only a flow of negative ions and, to a lesser and radially-decreasing extent, free electrons toward the positive electrode takes place. The inner two regions are known as the corona plasma. The inner region is an ionising plasma, the middle a non-ionising plasma. The outer region is known as the unipolar region. Negative coronas can only be sustained in fluids with electronegative molecules, to capture free electrons. Without the electronegative molecules capturing the free electrons, a simple path of electron flow of ionised gas exists between the two electrodes and an arc, or sp

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