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For other uses see fire
(disambiguation).
Small open fire
Fire is a rapid, self-sustaining oxidation process of
combustible gases ejected from a fuel. It starts by
subjecting the fuel to heat or another energy source, e.g. a match or lighter, and is sustained by the further release of heat
energy.
Controlling fire was one of humankind's first great achievements. It made possible
migration to colder climes which otherwise would have remained out of reach for colonization. It also allowed for cooking food and using flame and
heat to process materials. Archeology 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.
Fires and burning have often been used in religious sacrifices, as the smoke of the fire disperses into the heavens. Fire is one of the four classical elements, as well as one of the five Chinese elements. In Christianity, fire is a
symbol of the Holy Spirit, and is
also often used in descriptions of Hell.
The burning of wood is often the first association to the word fire, and
trees have since ancient times supplied much of the energy needed by humans. In the past,
metal smelting and charcoal production
consumed large quantities of wood for their production. Nowadays, large scale energy is usually not produced by fires of burning
wood, but has been replaced by hydrocarbon oil and coal, and in some cases nuclear energy or renewable energy sources.
Wood burning remains a heat source in many third world countries and where
other sources of energy are unavailable.
The glow of a flame is somewhat complex, due to a mix of black-body radiation emitted from soot, gas, and fuel particles (though the soot particles are too
small to behave like perfect blackbodies), and from 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 Fire Tetrahedron
There are four elements that maintain the combustion process, and the absence of any one of them will prevent a fire.
- The reducing agent (fuel) may be removed from the site of a fire to curb its spread. In forestry, controlled burns are used to keep the available fuel supply low, so that intense fires do not
occur. Sometimes you can stop the flow of a liquid or gas fuel. In the case of a burning pipeline, sometimes the flow of fuel can
simply be turned off.
- An oxidizer (usually oxygen) is needed to react with the fuel. Sand, foam, or gases which do not support combustion (such as carbon dioxide) may be used to stop the flow of oxygen to a fire (smother the flames). In particularly
violent fires, such as those of the Kuwaiti oil wells during the Gulf War, explosions may be used instead.
- Heat is what allows liquid fuels to be vaporized, and solid fuels to undergo pyrolysis. Solids and liquids do not burn directly, they must first be converted into a gas through pyrolysis or vaporization. Removal
of enough heat prevents fuels from burning. Water is particularly effective at removing
heat due to its high specific heat capacity.
- The chemical chain reaction is what perpetuates combustion; compounds such as halon extinguishing agents cause the chain reaction to be broken. The precise mechanism is not known, but it is
thought that the halogen radicals end the reactions that support combustion.
If an uncontrolled fire is underway, the removal of these elements is the job of firefighters. Fire safety engineers provide fire departments and
building designers with technical information about the best ways to remove enough of these four elements from modern buildings
and industries in order to prevent fires.
See also
External links
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