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Physics of fire Fire is simply glowing gas and other combustible particles such as carbon. It is not plasma, as it is not hot enough to reach such high ionization as is required of plasma (an 'electrically neutral, highly ionized gas composed of ions, electrons, and neutral particles'). This state of matter can be generated through focused concentrations of energy (such as fuel being exposed to an already open flame or to the sun's rays focused through a lens), or through an exothermic chemical reaction usually accompanied by intense heat released during a rapid loss of electrons from the combustible material (striking a match). Fire may be visible as a brilliant glow and/or flames and may produce smoke. Fires start when a flammable or combustible material with an adequate supply of oxygen or another oxidizer is subjected to enough heat. The common fire-causing sources of heat include a spark, another fire (such as an explosion, a fire in the oven or fireplace, or a lit match, lighter or cigarette) and sources of intense thermal radiation (such as sunlight, a flue, an incandescent light bulb or a radiant heater). Mechanical and electrical machinery may cause fire if combustible materials used on or located near the equipment are exposed to intense heat from Joule heating, friction or exhaust gas. Fires can sustain themselves by the further release of heat energy in the process of combustion and may propagate, provided there is a continuous supply of oxygen and fuel. Fires may become uncontrolled and cause great damage to and destruction of human life, animals, plants and property. Fire is extinguished when any of the elements of the so-called fire triangle—heat, oxygen, or fuel— are removed. The unburnable solid remains of a combustible material left after a fire are called ash. For more detailed information on the color of flames, see flame. A flame is an exothermic, self-sustaining, oxidizing chemical reaction producing energy and glowing gas, of which a very small portion is plasma. It consists of reacting gases and solids emitting visible and infrared light, the frequency spectrum of which is dependent on the chemical composition of the burning elements and intermediate reaction products. In many cases such as burning organic matter like wood or incomplete combustion of gas, incandescent solid particles, soot produces the familiar red-orange 'fire' color light. This light has a continuous spectrum. Complete combustion of gas has a dim blue color due to the emission of single wavelength radiations from various electron transitions in the excited molecules formed in the flame. Usually oxygen is involved, but hydrogen burning in chlorine produces a flame as well, producing the toxic acid hydrogen chloride (HCl). Other possible combinations producing flames, amongst many more, are fluorine and hydrogen, or hydrazine and nitrogen tetroxide. Recent discoveries by the National Aeronautics and Space Administration (NASA) of the United States also has found that gravity plays a role. Modifying the gravity causes different flame types. The glow of a flame is somewhat complex. Black-body radiation is emitted from soot, gas, and fuel particles, though the soot particles are too small to behave like perfect blackbodies. There is also photon emission by de-excited atoms and molecules in the gases. Much of the radiation is emitted in the visible and infrared bands. The color depends on temperature for the black-body radiation, and chemical makeup for the emission spectra. The dominant color in a flame changes with temperature. The photo of the forest fire is an excellent example of this variation. Near the ground, where most burning is occurring, it is white, the hottest color possible for organic material in general, or yellow. Above the yellow region, the color changes to orange, which is somewhat cooler, then red, which is cooler still. Above the red region, combustion no longer occurs, and the uncombusted carbon particles are visible as black smoke. The common distribution of a flame under normal gravity conditions depends on convection, as soot tends to rise to the top of a general flame, such as in a candle in normal gravity conditions, making it yellow. In microgravity or zero gravity, such as an environment in outer space, convection no longer occurs, and the flame becomes spherical, with a tendency to become more blue and more efficient (although they will go out if not moved steadily as the CO2 from combustion does not disperse in microgravity, and tends to smother the flame). There are several possible explanations for this difference, of which the most likely is that the temperature is evenly distributed enough that soot is not formed and complete combustion occurs. Experiments by NASA in microgravity reveal that diffusion flames in microgravity allow more soot to be completely oxidised after they are produced than diffusion flames on Earth, because of a series of mechanisms that behaved differently in microgravity when compared to normal gravity conditions. These discoveries have potential applications in applied science and industry, especially concerning fuel efficiency. Fire ecology is the study of the interaction of living things with fire. Controlling fire Controlling fire for the purposes of providing heat and light was one of mankind's first great achievements. The ability of fire making to generate heat and light made possible migration to colder climates and enabled people to cook food — a decisive step in the endless fight against disease. Smoke signals were an early use of fire for communication, and fire soon enabled advancements in metallurgy such as smelting and forging. Archaeology indicates that ancestors of modern humans such as Homo erectus seem to have been using controlled fire as early as some 790,000 years ago. The Cradle of Humankind site has evidence for controlled fire 1 million years ago. By the time of the Neolithic Revolution, during the introduction of grain based agriculture, people the world over used fire as a tool in landscape management. These fires were typically controlled burns or "cool fires", as opposed to uncontrolled "hot fires" that damage the soil. Such hot fires destroy plants and animals, and endanger communities. This is especially a problem in the forests of today where traditional burning is prevented in order to encourage the growth of timber crops. Cool fires are generally conducted in the spring and fall. They clear undergrowth, burning up biomass that could trigger a hot fire should it get too dense. They provide a greater variety of environments, which encourages game and plant diversity. For humans, they make dense, impassable forests traversable. The modern applications of fire are numerous. In its broadest sense, fire is used by nearly every human being on earth in a controlled setting every day. Owners of internal combustion vehicles use fire every time they drive. Thermal power stations provide electricity for a large percentage of humanity. However, fire is also used more directly; many nomadic peoples still use fire for cooking. It is also used for smoking, and as a weapon. In fact, the use of fire for warfare has a long history up to the present day. Hunter-gatherer groups around the world have been noted as using grass and forest fires to injure their enemies and destroy their ability to find food, so it can be assumed that fire has been used in warfare for as long as humans have existed. Homer detailed its use by Greek commandoes who hid in a wooden horse to burn Troy during the Trojan war. Later the Byzantine fleet used Greek fire to attack ships and men. In the Vietnam War, the Americans dropped a modern version, napalm, from the air. More recently many villages were burned during the Rwandan Genocide. Aerial bombing of cities, including firebombing, using incendiary bombs was also frequently used during World War II. Molotov cocktails are cheap to construct and are in common use as well. Typical temperatures of fires and flames Fire and religion See also fire worship. Fires and burning have often been used in religious rites and symbolism. Fire is one of the four classical elements, as well as one of the five Chinese elements. In Hinduism fire is one of five sacred elements of which all living creatures are comprised and is considered an eternal witness essential to sacred religious ceremonies. Members of the Prausian Tribe of Central Europe long used fire to burn wicker men with the living people from enemy tribes inside as a means of sacrifice to their gods. Fire is a symbol of Ahura Mazda, the god of the Zoroastrian religion. A Zoroastrian church is known as a Fire Temple. Fire is also an important part of Calcination, the fire operation in the art of alchemy. In Roman mythology, Vulcan is the god of fire. The analogue in Greek mythology is Hephaestus. In Greek mythology, Prometheus is the Titan chiefly honored for stealing fire from the gods in the stalk of a fennel plant and giving it to mortals for their use. In Judaism fire also has great significance. Candles are lit to usher in holidays and to separate Shabbat from the rest of the week, as well as to remember the dead. Another important fire symbol is the Eternal Flame, which was a fire kept in the First and Second Temples and will always be kept burning. In Christianity, fire is a symbol of the Holy Ghost. It is also often used in descriptions of Hell. Additionally, a fire is used in the Roman Catholic Mass during the Easter Vigil. In the Middle ages, the judgment of God was appealed to by the ordeal of fire. In Spain, bonfires as associated to the eve of Saint John the Evangelist, a Christianization of the summer solstice, especially in Alicante. In Valencia, Saint Joseph is celebrated by burning allegorical figures on the streets, a Christianization of the spring equinox. Fire was also a purifier. Giovanni da Pian del Carpine narrates that when he visited the Mongol Batu Khan, he was made to pass between two fires to remove possible witchcraft or poisons. Fire is sometimes associated with Halloween. Fire as a power source
Uncontrolled fire
Conflagrations See for more examples. Timeline: Fire protection and prevention
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