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Aqua regia

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Freshly prepared aqua regia is colorless, but it turns orange within seconds. Here, fresh aqua regia has been added to these NMR tubes to remove all traces of organic material.

Freshly prepared aqua regia to remove metal salt deposits.

Aqua regia (Latin for royal water) is a highly corrosive, fuming yellow or red solution. The mixture is formed by freshly mixing concentrated nitric acid and concentrated hydrochloric acid, usually in a volumetric ratio of 1:3 respectively. It is one of the few reagents that dissolves gold and platinum. It was so named because it can dissolve the so-called royal, or noble metals, although tantalum, iridium, and a few other metals are able to withstand it.

Contents [hide] 1 Applications 2 Chemistry 2.1 Dissolving gold 2.2 Dissolving platinum 2.3 Decomposition of aqua regia 3 History 4 In art and entertainment 4.1 Literature 5 See also 6 References 7 External links

[edit] Applications

Aqua regia is used in etching and in certain analytic procedures. It is also used in some laboratories to clean glassware of organic compounds and metal particles.

This method is preferred over the “traditional” chromic acid bath for cleaning NMR tubes, because no traces of paramagnetic chromium can remain to later ruin acquired spectra.^[1] Furthermore, chromic acid baths are discouraged because of the high toxicity of chromium and the potential for explosions. Aqua regia is itself very corrosive and has been implicated in several explosions as well due to mishandling and it should not be used unless gentler cleaning techniques such as the use of brushes, sonication, detergents, or milder oxidisers are inadequate.^[2]

Due to the reaction between its components resulting in its decomposition, aqua regia quickly loses its effectiveness. As such, its components should only be mixed immediately before use. While local regulations may vary, aqua regia may be disposed of by carefully neutralizing with an appropriate agent—such as sodium bicarbonate—before pouring down the sink. If there is a large amount of metal in solution with the acid, it may be preferable to carefully neutralize it, and absorb the solution with a solid material such as vermiculite before discarding it with solid waste. This practice should not be used when EPA regulated or otherwise toxic metals are present.

[edit] Chemistry

[edit] Dissolving gold

Aqua regia dissolves gold, even though neither constituent acid will do so alone, because, in combination, each acid performs a different task. Nitric acid is a powerful oxidizer, which will actually dissolve a virtually undetectable amount of gold, forming gold ions (Au³⁺). The hydrochloric acid provides a ready supply of chloride ions (Cl^-), which react with the gold to produce chloraurate anions, also in solution. The reaction with hydrochloric acid is an equilibrium reaction which favors formation of chloraurate anions (AuCl₄^-). This results in a removal of gold ions from solution and allows further oxidation of gold to take place, and so the gold is dissolved. In addition, gold may be oxidized by the free chlorine present in aqua regia. Appropriate equations are

Au (s) + 3 NO₃^- (aq) + 6 H⁺ (aq) → Au³⁺ (aq) + 3 NO₂ (g) + 3 H₂O (l) and

Au³⁺ (aq) + 4 Cl^- (aq) → AuCl₄^- (aq).

The oxidation reaction can also be written with nitric oxide as the product rather than nitrogen dioxide:

Au (s) + NO₃^- (aq) + 4 H⁺ (aq) → Au³⁺ (aq) + NO (g) + 2 H₂O (l).

[edit] Dissolving platinum

Similar equations can be written for platinum. As with gold, the oxidation reaction can be written with either nitric oxide or nitrogen dioxide as the nitrogen oxide product.

Pt (s) + 4 NO ₃^- (aq) + 8 H⁺ (aq) → Pt⁴⁺ (aq) + 4 NO₂ (g) + 4 H₂O (l)

3Pt (s) + 4 NO ₃^- (aq) + 16 H⁺ (aq) → 3Pt⁴⁺ (aq) + 4 NO (g) + 8 H₂O (l)

The oxidized platinum ion then reacts with chloride ions resulting in the chloroplatinate ion.

Pt⁴⁺ (aq) + 6 Cl^- (aq) → PtCl₆^2- (aq)

Experimental evidence reveals that the reaction of platinum with aqua regia is considerably more complex. The initial reactions produce a mixture of chloroplatinous acid (H₂PtCl₄) and nitrosoplatinic chloride ((NO)₂PtCl₄). The nitrosoplatinic chloride is a solid product. If full dissolution of the platinum is desired, repeated extractions of the residual solids with concentrated hydrochloric acid must be performed.

Pt (s) + 2 HNO₃ (aq) + 4 HCl (aq) → (NO)₂PtCl₄ (s) + 3 H₂O (l) + 1/2 O₂ (g)

(NO)₂PtCl₄ (s) + 2 HCl (aq) → H₂PtCl₄ (aq) + 2 NOCl (g)

The chloroplatinous acid can be oxidized to chloroplatinic acid by saturating the solution with chlorine while heating.

H₂PtCl₄ (aq) + Cl₂ (g) → H₂PtCl₆ (aq)

[edit] Decomposition of aqua regia

Upon mixing of concentrated hydrochloric acid and concentrated nitric acid, chemical reactions occur. These reactions result in the volatile products nitrosyl chloride and chlorine as evidenced by the fuming nature and characteristic yellow color of aqua regia. As the volatile products escape from solution, the aqua regia loses its potency.

HNO₃ (aq) + 3 HCl (aq) → NOCl (g) + Cl₂ (g) + 2 H₂O (l)

Nitrosyl chloride can further decompose into nitric oxide and chlorine. This dissociation is equilibrium-limited. Therefore, in addition to nitrosyl chloride and chlorine, the fumes over aqua regia contain nitric oxide.

2 NOCl (g) → 2 NO (g) + Cl₂ (g)

[edit] History

Jabir ibn Hayyan, medieval manuscript drawing, anonymous

Hydrochloric acid was first discovered around the year 800 by the alchemist Abu Musa Jabir ibn Hayyan (Geber) by mixing common salt with vitriol (sulfuric acid). Jabir’s invention of gold-dissolving aqua regia, consisting of hydrochloric acid and nitric acid, contributed to the effort of alchemists to find the philosopher’s stone.^[3]

When Germany invaded Denmark in World War II, the Hungarian chemist George de Hevesy dissolved the gold Nobel Prizes of Max von Laue and James Franck into aqua regia to prevent the Nazis from stealing them. He placed the resulting solution on a shelf in his laboratory at the Niels Bohr Institute. It was subsequently ignored by the Nazis who thought the jar—one of perhaps hundreds on the shelving—contained common chemicals. After the war, de Hevesy returned to find the solution undisturbed and precipitated the gold out of the acid. The gold was returned to the Royal Swedish Academy of Sciences and the Nobel Foundation who recast and presented the medals to Laue and Franck.^[4]

[edit] In art and entertainment

[edit] Literature

• Cryptonomicon, by Neal Stephenson – The fuel for the “Galvanick Lucipher” (a sort of specialized lantern) used by the butler Ghnxh on Qwghlm. Oddly, the mechanical fixings for the electric parts immersed in the aqua regia are described as being made of “hammered gold”.

• The Crying of Lot 49 by Thomas Pynchon – During The Courier’s Tragedy, the faithful servant Ercole pours aqua regia into a steel box around the traitor Domenico’s head.

• Octopussy, a James Bond film; Bond is provided by Q with a fountain pen containing a mixture of hydrochloric and nitric acids, which Bond utilizes to cut his way through metal prison bars.

Uranium

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This article is about the chemical element. For other uses, see Uranium (disambiguation).

92 protactinium ← uranium → neptunium Nd ↑ U ↓ (Uqb) Periodic table – Extended periodic table
General
Name, symbol, number	uranium, U, 92
Chemical series	actinides
Group, period, block	n/a, 7, f
Appearance	silvery gray metallic; corrodes to a spalling black oxide coat in air
Standard atomic weight	238.02891(3) g·mol−1
Electron configuration	[Rn] 5f3 6d1 7s2
Electrons per shell	2, 8, 18, 32, 21, 9, 2
Physical properties
Phase	solid
Density (near r.t.)	19.1 g·cm−3
Liquid density at m.p.	17.3 g·cm−3
Melting point	1405.3 K (1132.2 °C, 2070 °F)
Boiling point	4404 K (4131 °C, 7468 °F)
Heat of fusion	9.14 kJ·mol−1
Heat of vaporization	417.1 kJ·mol−1
Specific heat capacity	(25 °C) 27.665 J·mol−1·K−1
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 2325 2564 2859 3234 3727 4402
Atomic properties
Crystal structure	orthorhombic
Oxidation states	3+,4+,5+,6+[1] (weakly basic oxide)
Electronegativity	1.38 (Pauling scale)
Ionization energies	1st: 597.6 kJ·mol−1
	2nd: 1420 kJ·mol−1
Atomic radius	175 pm
Van der Waals radius	186 pm
Miscellaneous
Magnetic ordering	paramagnetic
Electrical resistivity	(0 °C) 0.280 µΩ·m
Thermal conductivity	(300 K) 27.5 W·m−1·K−1
Thermal expansion	(25 °C) 13.9 µm·m−1·K−1
Speed of sound (thin rod)	(20 °C) 3155 m/s
Young’s modulus	208 GPa
Shear modulus	111 GPa
Bulk modulus	100 GPa
Poisson ratio	0.23
CAS registry number	7440-61-1
Selected isotopes
Main article: Isotopes of uranium iso NA half-life DM DE (MeV) DP 232U syn 68.9 y α & SF 5.414 228Th 233U syn 159,200 y SF & α 4.909 229Th 234U 0.0054% 245,500 y SF & α 4.859 230Th 235U 0.7204% 7.038×108 y SF & α 4.679 231Th 236U syn 2.342×107 y SF & α 4.572 232Th 238U 99.2742% 4.468×109 y SF & α 4.270 234Th
References
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Uranium (pronounced /jʊˈreɪniəm/) is a silver-gray metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. It has 92 protons and 92 electrons, 6 of them valence electrons. It can have between 141 and 146 neutrons, with 146 (U-238) and 143 in its most common isotopes. Uranium has the highest atomic weight of the naturally occurring elements. Uranium is approximately 70% more dense than lead, but not as dense as gold or tungsten. It is weakly radioactive. It occurs naturally in low concentrations (a few parts per million) in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite (see uranium mining).

In nature, uranium atoms exist as uranium-238 (99.284%), uranium-235 (0.711%),^[2] and a very small amount of uranium-234 (0.0058%). Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years,^[3] making them useful in dating the age of the Earth (see uranium-thorium dating, uranium-lead dating and uranium-uranium dating).

Many contemporary uses of uranium exploit its unique nuclear properties. Uranium-235 has the distinction of being the only naturally occurring fissile isotope. Uranium-238 is both fissionable by fast neutrons, and fertile (capable of being transmuted to fissile plutonium-239 in a nuclear reactor). An artificial fissile isotope, uranium-233, can be produced from natural thorium and is also important in nuclear technology. While uranium-238 has a small probability to fission spontaneously or when bombarded with fast neutrons, the much higher probability of uranium-235 and to a lesser degree uranium-233 to fission when bombarded with slow neutrons generates the heat in nuclear reactors used as a source of power, and provides the fissile material for nuclear weapons. Both uses rely on the ability of uranium to produce a sustained nuclear chain reaction. Depleted uranium (uranium-238) is used in kinetic energy penetrators and armor plating.^[4]

Uranium is used as a colorant in uranium glass, producing orange-red to lemon yellow hues. It was also used for tinting and shading in early photography. The 1789 discovery of uranium in the mineral pitchblende is credited to Martin Heinrich Klaproth, who named the new element after the planet Uranus. Eugène-Melchior Péligot was the first person to isolate the metal, and its radioactive properties were uncovered in 1896 by Antoine Becquerel. Research by Enrico Fermi and others starting in 1934 led to its use as a fuel in the nuclear power industry and in Little Boy, the first nuclear weapon used in war. An ensuing arms race during the Cold War between the United States and the Soviet Union produced tens of thousands of nuclear weapons that used enriched uranium and uranium-derived plutonium. The security of those weapons and their fissile material following the breakup of the Soviet Union in 1991 is a concern for public health and safety.

Contents [hide] 1 Characteristics 2 Applications 2.1 Military 2.2 Civilian 3 History 3.1 Pre-discovery use 3.2 Discovery 3.3 Fission research 3.4 Bombs 3.5 Reactors 3.6 Naturally Occurring Nuclear Fission 3.7 Cold War legacy and waste 4 Occurrence 4.1 Biotic and abiotic 4.2 Production and mining 4.3 Resources and reserves 4.4 Supply 5 Compounds 5.1 Oxidation states and oxides 5.1.1 Oxides 5.1.2 Aqueous chemistry 5.1.3 Carbonates 5.1.4 The effect of pH 5.2 Hydrides, carbides and nitrides 5.3 Halides 6 Isotopes 6.1 Natural concentrations 6.2 Enrichment 7 Precautions 7.1 Exposure 7.2 Effects 8 See also 9 Notes 10 References 11 External links

[edit] Characteristics

An induced nuclear fission event involving uranium-235

When refined, uranium is a silvery white, weakly radioactive metal, which is slightly softer than steel,^[5] strongly electropositive and a poor electrical conductor.^[6] It is malleable, ductile, and slightly paramagnetic.^[5] Uranium metal has very high density, being approximately 70% denser than lead, but slightly less dense than gold.

Uranium metal reacts with almost all nonmetallic elements and their compounds, with reactivity increasing with temperature.^[7] Hydrochloric and nitric acids dissolve uranium, but nonoxidizing acids attack the element very slowly.^[6] When finely divided, it can react with cold water; in air, uranium metal becomes coated with a dark layer of uranium oxide.^[5] Uranium in ores is extracted chemically and converted into uranium dioxide or other chemical forms usable in industry.

Uranium was the first element that was found to be fissile. Upon bombardment with slow neutrons, its uranium-235 isotope will most of the time divide into two smaller nuclei, releasing nuclear binding energy and more neutrons. If these neutrons are absorbed by other uranium-235 nuclei, a nuclear chain reaction occurs and, if there is nothing to absorb some neutrons and slow the reaction, the reaction is explosive. As little as 15 lb (7 kg) of uranium-235 can be used to make an atomic bomb.^[8] The first atomic bomb worked by this principle (nuclear fission).

[edit] Applications

[edit] Military

Depleted uranium is used by various militaries as high-density penetrators.

The major application of uranium in the military sector is in high-density penetrators. This ammunition consists of depleted uranium (DU) alloyed with 1–2% other elements. At high impact speed, the density, hardness, and flammability of the projectile enable destruction of heavily armored targets. Tank armor and the removable armor on combat vehicles are also hardened with depleted uranium (DU) plates. The use of DU became a contentious political-environmental issue after the use of DU munitions by the US, UK and other countries during wars in the Persian Gulf and the Balkans raised questions of uranium compounds left in the soil (see Gulf War Syndrome).^[8]

Depleted uranium is also used as a shielding material in some containers used to store and transport radioactive materials.^[6] Other uses of DU include counterweights for aircraft control surfaces, as ballast for missile re-entry vehicles and as a shielding material.^[5] Due to its high density, this material is found in inertial guidance devices and in gyroscopic compasses.^[5] DU is preferred over similarly dense metals due to its ability to be easily machined and cast as well as its relatively low cost.^[9] Counter to popular belief, the main risk of exposure to DU is chemical poisoning by uranium oxide rather than radioactivity (uranium being only a weak alpha emitter).

During the later stages of World War II, the entire Cold War, and to a lesser extent afterwards, uranium has been used as the fissile explosive material to produce nuclear weapons. Two major types of fission bombs were built: a relatively simple device that uses uranium-235 and a more complicated mechanism that uses uranium-238-derived plutonium-239. Later, a much more complicated and far more powerful fusion bomb that uses a plutonium-based device in a uranium casing to cause a mixture of tritium and deuterium to undergo nuclear fusion was built.^[10]

[edit] Civilian

The most visible civilian use of uranium is as the thermal power source used in nuclear power plants.

The main use of uranium in the civilian sector is to fuel commercial nuclear power plants; by the time it is completely fissioned, one kilogram of uranium-235 can theoretically produce about 20 trillion joules of energy (2×10¹³ joules); as much electricity as 1500 tonnes of coal.^[4]

Commercial nuclear power plants use fuel that is typically enriched to around 3% uranium-235.^[4] The CANDU reactor is the only commercial reactor capable of using unenriched uranium fuel. Fuel used for United States Navy reactors is typically highly enriched in uranium-235 (the exact values are classified). In a breeder reactor, uranium-238 can also be converted into plutonium through the following reaction:^{[5] 238}U (n, gamma) → ²³⁹U -(beta) → ²³⁹Np -(beta) → ²³⁹Pu.

Uranium glass glowing under UV light

Uranium glass used as lead-in seals in a vacuum capacitor

Prior to the discovery of radiation, uranium was primarily used in small amounts for yellow glass and pottery glazes (such as uranium glass and in Fiestaware).

After Marie Curie discovered radium in uranium ore, a huge industry developed to mine uranium so as to extract the radium, which was used to make glow-in-the-dark paints for clock and aircraft dials.^[11] This left a prodigious quantity of uranium as a ‘waste product’, since it takes three metric tons of uranium to extract one gram of radium. This ‘waste product’ was diverted to the glazing industry, making uranium glazes very inexpensive and abundant. In addition to the pottery glazes, uranium tile glazes accounted for the bulk of the use, including common bathroom and kitchen tiles which can be colored green, yellow, mauve, black, blue, red and other colors with uranium.

Uranium was also used in photographic chemicals (esp. uranium nitrate as a toner),^[5] in lamp filaments, to improve the appearance of dentures, and in the leather and wood industries for stains and dyes. Uranium salts are mordants of silk or wool. Uranyl acetate and uranyl formate are used as electron-dense “stains” in transmission electron microscopy, to increase the contrast of biological specimens in ultrathin sections and in negative staining of viruses, isolated cell organelles and macromolecules.

The discovery of the radioactivity of uranium ushered in additional scientific and practical uses of the element. The long half-life of the isotope uranium-238 (4.51×10⁹ years) makes it well-suited for use in estimating the age of the earliest igneous rocks and for other types of radiometric dating (including uranium-thorium dating and uranium-lead dating). Uranium metal is used for X-ray targets in the making of high-energy X-rays.^[5]

[edit] History

[edit] Pre-discovery use

The use of uranium in its natural oxide form dates back to at least the year 79, when it was used to add a yellow color to ceramic glazes.^[5] Yellow glass with 1% uranium oxide was found in a Roman villa on Cape Posillipo in the Bay of Naples, Italy by R. T. Gunther of the University of Oxford in 1912.^[12] Starting in the late Middle Ages, pitchblende was extracted from the Habsburg silver mines in Joachimsthal, Bohemia (now Jáchymov in the Czech Republic) and was used as a coloring agent in the local glassmaking industry.^[13] In the early 19th century, the world’s only known source of uranium ores were these old mines.

[edit] Discovery

Antoine Henri Becquerel discovered the phenomenon of radioactivity by exposing a photographic plate to uranium (1896).

The discovery of the element is credited to the German chemist Martin Heinrich Klaproth. While he was working in his experimental laboratory in Berlin in 1789, Klaproth was able to precipitate a yellow compound (likely sodium diuranate) by dissolving pitchblende in nitric acid and neutralizing the solution with sodium hydroxide.^[13] Klaproth mistakenly assumed the yellow substance was the oxide of a yet-undiscovered element and heated it with charcoal to obtain a black powder, which he thought was the newly discovered metal itself (in fact, that powder was an oxide of uranium).^[13][14] He named the newly discovered element after the planet Uranus, which had been discovered eight years earlier by William Herschel.^[15]

In 1841, Eugène-Melchior Péligot, who was Professor of Analytical Chemistry at the Conservatoire National des Arts et Métiers (Central School of Arts and Manufactures) in Paris, isolated the first sample of uranium metal by heating uranium tetrachloride with potassium.^[16][13] Uranium was not seen as being particularly dangerous during much of the 19th century, leading to the development of various uses for the element. One such use for the oxide was the aforementioned but no longer secret coloring of pottery and glass.

Antoine Henri Becquerel discovered radioactivity by using uranium in 1896.^[7] Becquerel made the discovery in Paris by leaving a sample of a uranium salt on top of an unexposed photographic plate in a drawer and noting that the plate had become ‘fogged’.^[17] He determined that a form of invisible light or rays emitted by uranium had exposed the plate.

[edit] Fission research

Enrico Fermi (bottom left) and the rest of the team that initiated the first artificial nuclear chain reaction (1942).

A team led by Enrico Fermi in 1934 observed that bombarding uranium with neutrons produces the emission of beta rays (electrons or positrons; see beta particle).^[18] The fission products were at first mistaken for new elements of atomic numbers 93 and 94, which the Dean of the Faculty of Rome, Orso Mario Corbino, christened ausonium and hesperium, respectively.^{[19][20][21][22]} The experiments leading to the discovery of uranium’s ability to fission (break apart) into lighter elements and release binding energy were conducted by Otto Hahn and Fritz Strassmann^[18] in Hahn’s laboratory in Berlin. Lise Meitner and her nephew, physicist Otto Robert Frisch, published the physical explanation in February 1939 and named the process ‘nuclear fission‘.^[23] Soon after, Fermi hypothesized that the fission of uranium might release enough neutrons to sustain a fission reaction. Confirmation of this hypothesis came in 1939, and later work found that on average about 2 1/2 neutrons are released by each fission of the rare uranium isotope uranium-235.^[18] Further work found that the far more common uranium-238 isotope can be transmuted into plutonium, which, like uranium-235, is also fissionable by thermal neutrons.

On 2 December 1942, another team led by Enrico Fermi was able to initiate the first artificial nuclear chain reaction, Chicago Pile-1. Working in a lab below the stands of Stagg Field at the University of Chicago, the team created the conditions needed for such a reaction by piling together 400 tons (360 tonnes) of graphite, 58 tons (53 tonnes) of uranium oxide, and six tons (five and a half tonnes) of uranium metal.^[18] Later researchers found that such a chain reaction could either be controlled to produce usable energy or could be allowed to go out of control to produce an explosion more violent than anything possible using chemical explosives.

[edit] Bombs

The mushroom cloud over Hiroshima after the dropping of the uranium-based atomic bomb nicknamed ‘Little Boy‘ (1945)

Two major types of atomic bomb were developed in the Manhattan Project during World War II: a plutonium-based device (see Trinity test and ‘Fat Man‘) whose plutonium was derived from uranium-238, and a uranium-based device (codenamed ‘Little Boy‘) whose fissile material was highly enriched uranium. The uranium-based Little Boy device became the first nuclear weapon used in war when it was detonated over the Japanese city of Hiroshima on 6 August 1945. Exploding with a yield equivalent to 12,500 tonnes of TNT, the blast and thermal wave of the bomb destroyed nearly 50,000 buildings and killed approximately 75,000 people (see Atomic bombings of Hiroshima and Nagasaki).^[17]

[edit] Reactors

Four light bulbs lit with electricity generated from the first artificial electricity-producing nuclear reactor, EBR-I (1951)

Experimental Breeder Reactor I at the Idaho National Laboratory(INL) near Arco, Idaho became the first functioning artificial nuclear reactor on 20 December 1951. Initially, four 150-watt light bulbs were lit by the reactor, but improvements eventually enabled it to power the whole facility (later, the whole town of Arco became the first in the world to have all its electricity come from nuclear power).^[24] The world’s first commercial scale nuclear power station, Obninsk in the Soviet Union, began generation with its reactor AM-1 on 27 June 1954. Other early nuclear power plants were Calder Hall in England which began generation on 17 October 1956^[25] and the Shippingport Atomic Power Station in Pennsylvania which began on 26 May 1958. Nuclear power was used for the first time for propulsion by a submarine, the USS Nautilus, in 1954.^[18]

[edit] Naturally Occurring Nuclear Fission

Main article: Natural nuclear fission reactor

Fifteen ancient and no longer active natural nuclear fission reactors were found in three separate ore deposits at the Oklo mine in Gabon, West Africa in 1972. Discovered by French physicist Francis Perrin, they are collectively known as the Oklo Fossil Reactors. The ore they exist in is 1.7 billion years old; at that time, uranium-235 constituted about three percent of the total uranium on Earth.^[26] This is high enough to permit a sustained nuclear fission chain reaction to occur, providing other conditions are right. The ability of the surrounding sediment to contain the nuclear waste products in less than ideal conditions has been cited by the U.S. federal government as evidence of their claim that the Yucca Mountain facility could safely be a repository of waste for the nuclear power industry.^[26]

[edit] Cold War legacy and waste

U.S. and USSR/Russian nuclear weapons stockpiles, 1945–2006

During the Cold War between the Soviet Union and the United States, huge stockpiles of uranium were amassed and tens of thousands of nuclear weapons were created using enriched uranium and plutonium made from uranium.

Since the break-up of the Soviet Union in 1991, an estimated 600 tons (540 tonnes) of highly enriched weapons grade uranium (enough to make 40,000 nuclear warheads) have been stored in often inadequately guarded facilities in the Russian Federation and several other former Soviet states.^[8] Police in Asia, Europe, and South America on at least 16 occasions from 1993 to 2005 have intercepted shipments of smuggled bomb-grade uranium or plutonium, most of which was from ex-Soviet sources.^[8] From 1993 to 2005 the Material Protection, Control, and Accounting Program, operated by the federal government of the United States, spent approximately US $550 million to help safeguard uranium and plutonium stockpiles in Russia.^[8] The improvements made provided repairs and security enhancements at research and storage facilities. Scientific American reported in February 2006 that some of the facilities had been protected only by chain link fences which were in severe states of disrepair. According to an interview from the article, one facility had been storing samples of enriched (weapons grade) uranium in a broom closet prior to the improvement project; another had been keeping track of its stock of nuclear warheads using index cards kept in a shoe box.^[27]

Above-ground nuclear tests by the Soviet Union and the United States in the 1950s and early 1960s and by France into the 1970s and 1980s^[9] spread a significant amount of fallout from uranium daughter isotopes around the world.^[28] Additional fallout and pollution occurred from several nuclear accidents.

The Windscale fire at the Sellafield nuclear plant in 1957 spread iodine-131, a short lived radioactive isotope, over much of Northern England.

The Three Mile Island accident in 1979 released a small amount of iodine-131. The amounts released by the partial meltdown of the Three Mile Island power plant were minimal, and an environmental survey only found trace amounts in a few field mice dwelling nearby. As I-131 has a half life of slightly more than eight days, any danger posed by the radioactive material has long since passed for both of these incidents.

The Chernobyl disaster in 1986, however, was a complete core breach meltdown and partial detonation of the reactor, which ejected iodine-131 and strontium-90 over a large area of Europe. The 28 year half-life of strontium-90 means that only recently has some of the surrounding countryside around the reactor been deemed safe enough to be habitable.^[9] Since this is less than one half life after the accident, more than half the original release of strontium-90 will still be present. Many other radio active elements with half lives of many thousands of years were also released so use of the term “safe” is curious.

[edit] Occurrence

[edit] Biotic and abiotic

Main article: Uranium in the environment

Uraninite, also known as Pitchblende, is the most common ore mined to extract uranium.

Uranium is a naturally occurring element that can be found in low levels within all rock, soil, and water. Uranium is also the highest-numbered element to be found naturally in significant quantities on earth and is always found combined with other elements.^[5] Along with all elements having atomic weights higher than that of iron, it is only naturally formed in supernova explosions.^[29] The decay of uranium, thorium, and potassium-40 in the Earth’s mantle is thought to be the main source of heat^[30][31] that keeps the outer core liquid and drives mantle convection, which in turn drives plate tectonics.

Its average concentration in the Earth’s crust is (depending on the reference) 2 to 4 parts per million,^[6][9] or about 40 times as abundant as silver.^[7] The Earth’s crust from the surface to 25 km (15 mi) down is calculated to contain 10¹⁷ kg (2×10¹⁷ lb) of uranium while the oceans may contain 10¹³ kg (2×10¹³ lb).^[6] The concentration of uranium in soil ranges from 0.7 to 11 parts per million (up to 15 parts per million in farmland soil due to use of phosphate fertilizers), and 3 parts per billion of sea water is composed of the element.^[9]

It is more plentiful than antimony, tin, cadmium, mercury, or silver, and it is about as abundant as arsenic or molybdenum.^[5][9] It is found in hundreds of minerals including uraninite (the most common uranium ore), autunite, uranophane, torbernite, and coffinite.^[5] Significant concentrations of uranium occur in some substances such as phosphate rock deposits, and minerals such as lignite, and monazite sands in uranium-rich ores^[5] (it is recovered commercially from these sources with as little as 0.1% uranium^[7]).

Citrobacter species can have concentrations of uranium in their bodies 300 times higher than in the surrounding environment.

Some organisms, such as the lichen Trapelia involuta or microorganisms such as the bacterium Citrobacter, can absorb concentrations of uranium that are up to 300 times higher than in their environment.^[32] Citrobacter species absorb uranyl ions when given glycerol phosphate (or other similar organic phosphates). After one day, one gram of bacteria will encrust themselves with nine grams of uranyl phosphate crystals; this creates the possibility that these organisms could be used in bioremediation to decontaminate uranium-polluted water.^[13][33]

Plants absorb some uranium from the soil they are rooted in. Dry weight concentrations of uranium in plants range from 5 to 60 parts per billion, and ash from burnt wood can have concentrations up to 4 parts per million.^[13] Dry weight concentrations of uranium in food plants are typically lower with one to two micrograms per day ingested through the food people eat.^[13]

[edit] Production and mining

Main article: Uranium mining

The worldwide production of uranium in 2003 amounted to 41 429 tonnes, of which 25% was mined in Canada. Other important uranium mining countries are Australia, Russia, Niger, Namibia, Kazakhstan, Uzbekistan, South Africa, USA and Portugal.

Yellowcake is a concentrated mixture of uranium oxides that is further refined to extract pure uranium.

Uranium ore is mined in several ways: by open pit, underground, in-situ leaching, and borehole mining (see uranium mining).^[4] Low-grade uranium ore typically contains 0.1 to 0.25% of actual uranium oxides, so extensive measures must be employed to extract the metal from its ore.^[34] High-grade ores found in Athabasca Basin deposits in Saskatchewan, Canada can contain up to 70% uranium oxides, and therefore must be diluted with waste rock prior to milling, as the undilute stockpiled ore could become critical and start a nuclear reaction. Uranium ore is crushed and rendered into a fine powder and then leached with either an acid or alkali. The leachate is then subjected to one of several sequences of precipitation, solvent extraction, and ion exchange. The resulting mixture, called yellowcake, contains at least 75% uranium oxides. Yellowcake is then calcined to remove impurities from the milling process prior to refining and conversion.

Commercial-grade uranium can be produced through the reduction of uranium halides with alkali or alkaline earth metals.^[5] Uranium metal can also be made through electrolysis of KU₅ or UF₄, dissolved in a molten calcium chloride (CaCl₂) and sodium chloride (NaCl) solution.^[5] Very pure uranium can be produced through the thermal decomposition of uranium halides on a hot filament.^[5]

[edit] Resources and reserves

Current economic uranium resources will last for over 100 years at current consumption rates, while it is expected there is twice that amount awaiting discovery. With reprocessing and recycling, the reserves are good for thousands of years.^[35]. It is estimated that 5.5 million tonnes of uranium ore reserves are economically viable,^[35] while 35 million tonnes are classed as mineral resources (reasonable prospects for eventual economic extraction).^[36] An additional 4.6 billion tonnes of uranium are estimated to be in sea water (Japanese scientists in the 1980s showed that extraction of uranium from sea water using ion exchangers was feasible).^[37][38]

Exploration for uranium is continuing to increase with US$200 million being spent world wide in 2005, a 54% increase on the previous year..^[36]This trend has continued through 2006, when expenditure on exploration rocketed to total over $774 million, an increase of over 250% compared to 2004. The OECD Nuclear Energy Agency said exploration figures for 2007 would likely match those for 2006.^[35]

Australia has 40% of the world’s uranium ore reserves^[39] and the world’s largest single uranium deposit, located at the Olympic Dam Mine in South Australia.^[40] Almost all of the uranium production is exported, under strict International Atomic Energy Agency safeguards against use in nuclear weapons.

[edit] Supply

Uranium output in 2005

In 2005, seventeen countries produced concentrated uranium oxides, with Canada (27.9% of world production) and Australia (22.8%) being the largest producers and Kazakhstan (10.5%), Russia (8.0%), Namibia (7.5%), Niger (7.4%), Uzbekistan (5.5%), the United States (2.5%), Ukraine (1.9%) and China (1.7%) also producing significant amounts.^[41] Kazakhstan continues to increase production and may become the world’s largest producer of uranium by the year 2009 with an expected production of 12 826 tonnes, compared to Canada with 11 100 tonnes and Australia with 9 430 tonnes.^[42][43] The ultimate supply of uranium is believed to be very large and sufficient for at least the next 85 years^[36] although some studies indicate underinvestment in the late twentieth century may produce supply problems in the 21st century.^[44]

Some claim that production of uranium will peak similar to peak oil. Kenneth S. Deffeyes and Ian D. MacGregor point out that uranium deposits seem to be log-normal distributed. There is a 300-fold increase in the amount of uranium recoverable for each tenfold decrease in ore grade.”^[45] In other words, there is very little high grade ore and proportionately much more low grade ore.

[edit] Compounds

[edit] Oxidation states and oxides

[edit] Oxides

Triuranium octaoxide (diagram pictured) and uranium dioxide are the two most common uranium oxides.

Calcined uranium yellowcake as produced in many large mills contains a distribution of uranium oxidation species in various forms ranging from most oxidized to least oxidized. Particles with short residence times in a calciner will generally be less oxidized than particles that have long retention times or are recovered in the stack scrubber. While uranium content is referred to for U₃O₈ content, to do so is inaccurate and dates to the days of the Manhattan project when U₃O₈ was used as an analytical chemistry reporting standard.

Phase relationships in the uranium-oxygen system are highly complex. The most important oxidation states of uranium are uranium(IV) and uranium(VI), and their two corresponding oxides are, respectively, uranium dioxide (UO₂) and uranium trioxide (UO₃).^[46] Other uranium oxides such as uranium monoxide (UO), diuranium pentoxide (U₂O₅), and uranium peroxide (UO₄•2H₂O) are also known to exist.

The most common forms of uranium oxide are triuranium octaoxide (U₃O₈) and the aforementioned UO₂.^[47] Both oxide forms are solids that have low solubility in water and are relatively stable over a wide range of environmental conditions. Triuranium octaoxide is (depending on conditions) the most stable compound of uranium and is the form most commonly found in nature. Uranium dioxide is the form in which uranium is most commonly used as a nuclear reactor fuel.^[47] At ambient temperatures, UO₂ will gradually convert to U₃O₈. Because of their stability, uranium oxides are generally considered the preferred chemical form for storage or disposal.^[47]

[edit] Aqueous chemistry

Ions that represent the four different oxidation states of uranium are soluble and therefore can be studied in aqueous solutions. They are: U³⁺ (red), U⁴⁺ (green), UO₂⁺ (unstable), and UO₂²⁺ (yellow).^[48] A few solid and semi-metallic compounds such as UO and US exist for the formal oxidation state uranium(II), but no simple ions are known to exist in solution for that state. Ions of U³⁺ liberate hydrogen from water and are therefore considered to be highly unstable. The UO₂²⁺ ion represents the uranium(VI) state and is known to form compounds such as the carbonate, chloride and sulfate. UO₂²⁺ also forms complexes with various organic chelating agents, the most commonly encountered of which is uranyl acetate.^[48]

[edit] Carbonates

The Pourbaix diagram for uranium in a non-complexing aqueous medium (eg perchloric acid / sodium hydroxide).^[49]

The Pourbaix diagram for uranium in carbonate solution^[49]

The interactions of carbonate anions with uranium(VI) cause the Pourbaix diagram to change greatly when the medium is changed from water to a carbonate containing solution. It is interesting to note that while the vast majority of carbonates are insoluble in water (students are often taught that all carbonates other than those of alkali metals are insoluble in water), uranium carbonates are often soluble in water. This is due to the fact that a U(VI) cation is able to bind two terminal oxides and three or more carbonates to form anionic complexes.

[edit] The effect of pH

A diagram showing the relative concentrations of the different chemical forms of uranium in a non-complexing aqueous medium (eg perchloric acid / sodium hydroxide).^[49]

A diagram showing the relative concentrations of the different chemical forms of uranium in an aqueous carbonate solution.^[49]

The uranium fraction diagrams in the presence of carbonate illustrate this further: it may be seen that when the pH of a uranium(VI) solution is increased that the uranium is converted to a hydrated uranium oxide hydroxide and then at high pHs to an anionic hydroxide complex.

On addition of carbonate to the system the uranium is converted to a series of carbonate complexes when the pH is increased, one important overall effect of these reactions is to increase the solubility of the uranium in the range pH 6 to 8. This is important when considering the long term stability of used uranium dioxide nuclear fuels.

[edit] Hydrides, carbides and nitrides

Uranium metal heated to 250 to 300 °C (482 to 572 °F) reacts with hydrogen to form uranium hydride. Even higher temperatures will reversibly remove the hydrogen. This property makes uranium hydrides convenient starting materials to create reactive uranium powder along with various uranium carbide, nitride, and halide compounds.^[50] Two crystal modifications of uranium hydride exist: an α form that is obtained at low temperatures and a β form that is created when the formation temperature is above 250 °C.^[50]

Uranium carbides and uranium nitrides are both relatively inert semimetallic compounds that are minimally soluble in acids, react with water, and can ignite in air to form U₃O₈.^[50] Carbides of uranium include uranium monocarbide (UC), uranium dicarbide (UC₂), and diuranium tricarbide (U₂C₃). Both UC and UC₂ are formed by adding carbon to molten uranium or by exposing the metal to carbon monoxide at high temperatures. Stable below 1800 °C, U₂C₃ is prepared by subjecting a heated mixture of UC and UC₂ to mechanical stress.^[51] Uranium nitrides obtained by direct exposure of the metal to nitrogen include uranium mononitride (UN), uranium dinitride (UN₂), and diuranium trinitride (U₂N₃).^[51]

[edit] Halides

Uranium hexafluoride is the feedstock used to separate uranium-235 from natural uranium.

All uranium fluorides are created using uranium tetrafluoride (UF₄); UF₄ itself is prepared by hydrofluorination of uranium dioxide.^[50] Reduction of UF₄ with hydrogen at 1000 °C produces uranium trifluoride (UF₃). Under the right conditions of temperature and pressure, the reaction of solid UF₄ with gaseous uranium hexafluoride (UF₆) can form the intermediate fluorides of U₂F₉, U₄F₁₇, and UF₅.^[50]

At room temperatures, UF₆ has a high vapor pressure, making it useful in the gaseous diffusion process to separate highly valuable uranium-235 from the far more common uranium-238 isotope. This compound can be prepared from uranium dioxide and uranium hydride by the following process:^[50]

UO₂ + 4HF + heat (500 °C) → UF₄ + 2H₂O
UF₄ + F₂ + heat (350 °C) → UF₆

The resulting UF₆ white solid is highly reactive (by fluorination), easily sublimes (emitting a nearly perfect gas vapor), and is the most volatile compound of uranium known to exist.^[50]

One method of preparing uranium tetrachloride (UCl₄) is to directly combine chlorine with either uranium metal or uranium hydride. The reduction of UCl₄ by hydrogen produces uranium trichloride (UCl₃) while the higher chlorides of uranium are prepared by reaction with additional chlorine.^[50] All uranium chlorides react with water and air.

Bromides and iodides of uranium are formed by direct reaction of, respectively, bromine and iodine with uranium or by adding UH₃ to those element’s acids.^[50] Known examples include: UBr₃, UBr₄, UI₃, and UI₄. Uranium oxyhalides are water-soluble and include UO₂F₂, UOCl₂, UO₂Cl₂, and UO₂Br₂. Stability of the oxyhalides decrease as the atomic weight of the component halide increases.^[50]

[edit] Isotopes

Pie-graphs showing the relative proportions of uranium-238 (blue) and uranium-235 (red) at different levels of enrichment

[edit] Natural concentrations

Main article: Isotopes of uranium

Naturally occurring uranium is composed of three major isotopes, uranium-238 (99.28% natural abundance), uranium-235 (0.71%), and uranium-234 (0.0054%). All three isotopes are radioactive, creating radioisotopes, with the most abundant and stable being uranium-238 with a half-life of 4.51×10⁹ years (close to the age of the Earth), uranium-235 with a half-life of 7.13×10⁸ years, and uranium-234 with a half-life of 2.48×10⁵ years.^[52]

Uranium-238 is an α emitter, decaying through the 18-member uranium natural decay series into lead-206.^[7] The decay series of uranium-235 (also called actino-uranium) has 15 members that ends in lead-207.^[7] The constant rates of decay in these series makes comparison of the ratios of parent to daughter elements useful in radiometric dating. Uranium-234 decays to lead-206 through a series of short-lived intermediaries. Uranium-233 is made from thorium-232 by neutron bombardment;^[5] its decay series ends with thallium-205.

The isotope uranium-235 is important for both nuclear reactors and nuclear weapons because it is the only isotope existing in nature to any appreciable extent that is fissile, that is, can be broken apart by thermal neutrons.^[7] The isotope uranium-238 is also important because it absorbs neutrons to produce a radioactive isotope that subsequently decays to the isotope plutonium-239, which also is fissile.^[18]

[edit] Enrichment

Main article: Enriched uranium

Cascades of gas centrifuges are used to enrich uranium ore to concentrate its fissionable isotopes.

Enrichment of uranium ore through isotope separation to concentrate the fissionable uranium-235 is needed for use in nuclear weapons and most nuclear power plants with the exception of gas cooled reactors and pressurised heavy water reactors. A majority of neutrons released by a fissioning atom of uranium-235 must impact other uranium-235 atoms to sustain the nuclear chain reaction needed for these applications. The concentration and amount of uranium-235 needed to achieve this is called a ‘critical mass.’

To be considered ‘enriched’, the uranium-235 fraction has to be increased to significantly greater than its concentration in naturally occurring uranium. Enriched uranium typically has a uranium-235 concentration of between 3 and 5%.^[53] The process produces huge quantities of uranium that is depleted of uranium-235 and with a correspondingly increased fraction of uranium-238, called depleted uranium or ‘DU’. To be considered ‘depleted’, the uranium-235 isotope concentration has to have been decreased to significantly less than its natural concentration. Typically the amount of uranium-235 left in depleted uranium is 0.2% to 0.3%.^[54] As the price of uranium has risen since 2001, some enrichment tailings containing more than 0.35% uranium-235 are being considered for re-enrichment, driving the price of these depleted uranium hexafluoride stores above $130 per kilogram in July, 2007 from just $5 in 2001.^[54]

The gas centrifuge process, where gaseous uranium hexafluoride (UF₆) is separated by the difference in molecular weight between ²³⁵UF₆ and ²³⁸UF₆ using high-speed centrifuges, has become the cheapest and leading enrichment process (lighter UF₆ concentrates in the center of the centrifuge).^[17] The gaseous diffusion process was the previous leading method for enrichment and the one used in the Manhattan Project. In this process, uranium hexafluoride is repeatedly diffused through a silver-zinc membrane, and the different isotopes of uranium are separated by diffusion rate (uranium 238 is heavier and thus diffuses slightly slower than uranium-235).^[17] The molecular laser isotope separation method employs a laser beam of precise energy to sever the bond between uranium-235 and fluorine. This leaves uranium-238 bonded to fluorine and allows uranium-235 metal to precipitate from the solution.^[4] Another method is called liquid thermal diffusion.^[6]

[edit] Precautions

[edit] Exposure

A person can be exposed to uranium (or its radioactive daughters such as radon) by inhaling dust in air or by ingesting contaminated water and food. The amount of uranium in air is usually very small; however, people who work in factories that process phosphate fertilizers, live near government facilities that made or tested nuclear weapons, live or work near a modern battlefield where depleted uranium weapons have been used, or live or work near a coal-fired power plant, facilities that mine or process uranium ore, or enrich uranium for reactor fuel, may have increased exposure to uranium.^[55][56] Houses or structures that are over uranium deposits (either natural or man-made slag deposits) may have an increased incidence of exposure to radon gas.

Almost all uranium that is ingested is excreted during digestion, but up to 5% is absorbed by the body when the soluble uranyl ion is ingested while only 0.5% is absorbed when insoluble forms of uranium, such as its oxide, are ingested.^[13] However, soluble uranium compounds tend to quickly pass through the body whereas insoluble uranium compounds, especially when ingested via dust into the lungs, pose a more serious exposure hazard. After entering the bloodstream, the absorbed uranium tends to bioaccumulate and stay for many years in bone tissue because of uranium’s affinity for phosphates.^[13] Uranium is not absorbed through the skin, and alpha particles released by uranium cannot penetrate the skin.

[edit] Effects

One health risk from large intakes of uranium is toxic damage to the kidneys, because, in addition to being weakly radioactive, uranium is a toxic metal.^[57][58][13] Uranium is also a reproductive toxicant.^[59][60] Radiological effects are generally local because this is the nature of alpha radiation, the primary form from U-238 decay. Uranyl (UO₂⁺) ions, such as from uranium trioxide or uranyl nitrate and other hexavalent uranium compounds, have been shown to cause birth defects and immune system damage in laboratory animals.^[61] No human cancer has been seen as a result of exposure to natural or depleted uranium,^[62] but exposure to some of its decay products, especially radon, does pose a significant health threat.^[9] Exposure to strontium-90, iodine-131, and other fission products is unrelated to uranium exposure, but may result from medical procedures or exposure to spent reactor fuel or fallout from nuclear weapons.^[63] Although accidental inhalation exposure to a high concentration of uranium hexafluoride has resulted in human fatalities, those deaths were not associated with uranium itself.^[64] Finely divided uranium metal presents a fire hazard because uranium is pyrophoric, so small grains will ignite spontaneously in air at room temperature.^[5]

Science

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For other uses, see Science (disambiguation).

The Meissner effect causes a magnet to levitate above a high-temperature superconductor.

Science (from the Latin scientia, meaning “knowledge” or “to know”) is the effort to discover, and increase human understanding of how the physical world works. Through controlled methods, scientists use observable physical evidence of natural phenomena to collect data, and analyze this information to explain what and how things work. Such methods include experimentation that tries to simulate natural phenomena under controlled conditions and thought experiments. Knowledge in science is gained through research.

Contents [hide] 1 Etymology 2 History of science 3 History of usage of the word science 3.1 Distinguished from technology 4 Scientific method 4.1 Mathematics 5 Philosophy of science 6 Critiques 6.1 Science, pseudoscience and nonscience 6.2 Philosophical focus 6.3 The media and the scientific debate 6.4 Epistemological inadequacies 7 Scientific community 7.1 Fields 7.2 Institutions 7.3 Literature 8 See also 9 Notes 10 References 11 Further reading 12 External links

Etymology

DNA determines the genetic structure of all life on earth

The word science is derived from the Latin word scientia for knowledge, the nominal form of the verb scire, “to know”. The Proto-Indo-European (PIE) root that yields scire is *skei-, meaning to “cut, separate, or discern”. Other words from the same root include Sanskrit chyati, “he cuts off”, Greek schizo, “I split” (hence English schism, schizophrenia), Latin scindo, “I split” (hence English rescind).^[1] From the Middle Ages to the Enlightenment, science or scientia meant any systematic recorded knowledge.^[2] Science therefore had the same sort of very broad meaning that philosophy had at that time. In other languages, including French, Spanish, Portuguese, Italian, Polish and Russian, the word corresponding to science also carries this meaning.

History of science

Main article: History of science

History of usage of the word science

Well into the eighteenth century, science and natural philosophy were not quite synonymous, but only became so later with the direct use of what would become known formally as the scientific method, which was earlier developed during the Middle Ages and early modern period in Europe and the Middle East (see History of scientific method). Prior to the 18th century, however, the preferred term for the study of nature was natural philosophy, while English speakers most typically referred to the study of the human mind as moral philosophy. By contrast, the word “science” in English was still used in the 17th century to refer to the Aristotelian concept of knowledge which was secure enough to be used as a sure prescription for exactly how to do something. In this differing sense of the two words, the philosopher John Locke in An Essay Concerning Human Understanding wrote that “natural philosophy the study of nature is not capable of being made a science”.^[3]

By the early 1800s, natural philosophy had begun to separate from philosophy, though it often retained a very broad meaning. In many cases, science continued to stand for reliable knowledge about any topic, in the same way it is still used in the broad sense (see the introduction to this article) in modern terms such as library science, political science, and computer science. In the more narrow sense of science, as natural philosophy became linked to an expanding set of well-defined laws (beginning with Galileo’s laws, Kepler’s laws, and Newton’s laws for motion), it became more popular to refer to natural philosophy as natural science. Over the course of the nineteenth century, moreover, there was an increased tendency to associate science with study of the natural world (that is, the non-human world). This move sometimes left the study of human thought and society (what would come to be called social science) in a linguistic limbo by the end of the century and into the next.^[4]

Through the 19th century, many English speakers were increasingly differentiating science (meaning a combination of what we now term natural and biological sciences) from all other forms of knowledge in a variety of ways. The now-familiar expression “scientific method,” which refers to the prescriptive part of how to make discoveries in natural philosophy, was almost unused during the early part of the 19th century, but became widespread after the 1870s, though there was rarely totally agreement about just what it entailed.^[4] The word “scientist,” meant to refer to a systematically-working natural philosopher, (as opposed to an intuitive or empirically-minded one) was coined in 1833 by William Whewell.^[5] Discussion of scientists as a special group of people who did science, even if their attributes were up for debate, grew in the last half of the 19th century.^[4] Whatever people actually meant by these terms at first, they ultimately depicted science, in the narrow sense of the habitual use of the scientific method and the knowledge derived from it, as something deeply distinguished from all other realms of human endeavor.

By the twentieth century, the modern notion of science as a special brand of information about the world, practiced by a distinct group and pursued through a unique method, was essentially in place. It was used to give legitimacy to a variety of fields through such titles as “scientific” medicine, engineering, advertising, or motherhood.^[4] Over the 1900s, links between science and technology also grew increasingly strong.

Distinguished from technology

By the end of the century, it is arguable that technology had even begun to eclipse science as a term of public attention and praise. Scholarly studies of science have begun to refer to “technoscience” rather than science of technology separately. Meanwhile, such fields as biotechnology and nanotechnology are capturing the headlines. One author has suggested that, in the coming century, “science” may fall out of use, to be replaced by technoscience or even by some more exotic label such as “techknowledgy.”^[4]

Scientific method

Main article: Scientific method

The Bohr model of the atom, like many ideas in the history of science, was at first prompted by and later partially disproved by experiment.

The scientific method seeks to explain the events of nature in a reproducible way, and to use these reproductions to make useful predictions. It is done through observation of natural phenomena, and/or through experimentation that tries to simulate natural events under controlled conditions. It provides an objective process to find solutions to problems in a number of scientific and technological fields.^[6]

Based on observations of a phenomenon, a scientist may generate a model. This is an attempt to describe or depict the phenomenon in terms of a logical physical or mathematical representation. As empirical evidence is gathered, a scientist can suggest a hypothesis to explain the phenomenon. This description can be used to make predictions that are testable by experiment or observation using the scientific method. When a hypothesis proves unsatisfactory, it is either modified or discarded.

While performing experiments, Scientists may have a preference for one outcome over another, and it is important that this tendency does not bias their interpretation.^[7][8] A strict following of the scientific method attempts to minimize the influence of a scientist’s bias on the outcome of an experiment. This can be achieved by correct experimental design, and a thorough peer review of the experimental results as well as conclusions of a study.^[9][10] Once the experiment results are announced or published, an important cross-check can be the need to validate the results by an independent party.^[11]

Once a hypothesis has survived testing, it may become adopted into the framework of a scientific theory. This is a logically reasoned, self-consistent model or framework for describing the behavior of certain natural phenomena. A theory typically describes the behavior of much broader sets of phenomena than a hypothesis—commonly, a large number of hypotheses can be logically bound together by a single theory. These broader theories may be formulated using principles such as parsimony (e.g., “Occam’s Razor“). They are then repeatedly tested by analyzing how the collected evidence (facts) compares to the theory. When a theory survives a sufficiently large number of empirical observations, it then becomes a scientific generalization that can be taken as fully verified.

Despite the existence of well-tested theories, science cannot claim absolute knowledge of nature or the behavior of the subject or of the field of study due to epistemological problems that are unavoidable and preclude the discovery or establishment of absolute truth. Unlike a mathematical proof, a scientific theory is empirical, and is always open to falsification, if new evidence is presented. Even the most basic and fundamental theories may turn out to be imperfect if new observations are inconsistent with them. Critical to this process is making every relevant aspect of research publicly available, which allows ongoing review and repeating of experiments and observations by multiple researchers operating independently of one another. Only by fulfilling these expectations can it be determined how reliable the experimental results are for potential use by others.

Isaac Newton’s Newtonian law of gravitation is a famous example of an established law that was later found not to be universal—it does not hold in experiments involving motion at speeds close to the speed of light or in close proximity of strong gravitational fields. Outside these conditions, Newton’s Laws remain an excellent model of motion and gravity. Since general relativity accounts for all the same phenomena that Newton’s Laws do and more, general relativity is now regarded as a more comprehensive theory.^[12]

Mathematics

Data from the famous Michelson–Morley experiment

Mathematics is essential to many sciences. One important function of mathematics in science is the role it plays in the expression of scientific models. Observing and collecting measurements, as well as hypothesizing and predicting, often require extensive use of mathematics and mathematical models. Calculus may be the branch of mathematics most often used in science, but virtually every branch of mathematics has applications in science, including “pure” areas such as number theory and topology. Mathematics is fundamental to the understanding of the natural sciences and the social sciences, many of which also rely heavily on statistics.

Statistical methods, comprised of mathematical techniques for summarizing and exploring data, allow scientists to assess the level of reliability and the range of variation in experimental results. Statistical thinking also plays a fundamental role in many areas of science.

Computational science applies computing power to simulate real-world situations, enabling a better understanding of scientific problems than formal mathematics alone can achieve. According to the Society for Industrial and Applied Mathematics, computation is now as important as theory and experiment in advancing scientific knowledge.^[13]

Whether mathematics itself is properly classified as science has been a matter of some debate. Some thinkers see mathematicians as scientists, regarding physical experiments as inessential or mathematical proofs as equivalent to experiments. Others do not see mathematics as a science, since it does not require experimental test of its theories and hypotheses. In practice, mathematical theorems and formulas are obtained by logical derivations which presume axiomatic systems, rather than a combination of empirical observation and method of reasoning that has come to be known as scientific method. In general, mathematics is classified as formal science, while natural and social sciences are classified as empirical sciences.

Philosophy of science

Velocity-distribution data of a gas of rubidium atoms, confirming the discovery of a new phase of matter, the Bose–Einstein condensate.

Main article: Philosophy of science

The philosophy of science seeks to understand the nature and justification of scientific knowledge. It has proven difficult to provide a definitive account of the scientific method that can decisively serve to distinguish science from non-science. Thus there are legitimate arguments about exactly where the borders are, leading to the problem of demarcation. There is nonetheless a set of core precepts that have broad consensus among published philosophers of science and within the scientific community at large.

Science is reasoned-based analysis of sensation upon our awareness. As such, the scientific method cannot deduce anything about the realm of reality that is beyond what is observable by existing or theoretical means.^[14] When a manifestation of our reality previously considered supernatural is understood in the terms of causes and consequences, it acquires a scientific explanation.^[15]

Some of the findings of science can be very counter-intuitive. Atomic theory, for example, implies that a granite boulder which appears a heavy, hard, solid, grey object is actually a combination of subatomic particles with none of these properties, moving very rapidly in space where the mass is concentrated in a very small fraction of the total volume. Many of humanity’s preconceived notions about the workings of the universe have been challenged by new scientific discoveries. Quantum mechanics, particularly, examines phenomena that seem to defy our most basic postulates about causality and fundamental understanding of the world around us. Science is the branch of knowledge dealing with people and the understanding we have of our environment and how it works.

There are different schools of thought in the philosophy of scientific method. Methodological naturalism maintains that scientific investigation must adhere to empirical study and independent verification as a process for properly developing and evaluating natural explanations for observable phenomena. Methodological naturalism, therefore, rejects supernatural explanations, arguments from authority and biased observational studies. Critical rationalism instead holds that unbiased observation is not possible and a demarcation between natural and supernatural explanations is arbitrary; it instead proposes falsifiability as the landmark of empirical theories and falsification as the universal empirical method. Critical rationalism argues for the ability of science to increase the scope of testable knowledge, but at the same time against its authority, by emphasizing its inherent fallibility. It proposes that science should be content with the rational elimination of errors in its theories, not in seeking for their verification (such as claiming certain or probable proof or disproof; both the proposal and falsification of a theory are only of methodological, conjectural, and tentative character in critical rationalism). Instrumentalism rejects the concept of truth and emphasizes merely the utility of theories as instruments for explaining and predicting phenomena.

Critiques

Science, pseudoscience and nonscience

Main articles: Cargo cult science, Fringe science, Junk science, Pseudoscience, and Scientific misconduct

Any established body of knowledge which masquerades as science in an attempt to claim a legitimacy which it would not otherwise be able to achieve on its own terms is not science; it is often known as fringe- or alternative science. The most important of its defects is usually the lack of the carefully controlled and thoughtfully interpreted experiments which provide the foundation of the natural sciences and which contribute to their advancement. Another term, junk science, is often used to describe scientific theories or data which, while perhaps legitimate in themselves, are believed to be mistakenly used to support an opposing position. There is usually an element of political or ideological bias in the use of the term. Thus the arguments in favor of limiting the use of fossil fuels in order to reduce global warming are often characterized as junk science by those who do not wish to see such restrictions imposed, and who claim that other factors may well be the cause of global warming. A wide variety of commercial advertising (ranging from hype to outright fraud) would also fall into this category. Finally, there is just plain bad science, which is commonly used to describe well-intentioned but incorrect, obsolete, incomplete, or over-simplified expositions of scientific ideas.

The status of many bodies of knowledge as true sciences, has been a matter of debate. Discussion and debate abound in this topic with some fields like the social and behavioural sciences accused by critics of being unscientific. Many groups of people from academicians like Nobel Prize physicist Percy W. Bridgman,^[16] or Dick Richardson, Ph.D.—Professor of Integrative Biology at the University of Texas at Austin,^[17] to politicians like U.S. Senator Kay Bailey Hutchison and other co-sponsors,^[18] oppose giving their support or agreeing with the use of the label “science” in some fields of study and knowledge they consider non-scientific, ambiguous, or scientifically irrelevant compared with other fields. Karl Popper denied the existence of evidence^[19] and of scientific method.^[20] Popper holds that there is only one universal method, the negative method of trial and error. It covers not only all products of the human mind, including science, mathematics, philosophy, art and so on, but also the evolution of life.^[21] He also contributed to the Positivism dispute, a philosophical dispute between Critical rationalism (Popper,Albert) and the Frankfurt School (Adorno, Habermas) about the methodology of the social sciences.^[22]

Philosophical focus

Historian Jacques Barzun termed science “a faith as fanatical as any in history” and warned against the use of scientific thought to suppress considerations of meaning as integral to human existence.^[23] Many recent thinkers, such as Carolyn Merchant, Theodor Adorno and E. F. Schumacher considered that the 17th century scientific revolution shifted science from a focus on understanding nature, or wisdom, to a focus on manipulating nature, i.e. power, and that science’s emphasis on manipulating nature leads it inevitably to manipulate people, as well.^[24] Science’s focus on quantitative measures has led to critiques that it is unable to recognize important qualitative aspects of the world.^[24] It is not clear, however, if this kind of criticism is adequate to a vast number of non-experimental scientifics fields like Astronomy, Cosmology, Evolutionary Biology, Complexity Theory, Paleontology, Paleoanthropology, Archeology, Earth Sciences, Climatology, Ecology and other sciences, like Statistical Physics of irreversible non-linear systems, that emphasize systemic and historically contingent frozen accidents. Considerations about the philosophical impact of science to the discussion of the (or lack of) meaning in human existence are not supressed but strongly discussed in the literature of science divulgation, a movement sometimes called The Third Culture.

The implications of the ideological denial of ethics for the practice of science itself in terms of fraud, plagiarism, and data falsification, has been criticized by several academics. In “Science and Ethics”, the philosopher Bernard Rollin examines the ideology that denies the relevance of ethics to science, and argues in favor of making education in ethics part and parcel of scientific training.^[25]

The media and the scientific debate

The mass media face a number of pressures that can prevent them from accurately depicting competing scientific claims in terms of their credibility within the scientific community as a whole. Determining how much weight to give different sides in a scientific debate requires considerable expertise on the issue at hand.^[26] Few journalists have real scientific knowledge, and even beat reporters who know a great deal about certain scientific issues may know little about other ones they are suddenly asked to cover.^[27][28]

Epistemological inadequacies

Psychologist Carl Jung believed that though science attempted to understand all of nature, the experimental method used would pose artificial, conditional questions that evoke only partial answers.^[29] Robert Anton Wilson criticized science for using instruments to ask questions that produce answers only meaningful in terms of the instrument, and that there was no such thing as a completely objective vantage point from which to view the results of science.^[30]

Scientific community

Main article: Scientific community

The scientific community consists of the total body of scientists, its relationships and interactions. It is normally divided into “sub-communities” each working on a particular field within science.

Fields

Main article: Fields of science

Fields of science are commonly classified along two major lines: natural sciences, which study natural phenomena (including biological life), and social sciences, which study human behavior and societies. These groupings are empirical sciences, which means the knowledge must be based on observable phenomena and capable of being experimented for its validity by other researchers working under the same conditions.^[31] There are also related disciplines that are grouped into interdisciplinary and applied sciences, such as engineering and health science. Within these categories are specialized scientific fields that can include elements of other scientific disciplines but often possess their own terminology and body of expertise.^[32]

Mathematics, which is sometimes classified within a third group of science called formal science, has both similarities and differences with the natural and social sciences.^[31] It is similar to empirical sciences in that it involves an objective, careful and systematic study of an area of knowledge; it is different because of its method of verifying its knowledge, using a priori rather than empirical methods.^[31] Formal science, which also includes statistics and logic, is vital to the empirical sciences. Major advances in formal science have often led to major advances in the physical and biological sciences. The formal sciences are essential in the formation of hypotheses, theories, and laws,^[31] both in discovering and describing how things work (natural sciences) and how people think and act (social sciences).

Institutions

Louis XIV visiting the Académie des sciences in 1671.

Learned societies for the communication and promotion of scientific thought and experimentation have existed since the Renaissance period.^[33] The oldest surviving institution is the Accademia dei Lincei in Italy.^[34] National Academy of Sciences are distinguished institutions that exist in a number of countries, beginning with the British Royal Society in 1660^[35] and the French Académie des Sciences in 1666.^[36]

International scientific organizations, such as the International Council for Science, have since been formed to promote cooperation between the scientific communities of different nations. More recently, influential government agencies have been created to support scientific research, including the National Science Foundation in the U.S.

Other prominent organizations include the academies of science of many nations, CSIRO in Australia, Centre national de la recherche scientifique in France, Max Planck Society and Deutsche Forschungsgemeinschaft in Germany, and in Spain, CSIC.

Literature

Main article: Scientific literature

An enormous range of scientific literature is published.^[37] Scientific journals communicate and document the results of research carried out in universities and various other research institutions, serving as an archival record of science. The first scientific journals, Journal des Sçavans followed by the Philosophical Transactions, began publication in 1665. Since that time the total number of active periodicals has steadily increased. As of 1981, one estimate for the number of scientific and technical journals in publication was 11,500.^[38] While Pubmed lists almost 40,000, related to the medical sciences only.^[39]

Most scientific journals cover a single scientific field and publish the research within that field; the research is normally expressed in the form of a scientific paper. Science has become so pervasive in modern societies that it is generally considered necessary to communicate the achievements, news, and ambitions of scientists to a wider populace.

Science magazines such as New Scientist, Science & Vie and Scientific American cater to the needs of a much wider readership and provide a non-technical summary of popular areas of research, including notable discoveries and advances in certain fields of research. Science books engage the interest of many more people. Tangentially, the science fiction genre, primarily fantastic in nature, engages the public imagination and transmits the ideas, if not the methods, of science.

Recent efforts to intensify or develop links between science and non-scientific disciplines such as Literature or, more specifically, Poetry, include the Creative Writing <-> Science resource developed through the Royal Literary Fund.^[40]

[edit] Electric field

Electric” redirects here. For other uses, see Electric (disambiguation).

Lightning is one of the most dramatic effects of electricity

Electricity (from the Greek word ήλεκτρον, (elektron), meaning amber, and finally from New Latin ēlectricus, “amber-like”) is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena such as lightning and static electricity, but in addition, less familiar concepts such as the electromagnetic field and electromagnetic induction.

In general usage, the word ‘electricity’ is adequate to refer to a number of physical effects. However, in scientific usage, the term is vague, and these related, but distinct, concepts are better identified by more precise terms:

• Electric charge – a property of some subatomic particles, which determines their electromagnetic interactions. Electrically charged matter is influenced by, and produces, electromagnetic fields.
• Electric current – a movement or flow of electrically charged particles, typically measured in amperes.
• Electric field – an influence produced by an electric charge on other charges in its vicinity.
• Electric potential – the capacity of an electric field to do work, typically measured in volts.
• Electromagnetism – a fundamental interaction between the magnetic field and the presence and motion of an electric charge.

Electricity has been studied since antiquity, though scientific advances were not forthcoming until the seventeenth and eighteenth centuries. It would not be until the late nineteenth century, however, that engineers were able to put electricity to industrial and residential use. This period witnessed a rapid expansion in the development of electrical technology. Electricity’s extraordinary versatility as a source of energy means it can be put to an almost limitless set of applications which include transport, heating, lighting, communications, and computation. The backbone of modern industrial society is, and for the foreseeable future can be expected to remain, the use of electrical power.

^{Electric charge is a property of certain subatomic particles, which gives rise to and interacts with, the electromagnetic force, one of the four fundamental forces of nature. Charge originates in the atom, in which its most familiar carriers are the electron and proton. It is a conserved quantity, that is, the net charge within an isolated system will always remain constant regardless of any changes taking place within that system.[15] Within the system, charge may be transferred between bodies, either by direct contact, or by passing along a conducting material, such as a wire.[16] The informal term static electricity refers to the net presence (or ‘imbalance’) of charge on a body, usually caused when dissimilar materials are rubbed together, transferring charge from one to the other.}

Charge on a gold-leaf electroscope causes the leaves to visibly repel each other

The presence of charge gives rise to the electromagnetic force: charges exert a force on each other, an effect that was known, though not understood, in antiquity.^[17] A lightweight ball suspended from a string can be charged by touching it with a glass rod that has itself been charged by rubbing with a cloth. If a similar ball is charged by the same glass rod, it is found to repel the first: the charge acts to force the two balls apart. Two balls that are charged with a rubbed amber rod also repel each other. However, if one ball is charged by the glass rod, and the other by an amber rod, the two balls are found to attract each other. These phenomena were investigated in the late eighteenth century by Charles-Augustin de Coulomb, who deduced that charge manifests itself in two opposing forms, leading to the well-known axiom: like-charged objects repel and opposite-charged objects attract.^[17]

The force acts on the charged particles themselves, hence charge has a tendency to spread itself as evenly as possible over a conducting surface. The magnitude of the electromagnetic force, whether attractive or repulsive, is given by Coulomb’s law, which relates the force to the product of the charges and has an inverse-square relation to the distance between them.^[18][19] The electromagnetic force is very strong, second only in strength to the strong interaction,^[20] but unlike that force it operates over all distances.^[21] In comparison with the much weaker gravitational force, the electromagnetic force pushing two electrons apart is 10⁴² times that of the gravitational attraction pulling them together.^[22]

The charge on electrons and protons is opposite in sign, hence an amount of charge may be expressed as being either negative or positive. By convention, the charge carried by electrons is deemed negative, and that by protons positive, a custom that originated with the work of Benjamin Franklin.^[23] The amount of charge is usually given the symbol Q and expressed in coulombs;^[24] each electron carries the same charge of approximately −1.6022×10⁻¹⁹ coulomb. The proton has a charge that is equal and opposite, and thus +1.6022×10⁻¹⁹  coulomb. Charge is possessed not just by matter, but also by antimatter, each antiparticle bearing an equal and opposite charge to its corresponding particle.^[25]

Charge can be measured by a number of means, an early instrument being the gold-leaf electroscope, which although still in use for classroom demonstrations, has been superseded by the electronic electrometer.^[16]

Electric current

Main article: Current (electricity)

The movement of electric charge is known as an electric current, the intensity of which is usually measured in amperes. Current can consist of any moving charged particles; most commonly these are electrons, but any charge in motion constitutes a current.

By historical convention, a positive current is defined as having the same direction of flow as any positive charge it contains, or to flow from the most positive part of a circuit to the most negative part. Current defined in this manner is called conventional current. The motion of negatively-charged electrons around an electric circuit, one of the most familiar forms of current, is thus deemed positive in the opposite direction to that of the electrons.^[26] However, 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 needs to be explicitly stated.

An electric arc provides an energetic demonstration of electric current

The process by which electric current passes through a material is termed electrical conduction, and its nature varies with that of the charged particles and the material through which they are travelling. Examples of electric currents include metallic conduction, where electrons flow through a conductor such as metal, and electrolysis, where ions (charged atoms) flow through liquids. While the particles themselves can move quite slowly, sometimes with an average drift velocity only fractions of a millimetre per second,^[16] the electric field that drives them itself propagates at close to the speed of light, enabling electrical signals to pass rapidly along wires.^[27]

Current causes several observable effects, which historically were the means of recognising its presence. That water could be decomposed by the current from a voltaic pile was discovered by Nicholson and Carlisle in 1800, a process now known as electrolysis. Their work was greatly expanded upon by Michael Faraday in 1833.^[28] Current through a resistance causes localised heating, an effect James Prescott Joule studied mathematically in 1840.^[28] One of the most important discoveries relating to current was made accidentally by Hans Christian Ørsted in 1820, when, while preparing a lecture, he witnessed the current in a wire disturbing the needle of a magnetic compass.^[29] He had discovered electromagnetism, a fundamental interaction between electricity and magnetics.

In engineering or household applications, current is often described as being either direct current (DC) or alternating current (AC). These terms refer to how the current varies in time. Direct current, as produced by example from a battery and required by most electronic devices, is a unidirectional flow from the positive part of a circuit to the negative.^[30] If, as is most common, this flow is carried by electrons, they will be travelling in the opposite direction. Alternating current is any current that reverses direction repeatedly; almost always this takes the form of a sinusoidal wave.^[31] Alternating current thus pulses back and forth within a conductor without the charge moving any net distance over time. The time-averaged value of an alternating current is zero, but it delivers energy in first one direction, and then the reverse. Alternating current is affected by electrical properties that are not observed under steady state direct current, such as inductance and capacitance.^[32] These properties however can become important when circuitry is subjected to transients, such as when first energised.

Main article: Electric field

See also: Electrostatics

The concept of the electric field was introduced by Michael Faraday. An electric field is created by a charged body in the space that surrounds it, and results in a force exerted on any other charges placed within the field. The electric field acts between two charges in a similar manner to the way that the gravitational field acts between two masses, and like it, extends towards infinity and shows an inverse square relationship with distance.^[21] However, there is an important difference. Gravity always acts in attraction, drawing two masses together, while the electric field can result in either attraction or repulsion. Since large bodies such as planets generally carry no net charge, the electric field at a distance is usually zero. Thus gravity is the dominant force at distance in the universe, despite being much the weaker.^[22]

Field lines emanating from a positive charge above a plane conductor

An electric field generally varies in space,^[33] and its strength at any one point is defined as the force (per unit charge) that would be felt by a stationary, negligible charge if placed at that point.^[34] The conceptual charge, termed a ‘test charge‘, must be vanishingly small to prevent its own electric field disturbing the main field and must also be stationary to prevent the effect of magnetic fields. As the electric field is defined in terms of force, and force is a vector, so it follows that an electric field is also a vector, having both magnitude and direction. Specifically, it is a vector field.^[34]

The study of electric fields created by stationary charges is called electrostatics. The field may be visualised by a set of imaginary lines whose direction at any point is the same as that of the field. This concept was introduced by Faraday,^[35] whose term ‘lines of force‘ still sometimes sees use. The field lines are the paths that a point positive charge would seek to make as it was forced to move within the field; they are however an imaginary concept with no physical existence, and the field permeates all the intervening space between the lines.^[35] Field lines emanating from stationary charges have several key properties: first, that they originate at positive charges and terminate at negative charges; second, that they must enter any good conductor at right angles, and third, that they may never cross nor close in on themselves.^[36]

The principles of electrostatics are important when designing items of high-voltage equipment. There is a finite limit to the electric field strength that may withstood by any medium. Beyond this point, electrical breakdown occurs and an electric arc causes flashover between the charged parts. Air, for example, tends to arc at electric field strengths which exceed 30 kV per centimetre across small gaps. Over larger gaps, its breakdown strength is weaker, perhaps 1 kV per centimetre.^[37] The most visible natural occurrence of this is lightning, caused when charge becomes separated in the clouds by rising columns of air, and raises the electric field in the air to greater than it can withstand. The voltage of a large lightning cloud may be as high as 100 MV and have discharge energies as great as 250 kWh.^[38]

The field strength is greatly affected by nearby conducting objects, and it is particularly intense when it is forced to curve around sharply pointed objects. This principle is exploited in the lightning conductor, the sharp spike of which acts to encourage the lightning stroke to develop there, rather than to the building it serves to protect.^[39]

An electric field is zero inside a conductor. This is because the net charge on a conductor only exists on the surface. External electrostatic fields are always perpendicular to the conductors surface. Otherwise this would produce a force on the charge carriers inside the conductor and so the field would not be static as we assume.

[edit] Electric potential

Main article: Electric potential

See also: Voltage

A pair of AA cells. The + sign indicates the polarity of the potential differences between the battery terminals.

The concept of electric potential is closely linked to that of the electric field. A small charge placed within an electric field experiences a force, and to have brought that charge to that point against the force requires work. The electric potential at any point is defined as the energy required to bring a unit test charge from an infinite distance slowly to that point. It is usually measured in volts, and one volt is the potential for which one joule of work must be expended to bring a charge of one coulomb from infinity.^[40] This definition of potential, while formal, has little practical application, and a more useful concept is that of electric potential difference, and is the energy required to move a unit charge between two specified points. An electric field has the special property that it is conservative, which means that the path taken by the test charge is irrelevant: all paths between two specified points expend the same energy, and thus a unique value for potential difference may be stated.^[40] The volt is so strongly identified as the unit of choice for measurement and description of electric potential difference that the term voltage sees greater everyday usage.

For practical purposes, it is useful to define a common reference point to which potentials may be expressed and compared. While this could be at infinity, a much more useful reference is the Earth itself, which is assumed to be at the same potential everywhere. This reference point naturally takes the name earth or ground. Earth is assumed to be an infinite source of equal amounts of positive and negative charge, and is therefore electrically uncharged – and unchargeable.^[41]

Electric potential is a scalar quantity, that is, it has only magnitude and not direction. It may be viewed as analogous to temperature: as there is a certain temperature at every point in space, and the temperature gradient indicates the direction and magnitude of the driving force behind heat flow, similarly, there is an electric potential at every point in space, and its gradient, or field strength, indicates the direction and magnitude of the driving force behind charge movement. Equally, electric potential may be seen as analogous to height: just as a released object will fall through a difference in heights caused by a gravitational field, so a charge will ‘fall’ across the voltage caused by an electric field.^[42]

The electric field was formally defined as the force exerted per unit charge, but the concept of potential allows for a more useful and equivalent definition: the electric field is the local gradient of the electric potential. Usually expressed in volts per metre, the vector direction of the field is the line of greatest gradient of potential.^[16]

[edit] Electromagnetism

Main article: Electromagnetism

Magnetic field circles around a current

Ørsted’s discovery in 1821 that a magnetic field existed around all sides of a wire carrying an electric current indicated that there was a direct relationship between electricity and magnetism. Moreover, the interaction seemed different from gravitational and electrostatic forces, the two forces of nature then known. The force on the compass needle did not direct it to or away from the current-carrying wire, but acted at right angles to it.^[29] Ørsted’s slightly obscure words were that “the electric conflict acts in a revolving manner.” The force also depended on the direction of the current, for if the flow was reversed, then the force did too.^[43]

Ørsted did not fully understand his discovery, but he observed the effect was reciprocal: a current exerts a force on a magnet, and a magnetic field exerts a force on a current. The phenomenon was further investigated by Ampère, who discovered that two parallel current-carrying wires exerted a force upon each other: two wires conducting currents in the same direction are attracted to each other, while wires containing currents in opposite directions are forced apart.^[44] The interaction is mediated by the magnetic field each current produces and forms the basis for the international definition of the ampere.^[44]

The electric motor exploits an important effect of electromagnetism: a current through a magnetic field experiences a force at right angles to both the field and current

This relationship between magnetic fields and currents is extremely important, for it led to Michael Faraday’s invention of the electric motor in 1821. Faraday’s homopolar motor consisted of a permanent magnet sitting in a pool of mercury. A current was allowed through a wire suspended from a pivot above the magnet and dipped into the mercury. The magnet exerted a tangential force on the wire, making it circle around the magnet for as long as the current was maintained.^[45]

Experimentation by Faraday in 1831 revealed that a wire moving perpendicular to a magnetic field developed a potential difference between its ends. Further analysis of this process, known as electromagnetic induction, enabled him to state the principal, now known as Faraday’s law of induction, that the potential difference induced in a closed circuit is proportional to the rate of change of magnetic flux through the loop. Exploitation of this discovery enabled him to invent the first electrical generator in 1831, in which he converted the mechanical energy of a rotating copper disc to electrical energy.^[45] Faraday’s disc was inefficient and of no use as a practical generator, but it showed the possibility of generating electric power using magnetism, a possibility that would be taken up by those that followed on from his work.

Faraday’s and Ampère’s work showed that a time-varying magnetic field acted as a source of an electric field, and a time-varying electric field was a source of a magnetic field. Thus, when either field is changing in time, then a field of the other is necessarily induced.^[46] Such a phenomenon has the properties of a wave, and is naturally referred to as an electromagnetic wave. Electromagnetic waves were analysed theoretically by James Clerk Maxwell in 1864. Maxwell discovered a set of equations that could unambiguously describe the interrelationship between electric field, magnetic field, electric charge, and electric current. He could moreover prove that such a wave would necessarily travel at the speed of light, and thus light itself was a form of electromagnetic radiation. Maxwell’s Laws, which unify light, fields, and charge are one of the great milestones of theoretical physics.^[46]

[edit] Electric circuits

Main article: Electric circuit

A basic electric circuit. The voltage source V on the left drives a current I around the circuit, delivering electrical energy into the resistance R. From the resistor, the current returns to the source, completing the circuit.

An electric circuit is an interconnection of electric components, usually to perform some useful task, with a return path to enable the charge to return to its source.

The components in an electric circuit can take many forms, which can include elements such as resistors, capacitors, switches, transformers and electronics. Electronic circuits contain active components, usually semiconductors, and typically exhibit non-linear behavior, requiring complex analysis. The simplest electric components are those that are termed passive and linear: while they may temporarily store energy, they contain no sources of it, and exhibit linear responses to stimuli.^[47]

The resistor is perhaps the simplest of passive circuit elements: as its name suggests, it resists the current through it, dissipating its energy as heat. Ohm’s law is a basic law of circuit theory, stating that the current passing through a resistance is directly proportional to the potential difference across it. The ohm, the unit of resistance, was named in honour of Georg Ohm, and is symbolised by the Greek letter Ω. 1 Ω is the resistance that will produce a potential difference of one volt in response to a current of one amp.^[47]

The capacitor is a device capable of storing charge, and thereby storing electrical energy in the resulting field. Conceptually, it consists of two conducting plates separated by a thin insulating layer; in practice, thin metal foils are coiled together, increasing the surface area per unit volume and therefore the capacitance. The unit of capacitance is the farad, named after Michael Faraday, and given the symbol F: one farad is the capacitance that develops a potential difference of one volt when it stores a charge of one coulomb. A capacitor connected to a voltage supply initially causes a current as it accumulates charge; this current will however decay in time as the capacitor fills, eventually falling to zero. A capacitor will therefore not permit a steady state current, but instead blocks it.^[47]

The inductor is a conductor, usually a coil of wire, that stores energy in a magnetic field in response to the current through it. When the current changes, the magnetic field does too, inducing a voltage between the ends of the conductor. The induced voltage is proportional to the time rate of change of the current. The constant of proportionality is termed the inductance. The unit of inductance is the henry, named after Joseph Henry, a contemporary of Faraday. One henry is the inductance that will induce a potential difference of one volt if the current through it changes at a rate of one ampere per second.^[47] The inductor’s behaviour is in some regards converse to that of the capacitor: it will freely allow an unchanging current, but opposes a rapidly changing one.

[edit] Production and uses

[edit] Generation

Main article: Electricity generation

Wind power is of increasing importance in many countries

Thales’ experiments with amber rods were the first studies into the production of electrical energy. While this method, now known as the triboelectric effect, is capable of lifting light objects and even generating sparks, it is extremely inefficient.^[48] It was not until the invention of the voltaic pile in the eighteenth century that a viable source of electricity became available. The voltaic pile, and its modern descendant, the electrical battery, store energy chemically and make it available on demand in the form of electrical energy.^[48] The battery is a versatile and very common power source which is ideally suited to many applications, but its energy storage is finite, and once discharged it must be disposed of or recharged. For large electrical demands electrical energy must be generated and transmitted in bulk.

Electrical energy is usually generated by electro-mechanical generators driven by steam produced from fossil fuel combustion, or the heat released from nuclear reactions; or from other sources such as kinetic energy extracted from wind or flowing water. Such generators bear no resemblance to Faraday’s homopolar disc generator of 1831, but they still rely on his electromagnetic principle that a conductor linking a changing magnetic field induces a potential difference across its ends.^[49] The invention in the late nineteenth century of the transformer meant that electricity could be generated at centralised power stations, benefiting from economies of scale, and be transmitted across countries with increasing efficiency.^[50][51] Since electrical energy cannot easily be stored in quantities large enough to meet demands on a national scale, at all times exactly as much must be produced as is required.^[50] This requires electricity utilities to make careful predictions of their electrical loads, and maintain constant co-ordination with their power stations. A certain amount of generation must always be held in reserve to cushion an electrical grid against inevitable disturbances and losses.

Demand for electricity grows with great rapidity as a nation modernises and its economy develops. The United States showed a 12% increase in demand during each year of the first three decades of the twentieth century,^[52] a rate of growth that is now being experienced by emerging economies such as those of India or China.^[53][54] Historically, the growth rate for electricity demand has outstripped that for other forms of energy, such as coal.^[55]

Environmental concerns with electricity generation have led to an increased focus on generation from renewable sources, in particular from wind- and hydropower. While debate can be expected to continue over the environmental impact of different means of electricity production, its final form is relatively clean.^[56]

[edit] Uses

The light bulb, an early application of electricity, operates by Joule heating: the passage of current through resistance generating heat

Electricity is an extremely flexible form of energy, and has been adapted to a huge, and growing, number of uses.^[57] The invention of a practical incandescent light bulb in the 1870s led to lighting becoming one of the first publicly available applications of electrical power. Although electrification brought with it its own dangers, replacing the naked flames of gas lighting greatly reduced fire hazards within homes and factories.^[58] Public utilities were set up in many cities targeting the burgeoning market for electrical lighting.

The Joule heating effect employed in the light bulb also sees more direct use in electric heating. While this is versatile and controllable, it can be seen as wasteful, since most electrical generation has already required the production of heat at a power station.^[59] A number of countries, such as Denmark, have issued legislation restricting or banning the use of electric heating in new buildings.^[60] Electricity is however a highly practical energy source for refrigeration,^[61] with air conditioning representing a growing sector for electricity demand, the effects of which electricity utilities are increasingly obliged to accommodate.^[62]

Electricity is used within telecommunications, and indeed the electrical telegraph, demonstrated commercially in 1837 by Cooke and Wheatstone, was one of its earliest applications. With the construction of first intercontinental, and then transatlantic, telegraph systems in the 1860s, electricity had enabled communications in minutes across the globe. Optical fibre and satellite communication technology have taken a share of the market for communications systems, but electricity can be expected to remain an essential part of the process.

The effects of electromagnetism are most visibly employed in the electric motor, which provides a clean and efficient means of motive power. A stationary motor such as a winch is easily provided with a supply of power, but a motor that moves with its application, such as an electric vehicle, is obliged to either carry along a power source such as a battery, or by collecting current from a sliding contact such as a pantograph, placing restrictions on its range or performance.

Electronic devices make use of the transistor, perhaps one of the most important inventions of the twentieth century,^[63] and a fundamental building block of all modern circuitry. A modern integrated circuit may contain several billion miniaturised transistors in a region only a few centimetres square.^[64]

[edit] Electricity and the natural world

[edit] Physiological effects

Main article: Electric shock

A voltage applied to a human body causes an electric current through the tissues, and although the relationship is non-linear, the greater the voltage, the greater the current.^[65] The threshold for perception varies with the supply frequency and with the path of the current, but is about 1 mA for mains-frequency electricity.^[66] If the current is sufficiently high, it will cause muscle contraction, fibrillation of the heart, and tissue burns.^[65] The lack of any visible sign that a conductor is electrified makes electricity a particular hazard. The pain caused by an electric shock can be intense, leading electricity at times to be employed as a method of torture. Death caused by an electric shock is referred to as electrocution. Electrocution is still the means of judicial execution in some jurisdictions, though its use has become rarer in recent times.^[67]

[edit] Electrical phenomena in nature

The electric eel, Electrophorus electricus

Electricity is by no means a purely human invention, and may be observed in several forms in nature, a prominent manifestation of which is lightning. The Earth’s magnetic field is thought to arise from a natural dynamo of circulating currents in the planet’s core.^[68] Certain crystals, such as quartz, or even sugarcane, generate a potential difference across their faces when subjected to external pressure.^[69] This phenomenon is known as piezoelectricity, from the Greek piezein (πιέζειν), meaning to press, and was discovered in 1880 by Pierre and Jacques Curie. The effect is reciprocal, and when a piezoelectric material is subjected to an electric field, a small change in physical dimensions take place.^[69]

Some organisms, such as sharks, are able to detect and respond to changes in electric fields, an ability known as electroreception,^[70] while others, termed electrogenic, are able to generate voltages themselves to serve as a predatory or defensive weapon.^[3] The order Gymnotiformes, of which the best known example is the electric eel, detect or stun their prey via high voltages generated from modified muscle cells called electrocytes.^[4][3] All animals transmit information along their cell membranes with voltage pulses called action potentials, whose functions include communication by the nervous system between neurons and muscles.^[71] (Because of this principle, an electric shock can induce temporary or permanent paralysis by “overloading” the nervous system.) They are also responsible for coordinating activities in certain plants.^[71]