A thermostat is a component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint. Thermostats are used in any device or system that heats or cools to a setpoint temperature, examples include building heating, central heating, air conditioners, HVAC systems, water heaters, as well as kitchen equipment including ovens and refrigerators and medical and scientific incubators. In scientific literature, these devices are broadly classified as thermostatically controlled loads. Thermostatically controlled loads comprise 50% of the overall electricity demand in the United States. A thermostat operates as a "closed loop" control device, as it seeks to reduce the error between the desired and measured temperatures. Sometimes a thermostat combines both the sensing and control action elements of a controlled system, such as in an automotive thermostat; the word thermostat is derived from the Greek words θερμός thermos, "hot" and στατός statos, "standing, stationary".
A thermostat exerts control by switching heating or cooling devices on or off, or by regulating the flow of a heat transfer fluid as needed, to maintain the correct temperature. A thermostat can be the main control unit for a heating or cooling system, in applications ranging from ambient air control, to such as automotive coolant control. Thermostats are used in any device or system that heats or cools to a setpoint temperature, examples include building heating, central heating, air conditioners, as well as kitchen equipment including ovens and refrigerators and medical and scientific incubators. Thermostats use different types of sensors to measure the temperature. In one form, the mechanical thermostat, a bimetallic strip in the form of a coil directly operates electrical contacts that control the heating or cooling source. Electronic thermostats, use a thermistor or other semiconductor sensor that requires amplification and processing to control the heating or cooling equipment. A thermostat is an example of a "bang-bang controller" as the heating or cooling equipment output is not proportional to the difference between actual temperature and the temperature setpoint.
Instead, the heating or cooling equipment runs at full capacity until the set temperature is reached shuts off. Increasing the difference between the thermostat setting and the desired temperature therefore does not change the time to achieve the desired temperature; the rate at which the target system temperature can change is determined both by the capacity of the heating or cooling equipment to add or remove heat to or from a target system and the capacity of the target system to store heat. To prevent excessively rapid cycling of the equipment when the temperature is near the setpoint, a thermostat can include some hysteresis. Instead of changing from "on" to "off" and vice versa at the set temperature, a thermostat with hysteresis will not switch until the temperature has changed a little past the set temperature point. For example, a refrigerator set to 2°C might not start the cooling compressor until its food compartment's temperature reaches 3°C, will keep it running until the temperature has been lowered to 1 °C.
This reduces the risk of equipment wear from too frequent switching, although it introduces a target system temperature oscillation of a certain magnitude. To improve the comfort of the occupants of heated or air-conditioned spaces, bimetal sensor thermostats can include an "anticipator" system to warm the temperature sensor while the heating equipment is operating, or to warm the sensor when the cooling system is not operating; when adjusted this reduces any excessive hysteresis in the system and reduces the magnitude of temperature variations. Electronic thermostats have an electronic equivalent. Early technologies included mercury thermometers with electrodes inserted directly through the glass, so that when a certain temperature was reached the contacts would be closed by the mercury; these were accurate to within a degree of temperature. Common sensor technologies in use today include: Bimetallic electrical sensors. Expanding wax pellets Electronic thermistors and semiconductor devices Electrical thermocouplesThese may control the heating or cooling apparatus using: Direct mechanical control Electrical signals Pneumatic signals Possibly the earliest recorded examples of thermostat control were built by the Dutch innovator Cornelis Drebbel around 1620 in England.
He invented a mercury thermostat to regulate the temperature of a chicken incubator. This is one of the first recorded feedback-controlled devices. Modern thermostat control was developed in the 1830s by Andrew Ure, a Scottish chemist, who invented the bi-metallic thermostat; the textile mills of the time needed a constant and steady temperature to operate optimally, so to achieve this Ure designed the bimetallic thermostat, which would bend as one of the metals expanded in response to the increased temperature and cut off the energy supply. Warren S. Johnson of Wisconsin patented a bi-metal room thermostat in 1883, two years filed a patent for the first multi-zone thermostatic control system. Albert Butz invented the electric thermostat and patented it in 1886. One of the first industrial uses of the thermostat was in the regulation of the temperature in poultry incubators. Charles Hearson, a British engineer, designed the first modern incubator for eggs, taken up for use on poultry farms in 1879.
The incubators incorporated an accurate thermostat to regulate the temperature so as to simulate the experience of an egg being hatched naturally. This covers only devices which both sense and control using pu
Steam is water in the gas phase, formed when water boils or evaporates. Steam is invisible. At lower pressures, such as in the upper atmosphere or at the top of high mountains, water boils at a lower temperature than the nominal 100 °C at standard pressure. If heated further it becomes superheated steam; the enthalpy of vaporization is the energy required to turn water into the gaseous form when it increases in volume by 1,700 times at standard temperature and pressure. Piston type steam engines played a central role to the Industrial Revolution and modern steam turbines are used to generate more than 80% of the world's electricity. If liquid water comes in contact with a hot surface or depressurizes below its vapor pressure, it can create a steam explosion. Steam is traditionally created by heating a boiler via burning coal and other fuels, but it is possible to create steam with solar energy. Water vapor that includes water droplets is described as wet steam; as wet steam is heated further, the droplets evaporate, at a high enough temperature all of the water evaporates and the system is in vapor–liquid equilibrium.
Superheated steam is steam at a temperature higher than its boiling point for the pressure, which only occurs where all liquid water has evaporated or has been removed from the system. Steam tables contain thermodynamic data for water/steam and are used by engineers and scientists in design and operation of equipment where thermodynamic cycles involving steam are used. Additionally, thermodynamic phase diagrams for water/steam, such as a temperature-entropy diagram or a Mollier diagram shown in this article, may be useful. Steam charts are used for analysing thermodynamic cycles. In agriculture, steam is used for soil sterilization to avoid the use of harmful chemical agents and increase soil health. Steam's capacity to transfer heat is used in the home: for cooking vegetables, steam cleaning of fabric and flooring, for heating buildings. In each case, water is heated in a boiler, the steam carries the energy to a target object. Steam is used in ironing clothes to add enough humidity with the heat to take wrinkles out and put intentional creases into the clothing.
As of 2000 around 90% of all electricity was generated using steam as the working fluid, nearly all by steam turbines. In electric generation, steam is condensed at the end of its expansion cycle, returned to the boiler for re-use. However, in cogeneration, steam is piped into buildings through a district heating system to provide heat energy after its use in the electric generation cycle; the world's biggest steam generation system is the New York City steam system, which pumps steam into 100,000 buildings in Manhattan from seven cogeneration plants. In other industrial applications steam is used for energy storage, introduced and extracted by heat transfer through pipes. Steam is a capacious reservoir for thermal energy because of water's high heat of vaporization. Fireless steam locomotives were steam locomotives that operated from a supply of steam stored on board in a large tank resembling a conventional locomotive's boiler; this tank was filled by process steam, as is available in many sorts of large factory, such as paper mills.
The locomotive's propulsion used connecting rods, as for a typical steam locomotive. These locomotives were used in places where there was a risk of fire from a boiler's firebox, but were used in factories that had a plentiful supply of steam to spare. Owing to its low molecular mass, steam is an effective lifting gas, providing 60% as much lift as helium and twice as much as hot air, it is not flammable, unlike hydrogen, is cheap and abundant, unlike helium. The required heat, leads to condensation problems and requires an insulated envelope; these factors have limited its use thus far to demonstration projects. Steam engines and steam turbines use the expansion of steam to drive a piston or turbine to perform mechanical work; the ability to return condensed steam as water-liquid to the boiler at high pressure with little expenditure of pumping power is important. Condensation of steam to water occurs at the low-pressure end of a steam turbine, since this maximizes the energy efficiency, but such wet-steam conditions must be limited to avoid excessive turbine blade erosion.
Engineers use an idealised thermodynamic cycle, the Rankine cycle, to model the behavior of steam engines. Steam turbines are used in the production of electricity. An autoclave, which uses steam under pressure, is used in microbiology laboratories and similar environments for sterilization. Steam dry steam, may be used for antimicrobial cleaning to the levels of sterilization. Steam is a non-toxic antimicrobial agent. Steam is used in piping for utility lines, it is used in jacketing and tracing of piping to maintain the uniform temperature in pipelines and vessels. Steam is used in the process of wood killing insects and increasing plasticity. Steam is used to accentuate drying in prefabricates. Care should be taken since concrete produces heat during hydration and additional heat from the steam could be detrimental to hardening reaction processes of the concrete. Used in cleaning of fibers and other materials, sometimes in preparation for painting. Steam is useful in melting hardened grease and oil resid
Wool is the textile fiber obtained from sheep and other animals, including cashmere and mohair from goats, qiviut from muskoxen, from hide and fur clothing from bison, angora from rabbits, other types of wool from camelids. Wool consists of protein together with a few percent lipids. In this regard it is chemically quite distinct from the more dominant textile, cellulose. Wool is produced by follicles; these follicles are located in the upper layer of the skin called the epidermis and push down into the second skin layer called the dermis as the wool fibers grow. Follicles can be classed as either secondary follicles. Primary follicles produce three types of fiber: kemp, medullated fibers, true wool fibers. Secondary follicles only produce true wool fibers. Medullated fibers share nearly identical characteristics to hair and are long but lack crimp and elasticity. Kemp fibers are coarse and shed out. Wool's scaling and crimp make it easier to spin the fleece by helping the individual fibers attach to each other, so they stay together.
Because of the crimp, wool fabrics have greater bulk than other textiles, they hold air, which causes the fabric to retain heat. Wool has a high specific thermal resistance, so it impedes heat transfer in general; this effect has benefited desert peoples, as Tuaregs use wool clothes for insulation. Felting of wool occurs upon hammering or other mechanical agitation as the microscopic barbs on the surface of wool fibers hook together. Wool has several qualities that distinguish it from hair/fur: it is crimped and elastic; the amount of crimp corresponds to the fineness of the wool fibers. A fine wool like Merino may have up to 100 crimps per inch, while coarser wool like karakul may have as few as one or two. In contrast, hair has little if any scale and no crimp, 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, including the famous tweed cloth of Scotland.
Wool fibers absorb moisture, but are not hollow. Wool can absorb one-third of its own weight in water. Wool absorbs sound like many other fabrics, it is a creamy white color, although some breeds of sheep produce natural colors, such as black, brown and random mixes. Wool ignites at a higher temperature than some synthetic fibers, it has a lower rate of flame spread, a lower rate of heat release, a lower heat of combustion, does not melt or drip. Wool carpets are specified for high safety environments, such as trains and aircraft. Wool is specified for garments for firefighters and others in occupations where they are exposed to the likelihood of fire. Wool causes an allergic reaction in some people. Sheep shearing is the process. After shearing, the wool is separated into four main categories: fleece, broken and locks; the quality of fleeces is determined by a technique known as wool classing, whereby a qualified person, called a wool classer, groups wools of similar gradings together to maximize the return for the farmer or sheep owner.
In Australia before being auctioned, all Merino fleece wool is objectively measured for micron, staple length, staple strength, sometimes color and comfort factor. Wool straight off a sheep, known as "greasy wool" or "wool in the grease", contains a high level of valuable lanolin, as well as the sheep's dead skin and sweat residue, also contains pesticides and vegetable matter from the animal's environment. Before the wool can be used for commercial purposes, it must be scoured, a process of cleaning the greasy wool. Scouring may be as simple as a bath in warm water or as complicated as an industrial process using detergent and alkali in specialized equipment. In north west England, special potash pits were constructed to produce potash used in the manufacture of a soft soap for scouring locally produced white wool. In commercial wool, vegetable matter is removed by chemical carbonization. In less-processed wools, vegetable matter may be removed by hand and some of the lanolin left intact through the use of gentler detergents.
This semigrease wool can be worked into yarn and knitted into water-resistant mittens or sweaters, such as those of the Aran Island fishermen. Lanolin removed from wool is used in cosmetic products, such as hand creams. Raw wool has many impurities; the sheep's body yields many types of wool with differing strengths, length of staple and impurities. The raw wool is processed into'top'.'Worsted top' requires strong straight and parallel fibres. The quality of wool is determined by its fiber diameter, yield and staple strength. Fiber diameter is the single most important wool characteristic determining price. Merino wool is 3–5 inches in length and is fine; the finest and most valuable wool comes from Merino hoggets. Wool taken from sheep produced for meat is more coarse, has fibers 1.5 to 6 in in length. Damage or breaks in the wool can occur if the sheep is stressed whil
A shield is a piece of personal armour held in the hand or mounted on the wrist or forearm. Shields are used to intercept specific attacks, whether from close-ranged weaponry or projectiles such as arrows, by means of active blocks, as well as to provide passive protection by closing one or more lines of engagement during combat. Shields vary in size and shape, ranging from large panels that protect the user's whole body to small models that were intended for hand-to-hand-combat use. Shields vary a great deal in thickness. Shields vary in shape, ranging in roundness to angularity, proportional length and width and edge pattern. In prehistory and during the era of the earliest civilisations, shields were made of wood, animal hide, woven reeds or wicker. In classical antiquity, the Barbarian Invasions and the Middle Ages, they were constructed of poplar tree, lime or another split-resistant timber, covered in some instances with a material such as leather or rawhide and reinforced with a metal boss, rim or banding.
They were carried by foot soldiers and cavalry. Depending on time and place, shields could be round, square, triangular, bilabial or scalloped. Sometimes they took on the form of kites or flatirons, or had rounded tops on a rectangular base with an eye-hole, to look through when used with combat; the shield was held by straps that went over or around the user's arm. Shields were decorated with a painted pattern or an animal representation to show their army or clan; these designs developed into systematized heraldic devices during the High Middle Ages for purposes of battlefield identification. After the introduction of gunpowder and firearms to the battlefield, shields continued to be used by certain groups. In the 18th century, for example, Scottish Highland fighters liked to wield small shields known as targes, as late as the 19th century, some non-industrialized peoples employed them when waging war. In the 20th and 21st century, shields have been used by military and police units that specialize in anti-terrorist actions, hostage rescue, riot control and siege-breaking.
The modern term refers to a device, held in the hand or attached to the arm, as opposed to an armored suit or a bullet-proof vest. Shields are sometimes mounted on vehicle-mounted weapons to protect the operator; the oldest form of shield was a protection device designed to block attacks by hand weapons, such as swords and maces, or ranged weapons like sling-stones and arrows. Shields have varied in construction over time and place. Sometimes shields were made of metal. Many surviving examples of metal shields are felt to be ceremonial rather than practical, for example the Yetholm-type shields of the Bronze Age, or the Iron Age Battersea shield. Size and weight varied greatly. Armored warriors relying on speed and surprise would carry light shields that were either small or thin. Heavy troops might be equipped with robust shields. Many had a strap called a guige that allowed them to be slung over the user's back when not in use or on horseback. During the 14th–13th century BC, the Sards or Shardana, working as mercenaries for the Egyptian pharaoh Ramses II, utilized either large or small round shields against the Hittites.
The Mycenaean Greeks used two types of shields: the "figure-of-eight" shield and a rectangular "tower" shield. These shields were made from a wicker frame and reinforced with leather. Covering the body from head to foot, the figure-of-eight and tower shield offered most of the warrior's body a good deal of protection in head-to-head combat; the Ancient Greek hoplites used a round, bowl-shaped wooden shield, reinforced with bronze and called an aspis. Another name for this type of shield is a hoplon; the hoplon shield inspired the name for hoplite soldiers. The hoplon was the longest-lasting and most famous and influential of all of the ancient Greek shields; the Spartans used the aspis to create the Greek phalanx formation. Their shields offered protection not only for their comrades to their left. Examples of Germanic wooden shields circa 350 BC – 500 AD survive from weapons sacrifices in Danish bogs; the armored Roman legionaries carried large shields that could provide far more protection, but made swift movement a little more difficult.
The scutum had an oval shape, but the curved tops and sides were cut to produce the familiar rectangular shape most seen in the early Imperial legions. Famously, the Romans used their shields to create a tortoise-like formation called a testudo in which entire groups of soldiers would be enclosed in an armoured box to provide protection against missiles. Many ancient shield designs featured incuts of another; this was done to accommodate the shaft of a spear, thus facilitating tactics requiring the soldiers to stand close together forming a wall of shields. Typical in the early European Middle Ages were round shields with light, non-splitting wood like linden, alder or poplar reinforced with leather cover on one or both sides and metal rims, encircling a metal s
The Philippines the Republic of the Philippines, is an archipelagic country in Southeast Asia. Situated in the western Pacific Ocean, it consists of about 7,641 islands that are categorized broadly under three main geographical divisions from north to south: Luzon and Mindanao; the capital city of the Philippines is Manila and the most populous city is Quezon City, both part of Metro Manila. Bounded by the South China Sea on the west, the Philippine Sea on the east and the Celebes Sea on the southwest, the Philippines shares maritime borders with Taiwan to the north, Vietnam to the west, Palau to the east, Malaysia and Indonesia to the south; the Philippines' location on the Pacific Ring of Fire and close to the equator makes the Philippines prone to earthquakes and typhoons, but endows it with abundant natural resources and some of the world's greatest biodiversity. The Philippines has an area of 300,000 km2, according to the Philippines Statistical Authority and the WorldBank and, as of 2015, had a population of at least 100 million.
As of January 2018, it is the eighth-most populated country in Asia and the 12th most populated country in the world. 10 million additional Filipinos lived overseas, comprising one of the world's largest diasporas. Multiple ethnicities and cultures are found throughout the islands. In prehistoric times, Negritos were some of the archipelago's earliest inhabitants, they were followed by successive waves of Austronesian peoples. Exchanges with Malay, Indian and Chinese nations occurred. Various competing maritime states were established under the rule of datus, rajahs and lakans; the arrival of Ferdinand Magellan, a Portuguese explorer leading a fleet for the Spanish, in Homonhon, Eastern Samar in 1521 marked the beginning of Hispanic colonization. In 1543, Spanish explorer Ruy López de Villalobos named the archipelago Las Islas Filipinas in honor of Philip II of Spain. With the arrival of Miguel López de Legazpi from Mexico City, in 1565, the first Hispanic settlement in the archipelago was established.
The Philippines became part of the Spanish Empire for more than 300 years. This resulted in Catholicism becoming the dominant religion. During this time, Manila became the western hub of the trans-Pacific trade connecting Asia with Acapulco in the Americas using Manila galleons; as the 19th century gave way to the 20th, the Philippine Revolution followed, which spawned the short-lived First Philippine Republic, followed by the bloody Philippine–American War. The war, as well as the ensuing cholera epidemic, resulted in the deaths of thousands of combatants as well as tens of thousands of civilians. Aside from the period of Japanese occupation, the United States retained sovereignty over the islands until after World War II, when the Philippines was recognized as an independent nation. Since the unitary sovereign state has had a tumultuous experience with democracy, which included the overthrow of a dictatorship by a non-violent revolution; the Philippines is a founding member of the United Nations, World Trade Organization, Association of Southeast Asian Nations, the Asia-Pacific Economic Cooperation forum, the East Asia Summit.
It hosts the headquarters of the Asian Development Bank. The Philippines is considered to be an emerging market and a newly industrialized country, which has an economy transitioning from being based on agriculture to one based more on services and manufacturing. Along with East Timor, the Philippines is one of Southeast Asia's predominantly Christian nations; the Philippines was named in honor of King Philip II of Spain. Spanish explorer Ruy López de Villalobos, during his expedition in 1542, named the islands of Leyte and Samar Felipinas after the then-Prince of Asturias; the name Las Islas Filipinas would be used to cover all the islands of the archipelago. Before that became commonplace, other names such as Islas del Poniente and Magellan's name for the islands San Lázaro were used by the Spanish to refer to the islands; the official name of the Philippines has changed several times in the course of its history. During the Philippine Revolution, the Malolos Congress proclaimed the establishment of the República Filipina or the Philippine Republic.
From the period of the Spanish–American War and the Philippine–American War until the Commonwealth period, American colonial authorities referred to the country as the Philippine Islands, a translation of the Spanish name. Since the end of World War II, the official name of the country has been the Republic of the Philippines. Philippines has gained currency as the common name since being the name used in Article VI of the 1898 Treaty of Paris, with or without the definite article. Discovery in 2018 of stone tools and fossils of butchered animal remains in Rizal, Kalinga has pushed back evidence of early hominins in the archipelago to as early as 709,000 years. However, the metatarsal of the Callao Man, reliably dated by uranium-series dating to 67,000 years ago remains the oldest human remnant found in the archipelago to date; this distinction belonged to the Tabon Man of Palawan, carbon-dated to around 26,500 years ago. Negritos were among the archipelago's earliest inhabitants, but their first settlement in the Philippines has not been reliably dated.
There are several opposing theories regarding the origins of ancient Filipinos. F. Landa Jocano theorizes. Wilhelm Solheim's Island Origin Theory postulates that the peopling of the archipelago transpired via trade networks originating in the Sundaland area around
China the People's Republic of China, is a country in East Asia and the world's most populous country, with a population of around 1.404 billion. Covering 9,600,000 square kilometers, it is the third- or fourth-largest country by total area. Governed by the Communist Party of China, the state exercises jurisdiction over 22 provinces, five autonomous regions, four direct-controlled municipalities, the special administrative regions of Hong Kong and Macau. China emerged as one of the world's earliest civilizations, in the fertile basin of the Yellow River in the North China Plain. For millennia, China's political system was based on hereditary monarchies, or dynasties, beginning with the semi-legendary Xia dynasty in 21st century BCE. Since China has expanded, re-unified numerous times. In the 3rd century BCE, the Qin established the first Chinese empire; the succeeding Han dynasty, which ruled from 206 BC until 220 AD, saw some of the most advanced technology at that time, including papermaking and the compass, along with agricultural and medical improvements.
The invention of gunpowder and movable type in the Tang dynasty and Northern Song completed the Four Great Inventions. Tang culture spread in Asia, as the new Silk Route brought traders to as far as Mesopotamia and Horn of Africa. Dynastic rule ended in 1912 with the Xinhai Revolution; the Chinese Civil War resulted in a division of territory in 1949, when the Communist Party of China established the People's Republic of China, a unitary one-party sovereign state on Mainland China, while the Kuomintang-led government retreated to the island of Taiwan. The political status of Taiwan remains disputed. Since the introduction of economic reforms in 1978, China's economy has been one of the world's fastest-growing with annual growth rates above 6 percent. According to the World Bank, China's GDP grew from $150 billion in 1978 to $12.24 trillion by 2017. Since 2010, China has been the world's second-largest economy by nominal GDP and since 2014, the largest economy in the world by purchasing power parity.
China is the world's largest exporter and second-largest importer of goods. China is a recognized nuclear weapons state and has the world's largest standing army and second-largest defense budget; the PRC is a permanent member of the United Nations Security Council as it replaced the ROC in 1971, as well as an active global partner of ASEAN Plus mechanism. China is a leading member of numerous formal and informal multilateral organizations, including the Shanghai Cooperation Organization, WTO, APEC, BRICS, the BCIM, the G20. In recent times, scholars have argued that it will soon be a world superpower, rivaling the United States; the word "China" has been used in English since the 16th century. It is not a word used by the Chinese themselves, it has been traced through Portuguese and Persian back to the Sanskrit word Cīna, used in ancient India."China" appears in Richard Eden's 1555 translation of the 1516 journal of the Portuguese explorer Duarte Barbosa. Barbosa's usage was derived from Persian Chīn, in turn derived from Sanskrit Cīna.
Cīna was first used including the Mahābhārata and the Laws of Manu. In 1655, Martino Martini suggested that the word China is derived from the name of the Qin dynasty. Although this derivation is still given in various sources, it is complicated by the fact that the Sanskrit word appears in pre-Qin literature; the word may have referred to a state such as Yelang. The meaning transferred to China as a whole; the origin of the Sanskrit word is still a matter of debate, according to the Oxford English Dictionary. The official name of the modern state is the "People's Republic of China"; the shorter form is "China" Zhōngguó, from zhōng and guó, a term which developed under the Western Zhou dynasty in reference to its royal demesne. It was applied to the area around Luoyi during the Eastern Zhou and to China's Central Plain before being used as an occasional synonym for the state under the Qing, it was used as a cultural concept to distinguish the Huaxia people from perceived "barbarians". The name Zhongguo is translated as "Middle Kingdom" in English.
Archaeological evidence suggests that early hominids inhabited China between 2.24 million and 250,000 years ago. The hominid fossils of Peking Man, a Homo erectus who used fire, were discovered in a cave at Zhoukoudian near Beijing; the fossilized teeth of Homo sapiens have been discovered in Fuyan Cave in Hunan. Chinese proto-writing existed in Jiahu around 7000 BCE, Damaidi around 6000 BCE, Dadiwan from 5800–5400 BCE, Banpo dating from the 5th millennium BCE; some scholars have suggested. According to Chinese tradition, the first dynasty was the Xia, which emerged around 2100 BCE; the dynasty was considered mythical by historians until scientific excavations found early Bronze Age sites at Erlitou, Henan in 1959. It remains unclear whether these sites are the remains of the Xia dynasty or of another culture from the same period; the succeeding Shang dynasty is the earliest to be confirmed by contemporary records. The Shang ruled the plain of the Yellow River in eastern China from the 17th to the 11th century BCE.
Their oracle bone script
A polymer is a large molecule, or macromolecule, composed of many repeated subunits. Due to their broad range of properties, both synthetic and natural polymers play essential and ubiquitous roles in everyday life. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function. Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers, their large molecular mass relative to small molecule compounds produces unique physical properties, including toughness, a tendency to form glasses and semicrystalline structures rather than crystals. The terms polymer and resin are synonymous with plastic; the term "polymer" derives from the Greek word πολύς and μέρος, refers to a molecule whose structure is composed of multiple repeating units, from which originates a characteristic of high relative molecular mass and attendant properties. The units composing polymers derive or conceptually, from molecules of low relative molecular mass.
The term was coined in 1833 by Jöns Jacob Berzelius, though with a definition distinct from the modern IUPAC definition. The modern concept of polymers as covalently bonded macromolecular structures was proposed in 1920 by Hermann Staudinger, who spent the next decade finding experimental evidence for this hypothesis. Polymers are studied in the fields of biophysics and macromolecular science, polymer science. Products arising from the linkage of repeating units by covalent chemical bonds have been the primary focus of polymer science. Polyisoprene of latex rubber is an example of a natural/biological polymer, the polystyrene of styrofoam is an example of a synthetic polymer. In biological contexts all biological macromolecules—i.e. Proteins, nucleic acids, polysaccharides—are purely polymeric, or are composed in large part of polymeric components—e.g. Isoprenylated/lipid-modified glycoproteins, where small lipidic molecules and oligosaccharide modifications occur on the polyamide backbone of the protein.
The simplest theoretical models for polymers are ideal chains. Polymers are of two types: occurring and synthetic or man made. Natural polymeric materials such as hemp, amber, wool and natural rubber have been used for centuries. A variety of other natural polymers exist, such as cellulose, the main constituent of wood and paper; the list of synthetic polymers in order of worldwide demand, includes polyethylene, polystyrene, polyvinyl chloride, synthetic rubber, phenol formaldehyde resin, nylon, polyacrylonitrile, PVB, many more. More than 330 million tons of these polymers are made every year. Most the continuously linked backbone of a polymer used for the preparation of plastics consists of carbon atoms. A simple example is polyethylene. Many other structures do exist. Oxygen is commonly present in polymer backbones, such as those of polyethylene glycol, DNA. Polymerization is the process of combining many small molecules known as monomers into a covalently bonded chain or network. During the polymerization process, some chemical groups may be lost from each monomer.
This happens in the polymerization of PET polyester. The monomers are terephthalic acid and ethylene glycol but the repeating unit is —OC—C6H4—COO—CH2—CH2—O—, which corresponds to the combination of the two monomers with the loss of two water molecules; the distinct piece of each monomer, incorporated into the polymer is known as a repeat unit or monomer residue. Laboratory synthetic methods are divided into two categories, step-growth polymerization and chain-growth polymerization; the essential difference between the two is that in chain growth polymerization, monomers are added to the chain one at a time only, such as in polyethylene, whereas in step-growth polymerization chains of monomers may combine with one another directly, such as in polyester. Newer methods, such as plasma polymerization do not fit neatly into either category. Synthetic polymerization reactions may be carried out without a catalyst. Laboratory synthesis of biopolymers of proteins, is an area of intensive research. There are three main classes of biopolymers: polysaccharides and polynucleotides.
In living cells, they may be synthesized by enzyme-mediated processes, such as the formation of DNA catalyzed by DNA polymerase. The synthesis of proteins involves multiple enzyme-mediated processes to transcribe genetic information from the DNA to RNA and subsequently translate that information to synthesize the specified protein from amino acids; the protein may be modified further following translation in order to provide appropriate structure and functioning. There are other biopolymers such as rubber, suberin and lignin. Occurring polymers such as cotton and rubber were familiar materials for years before synthetic polymers such as polyethene and perspex appeared on the market. Many commercially important polymers are synthesized by chemical modification of occurring polymers. Prominent examples inclu