Geothermal power (from the Greek words geo, meaning earth, and thermal, meaning heat) is energy generated by heat stored beneath the Earth's surface or the collection of absorbed heat derived from underground in the atmosphere and oceans. Prince Piero Ginori Conti tested the first geothermal generator on 4 July 1904, at the Larderello dry steam field in Italy.[1] The largest group of geothermal power plants in the world is located in The Geysers, a geothermal field in California.[2] As of 2007, geothermal power supplies less than 1% of the world's energy.[3]
Advantages
Krafla Geothermal Station in northeast IcelandGeothermal energy offers a number of advantages over traditional fossil fuel based sources, primarily that the heat source requires no purchase of fuel. From an environmental standpoint, emissions of undesirable substances are small.[4] It is also nearly sustainable because the heat extraction is small compared to the size of the heat reservoir, which may also receive some heat replenishment from greater depths. In addition, geothermal power plants are unaffected by changing weather conditions.[5] Geothermal power plants work continuously, day and night, making them base load power plants. From an economic view, geothermal energy is extremely price competitive in some areas and reduces reliance on fossil fuels and their inherent price unpredictability.[6] It also offers a degree of scalability: a large geothermal plant can power entire cities while smaller power plants can supply more remote sites such as rural villages.[7]
[edit] Disadvantages
From an engineering perspective, the geothermal fluid is corrosive, and worse, is at a relatively low temperature (compared to steam from boilers), which by the laws of thermodynamics limits the efficiency of heat engines in extracting useful energy as in the generation of electricity. Much of the heat energy is lost, unless there is also a local use for low-temperature heat, such as greenhouses or timber mills or district heating, etc.
There are several environmental concerns behind geothermal energy. Construction of the power plants can adversely affect land stability in the surrounding region. This is mainly a concern with Enhanced Geothermal Systems, where water is injected into hot dry rock where no water was before.[8] Dry steam and flash steam power plants also emit low levels of carbon dioxide, nitric oxide, and sulfur, although at roughly 5% of the levels emitted by fossil fuel power plants.[7] However, geothermal plants can be built with emissions-controlling systems that can inject these substances back into the earth, thereby reducing carbon emissions to less than 0.1% of those from fossil fuel power plants.[9] Hot water from geothermal sources will contain trace amounts of dangerous elements such as mercury, arsenic, antimony, etc. which if disposed of into rivers can render their water unsafe to drink.
Although geothermal sites are capable of providing heat for many decades, eventually specific locations may cool down. It is likely that in these locations, the system was designed too large for the site, since there is only so much energy that can be stored and replenished in a given volume of earth. Some interpret this as meaning a specific geothermal location can undergo depletion, and question whether geothermal energy is truly renewable. For example, the world's second-oldest geothermal generator at Wairakei has reduced production. If left alone, however, these places will recover some of their lost heat, as the mantle has vast heat reserves[citation needed]. An assessment of the total potential for electricity production from the high-temperature geothermal fields in Iceland gives a value of about 1500 TWh (total) or 15 TWh per year over a 100 year period. The electricity production capacity from geothermal fields is now only 1.3 TWh per year. [10]
[edit] Potential
If heat recovered by ground source heat pumps is included, the non-electric generating capacity of geothermal energy is estimated at more than 100 GW (gigawatts of thermal power) and is used commercially in over 70 countries. During 2005, contracts were placed for an additional 0.5 GW of capacity in the United States, while there were also plants under construction in 11 other countries.[11]
Estimates of exploitable worldwide geothermal energy resources vary considerably. According to a 1999 study, it was thought that this might amount to between 65 and 138 GW of electrical generation capacity 'using enhanced technology'.[12]
A 2006 report by MIT, that took into account the use of Enhanced Geothermal Systems (EGS), concluded that it would be affordable to generate 100 GWe (gigawatts of electricity) or more by 2050 in the United States alone, for a maximum investment of 1 billion US dollars in research and development over 15 years.[11]
The MIT report calculated the world's total EGS resources to be over 13,000 ZJ. Of these, over 200 ZJ would be extractable, with the potential to increase this to over 2,000 ZJ with technology improvements - sufficient to provide all the world's energy needs for several millennia.[11]
The key characteristic of an EGS (also called a Hot Dry Rock system), is that it reaches at least 10 km down into hard rock. At a typical site two holes would be bored and the deep rock between them fractured. Water would be pumped down one and steam would come up the other. The MIT report estimated that there was enough energy in hard rocks 10 km below the United States to supply all the world's current needs for 30,000 years. [11]
Drilling at this depth is now possible in the petroleum industry, albeit it is expensive. (Exxon announced an 11 km hole at the Chayvo field, Sakhalin. Lloyds List 1/5/07 p 6) Wells drilled to depths greater than 4000 metres generally incur drilling costs in the tens of millions of dollars. The technological challenges are to drill wide bores at low cost and to break rock over larger volumes. Apart from the energy used to make the bores, the process releases no greenhouse gases.
Other important countries considered high in potential for development are the People's Republic of China, Hungary, Mexico, Iceland, and New Zealand. There are a number of potential sites being developed or evaluated in South Australia that are several kilometres in depth.
Thursday, July 31, 2008
Hydroelectricity
A hydroelectric dam and plant on the Muskegon river in MichiganMain article: Hydroelectricity
Hydroelectric dams impound a reservoir of water and release it through one or more water turbines to generate electricity.
[edit] Pumped storage
A pumped storage hydroelectric power plant is a net consumer of energy but decreases the price of electricity. Water is pumped to a high reservoir during the night when the demand, and price, for electricity is low. During hours of peak demand, when the price of electricity is high, the stored water is released to produce electric power.
[edit] Solar
Main article: Solar power
A control room of a waste incineration power plantA solar photovoltaic power plant converts sunlight into electrical energy, which may need conversion to alternating current for transmission to users. This type of plant does not use rotating machines for energy conversion. Solar thermal electric plants are another type of solar power plant. They direct sunlight using either parabolic troughs or heliostats. Parabolic troughs direct sunlight onto a pipe containing a heat transfer fluid, such as oil, which is then used to boil water, which turns the generator. The central tower type of power plant uses hundreds or thousands of mirrors, depending on size, to direct sunlight onto a receiver on top of a tower. Again, the heat is used to produce steam to turn turbines. There is yet another type of solar thermal electric plant. The sunlight strikes the bottom of the pond, warming the lowest layer which is prevented from rising by a salt gradient. A Rankine cycle engine exploits the temperature difference in the layers to produce electricity. Not many solar thermal electric plants have been built. Most of them can be found in the Mojave Desert, although Sandia National Laboratory, Israel and Spain have also built a few plants.
[edit] Wind
Main article: Wind power
Wind turbine in front of a thermal power station in Amsterdam, NetherlandsWind turbines can be used to generate electricity in areas with strong, steady winds. Many different designs have been used in the past, but almost all modern turbines being produced today use a three-bladed, upwind design. Grid-connected wind turbines now being built are much larger than the units installed during the 1970s, and so produce power more cheaply and reliably than earlier models. With larger turbines (on the order of one megawatt), the blades move more slowly than older, smaller, units, which makes them less visually distracting and safer for airborne animals. However, the old turbines can still be seen at some wind farms, particularly at Altamont Pass and Tehachapi Pass.
[edit] Operations
The power station operator has several duties in the electrical generating facility. Operators are responsible for the safety of the work crews that frequently do repairs on the mechanical and electrical equipment. They maintain the equipment with periodic inspections and logs temperatures, pressures and other important information on regular intervals. Operators are responsible for starting and stopping the generators depending on need. They are able to synchronize and adjust the voltage output of the added generation with the running electrical system without upsetting the system. They must know the electrical and mechanical systems in order to troubleshoot problems in the facility and add to the reliability of the facility. Operators must be able to respond to an emergency and know the procedures in place to deal with it.
A hydroelectric dam and plant on the Muskegon river in MichiganMain article: Hydroelectricity
Hydroelectric dams impound a reservoir of water and release it through one or more water turbines to generate electricity.
[edit] Pumped storage
A pumped storage hydroelectric power plant is a net consumer of energy but decreases the price of electricity. Water is pumped to a high reservoir during the night when the demand, and price, for electricity is low. During hours of peak demand, when the price of electricity is high, the stored water is released to produce electric power.
[edit] Solar
Main article: Solar power
A control room of a waste incineration power plantA solar photovoltaic power plant converts sunlight into electrical energy, which may need conversion to alternating current for transmission to users. This type of plant does not use rotating machines for energy conversion. Solar thermal electric plants are another type of solar power plant. They direct sunlight using either parabolic troughs or heliostats. Parabolic troughs direct sunlight onto a pipe containing a heat transfer fluid, such as oil, which is then used to boil water, which turns the generator. The central tower type of power plant uses hundreds or thousands of mirrors, depending on size, to direct sunlight onto a receiver on top of a tower. Again, the heat is used to produce steam to turn turbines. There is yet another type of solar thermal electric plant. The sunlight strikes the bottom of the pond, warming the lowest layer which is prevented from rising by a salt gradient. A Rankine cycle engine exploits the temperature difference in the layers to produce electricity. Not many solar thermal electric plants have been built. Most of them can be found in the Mojave Desert, although Sandia National Laboratory, Israel and Spain have also built a few plants.
[edit] Wind
Main article: Wind power
Wind turbine in front of a thermal power station in Amsterdam, NetherlandsWind turbines can be used to generate electricity in areas with strong, steady winds. Many different designs have been used in the past, but almost all modern turbines being produced today use a three-bladed, upwind design. Grid-connected wind turbines now being built are much larger than the units installed during the 1970s, and so produce power more cheaply and reliably than earlier models. With larger turbines (on the order of one megawatt), the blades move more slowly than older, smaller, units, which makes them less visually distracting and safer for airborne animals. However, the old turbines can still be seen at some wind farms, particularly at Altamont Pass and Tehachapi Pass.
[edit] Operations
The power station operator has several duties in the electrical generating facility. Operators are responsible for the safety of the work crews that frequently do repairs on the mechanical and electrical equipment. They maintain the equipment with periodic inspections and logs temperatures, pressures and other important information on regular intervals. Operators are responsible for starting and stopping the generators depending on need. They are able to synchronize and adjust the voltage output of the added generation with the running electrical system without upsetting the system. They must know the electrical and mechanical systems in order to troubleshoot problems in the facility and add to the reliability of the facility. Operators must be able to respond to an emergency and know the procedures in place to deal with it.
POWER STATIONS AND ELECTRICITY
A power station (also referred to as generating station or power plant) is an industrial facility for the generation of electric power.[1][2][3]
Power plant is also used to refer to the engine in ships, aircraft and other large vehicles. Some prefer to use the term energy center because it more accurately describes what the plants do, which is the conversion of other forms of energy, like chemical energy, gravitational potential energy or heat energy into electrical energy. However, power plant is the most common term in the U.S., while elsewhere power station and power plant are both widely used, power station prevailing in many Commonwealth countries and especially in the United Kingdom.
At the center of nearly all power stations is a generator, a rotating machine that converts mechanical energy into electrical energy by creating relative motion between a magnetic field and a conductor. The energy source harnessed to turn the generator varies widely. It depends chiefly on what fuels are easily available and the types of technology that the power company has access to.
Thermal power stations
Rotor of a modern steam turbine, used in power stationMain article: Thermal power station
In thermal power stations, mechanical power is produced by a heat engine, which transforms thermal energy, often from combustion of a fuel, into rotational energy. Most thermal power stations produce steam, and these are sometimes called steam power stations. About 80% of all electric power is generated by use of steam turbines.[citation needed] Not all thermal energy can be transformed to mechanical power, according to the second law of thermodynamics. Therefore, there is always heat lost to the environment. If this loss is employed as useful heat, for industrial processes or district heating, the power plant is referred to as a cogeneration power plant or CHP (combined heat-and-power) plant. In countries where district heating is common, there are dedicated heat plants called heat-only boiler stations. An important class of power stations in the Middle East uses byproduct heat for desalination of water.
[edit] Classification
CHP plant in Warsaw, Poland
Geothermal power station in Iceland
Coal Power Station in Tampa FL
480 megawatt GE H series power generation gas turbineThermal power plants are classified by the type of fuel and the type of prime mover installed.
[edit] By fuel
Nuclear power plants[4] use a nuclear reactor's heat to operate a steam turbine generator.
Fossil fuelled power plants may also use a steam turbine generator or in the case of natural gas fired plants may use a combustion turbine.
Geothermal power plants use steam extracted from hot underground rocks.
Renewable energy plants may be fuelled by waste from sugar cane, municipal solid waste, landfill methane, or other forms of biomass.
In integrated steel mills, blast furnace exhaust gas is a low-cost, although low-energy-density, fuel.
Waste heat from industrial processes is occasionally concentrated enough to use for power generation, usually in a steam boiler and turbine.
[edit] By prime mover
Steam turbine plants use the dynamic pressure generated by expanding steam to turn the blades of a turbine. Almost all large non-hydro plants use this system.
Gas turbine plants use the dynamic pressure from flowing gases to directly operate the turbine. Natural-gas fuelled turbine plants can start rapidly and so are used to supply "peak" energy during periods of high demand, though at higher cost than base-loaded plants. These may be comparatively small units, and sometimes completely unmanned, being remotely operated. This type was pioneered by the UK, Princetown[5] being the world's first, commissioned in 1959.
Combined cycle plants have both a gas turbine fired by natural gas, and a steam boiler and steam turbine which use the exhaust gas from the gas turbine to produce electricity. This greatly increases the overall efficiency of the plant, and many new baseload power plants are combined cycle plants fired by natural gas.
Internal combustion Reciprocating engines are used to provide power for isolated communities and are frequently used for small cogeneration plants. Hospitals, office buildings, industrial plants, and other critical facilities also use them to provide backup power in case of a power outage. These are usually fuelled by diesel oil, heavy oil, natural gas and landfill gas.
Microturbines, Stirling engine and internal combustion reciprocating engines are low cost solutions for using opportunity fuels, such as landfill gas, digester gas from water treatment plants and waste gas from oil production.
A power station (also referred to as generating station or power plant) is an industrial facility for the generation of electric power.[1][2][3]
Power plant is also used to refer to the engine in ships, aircraft and other large vehicles. Some prefer to use the term energy center because it more accurately describes what the plants do, which is the conversion of other forms of energy, like chemical energy, gravitational potential energy or heat energy into electrical energy. However, power plant is the most common term in the U.S., while elsewhere power station and power plant are both widely used, power station prevailing in many Commonwealth countries and especially in the United Kingdom.
At the center of nearly all power stations is a generator, a rotating machine that converts mechanical energy into electrical energy by creating relative motion between a magnetic field and a conductor. The energy source harnessed to turn the generator varies widely. It depends chiefly on what fuels are easily available and the types of technology that the power company has access to.
Thermal power stations
Rotor of a modern steam turbine, used in power stationMain article: Thermal power station
In thermal power stations, mechanical power is produced by a heat engine, which transforms thermal energy, often from combustion of a fuel, into rotational energy. Most thermal power stations produce steam, and these are sometimes called steam power stations. About 80% of all electric power is generated by use of steam turbines.[citation needed] Not all thermal energy can be transformed to mechanical power, according to the second law of thermodynamics. Therefore, there is always heat lost to the environment. If this loss is employed as useful heat, for industrial processes or district heating, the power plant is referred to as a cogeneration power plant or CHP (combined heat-and-power) plant. In countries where district heating is common, there are dedicated heat plants called heat-only boiler stations. An important class of power stations in the Middle East uses byproduct heat for desalination of water.
[edit] Classification
CHP plant in Warsaw, Poland
Geothermal power station in Iceland
Coal Power Station in Tampa FL
480 megawatt GE H series power generation gas turbineThermal power plants are classified by the type of fuel and the type of prime mover installed.
[edit] By fuel
Nuclear power plants[4] use a nuclear reactor's heat to operate a steam turbine generator.
Fossil fuelled power plants may also use a steam turbine generator or in the case of natural gas fired plants may use a combustion turbine.
Geothermal power plants use steam extracted from hot underground rocks.
Renewable energy plants may be fuelled by waste from sugar cane, municipal solid waste, landfill methane, or other forms of biomass.
In integrated steel mills, blast furnace exhaust gas is a low-cost, although low-energy-density, fuel.
Waste heat from industrial processes is occasionally concentrated enough to use for power generation, usually in a steam boiler and turbine.
[edit] By prime mover
Steam turbine plants use the dynamic pressure generated by expanding steam to turn the blades of a turbine. Almost all large non-hydro plants use this system.
Gas turbine plants use the dynamic pressure from flowing gases to directly operate the turbine. Natural-gas fuelled turbine plants can start rapidly and so are used to supply "peak" energy during periods of high demand, though at higher cost than base-loaded plants. These may be comparatively small units, and sometimes completely unmanned, being remotely operated. This type was pioneered by the UK, Princetown[5] being the world's first, commissioned in 1959.
Combined cycle plants have both a gas turbine fired by natural gas, and a steam boiler and steam turbine which use the exhaust gas from the gas turbine to produce electricity. This greatly increases the overall efficiency of the plant, and many new baseload power plants are combined cycle plants fired by natural gas.
Internal combustion Reciprocating engines are used to provide power for isolated communities and are frequently used for small cogeneration plants. Hospitals, office buildings, industrial plants, and other critical facilities also use them to provide backup power in case of a power outage. These are usually fuelled by diesel oil, heavy oil, natural gas and landfill gas.
Microturbines, Stirling engine and internal combustion reciprocating engines are low cost solutions for using opportunity fuels, such as landfill gas, digester gas from water treatment plants and waste gas from oil production.
Records and locations
On average, lightning flashes occur on earth about 100 times every second. 80% of these flashes are in-cloud and 20% are cloud-to-ground.[citation needed] For most landmasses, lightning strikes most often during the summer, limiting the strike numbers. The spot with the most lightning lies deep in the mountains of eastern Democratic Republic of the Congo, near the small village of Kifuka which has an elevation of 3,200 feet (975 m). Thunderbolts pelt this land, and each year on average, 158 bolts occur over each square kilometer (equivalent to 10 city-blocks square).[76] Singapore has one of the highest rates of lightning activity in the world.[77] The city of Teresina in northern Brazil has the third-highest rate of occurrences of lightning strikes in the world. The surrounding region is referred to as the Chapada do Corisco ("Flash Lightning Flatlands").[78] In the US, Central Florida sees more lightning than any other area. For example, in what is called "Lightning Alley", an area from Tampa, to Orlando, there are as many as 50 strikes per square mile (about 20 per km²) per year.[79][80] The Empire State Building is struck by lightning on average 23 times each year, and was once struck 8 times in 24 minutes.[81]
Lightning as viewed from the Empire State BuildingRoy Sullivan held a Guinness World Record after surviving 7 different lightning strikes across 35 years.[82]
In July 2007, lightning killed up to 30 people when it struck a remote mountain village Ushari Dara in northwestern Pakistan.[83]
Lightning can also strike indoor pools, directed into the pump by electrical circuits from outdoor power poles. Such strikes could potentially kill people who are swimming or walking on wet floors around a pool. In 2000, lightning killed two boys in an outdoor pool in Florida.[84]
A single lightning strike can have a potential of a billion volts and deliver 100,000 amperes of current. If a bolt directly hits a marine animal swimming on the surface, it will undoubtedly hurt or kill the animal. Lightning strikes have killed or injured people on the surface more than 30 yards away.[85]
On 31 October 2005, sixty-eight dairy cows, all in full milk, died on a farm at Fernbrook on the Waterfall Way near Dorrigo, New South Wales after being struck by lightning. Three others were paralysed for several hours but they later made a full recovery. The cows were sheltering under a tree when it was struck by lightning and the electricity spread onto the surrounding soil killing the animals.[86]
Lightning rarely strikes the open ocean, although some sea regions are lightning "hot spots." Winter storms passing off the east coast of the United States often erupt with electrical activity when they cross the warm waters of the Gulf Stream.[85]The Gulf Stream, for example, has roughly as many lightning strikes as the southern plains of the USA.
[edit] Lightning detection
Lightning discharges generate a wide range of electromagnetic radiations, including radio-frequency pulses. The times at which a pulse from a given lightning discharge arrive at several receivers can be used to locate the source of the discharge. The United States federal government has constructed a nation-wide grid of such lightning detectors, allowing lightning discharges to be tracked in real time throughout the continental U.S.[87][88]
In addition to ground-based lightning detection, several instruments aboard satellites have been constructed to observe lightning distribution. These include the Optical Transient Detector (OTD), aboard OrbView-1 satellite launched on April 3, 1995, and the subsequent Lightning Imaging Sensor (LIS) aboard TRMM launched on November 28, 1997.[89][90][91]
For more information, see Lightning detector.
[edit] Most spectacular lightning strike incidences
Several spectacular lightning incidences have occurred, either with people killed or great damage caused. The following incomplete list shows some cases:
Please help improve this article or section by expanding it. Further information might be found on the talk page or at requests for expansion. (March 2008)
This section needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (June 2008)
1902: A lightning strike damaged the upper section of the Eiffel Tower, requiring the reconstruction of its top[92]
December 8th, 1963: Pan Am Flight 214 crashed as result of a lightning strike, and 81 people were killed.
July 1970, the central mast of the Orlunda radio transmitter collapsed after a lightning strike destroyed its basement insulator.
December 24th, 1971: LANSA Flight 508 crashed as a result of lightning in Peru, with 91 people killed.[93]
August 2004: Lightning strike killed 31 jersey cows sheltering under a tree in Denmark.[94]
On average, lightning flashes occur on earth about 100 times every second. 80% of these flashes are in-cloud and 20% are cloud-to-ground.[citation needed] For most landmasses, lightning strikes most often during the summer, limiting the strike numbers. The spot with the most lightning lies deep in the mountains of eastern Democratic Republic of the Congo, near the small village of Kifuka which has an elevation of 3,200 feet (975 m). Thunderbolts pelt this land, and each year on average, 158 bolts occur over each square kilometer (equivalent to 10 city-blocks square).[76] Singapore has one of the highest rates of lightning activity in the world.[77] The city of Teresina in northern Brazil has the third-highest rate of occurrences of lightning strikes in the world. The surrounding region is referred to as the Chapada do Corisco ("Flash Lightning Flatlands").[78] In the US, Central Florida sees more lightning than any other area. For example, in what is called "Lightning Alley", an area from Tampa, to Orlando, there are as many as 50 strikes per square mile (about 20 per km²) per year.[79][80] The Empire State Building is struck by lightning on average 23 times each year, and was once struck 8 times in 24 minutes.[81]
Lightning as viewed from the Empire State BuildingRoy Sullivan held a Guinness World Record after surviving 7 different lightning strikes across 35 years.[82]
In July 2007, lightning killed up to 30 people when it struck a remote mountain village Ushari Dara in northwestern Pakistan.[83]
Lightning can also strike indoor pools, directed into the pump by electrical circuits from outdoor power poles. Such strikes could potentially kill people who are swimming or walking on wet floors around a pool. In 2000, lightning killed two boys in an outdoor pool in Florida.[84]
A single lightning strike can have a potential of a billion volts and deliver 100,000 amperes of current. If a bolt directly hits a marine animal swimming on the surface, it will undoubtedly hurt or kill the animal. Lightning strikes have killed or injured people on the surface more than 30 yards away.[85]
On 31 October 2005, sixty-eight dairy cows, all in full milk, died on a farm at Fernbrook on the Waterfall Way near Dorrigo, New South Wales after being struck by lightning. Three others were paralysed for several hours but they later made a full recovery. The cows were sheltering under a tree when it was struck by lightning and the electricity spread onto the surrounding soil killing the animals.[86]
Lightning rarely strikes the open ocean, although some sea regions are lightning "hot spots." Winter storms passing off the east coast of the United States often erupt with electrical activity when they cross the warm waters of the Gulf Stream.[85]The Gulf Stream, for example, has roughly as many lightning strikes as the southern plains of the USA.
[edit] Lightning detection
Lightning discharges generate a wide range of electromagnetic radiations, including radio-frequency pulses. The times at which a pulse from a given lightning discharge arrive at several receivers can be used to locate the source of the discharge. The United States federal government has constructed a nation-wide grid of such lightning detectors, allowing lightning discharges to be tracked in real time throughout the continental U.S.[87][88]
In addition to ground-based lightning detection, several instruments aboard satellites have been constructed to observe lightning distribution. These include the Optical Transient Detector (OTD), aboard OrbView-1 satellite launched on April 3, 1995, and the subsequent Lightning Imaging Sensor (LIS) aboard TRMM launched on November 28, 1997.[89][90][91]
For more information, see Lightning detector.
[edit] Most spectacular lightning strike incidences
Several spectacular lightning incidences have occurred, either with people killed or great damage caused. The following incomplete list shows some cases:
Please help improve this article or section by expanding it. Further information might be found on the talk page or at requests for expansion. (March 2008)
This section needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (June 2008)
1902: A lightning strike damaged the upper section of the Eiffel Tower, requiring the reconstruction of its top[92]
December 8th, 1963: Pan Am Flight 214 crashed as result of a lightning strike, and 81 people were killed.
July 1970, the central mast of the Orlunda radio transmitter collapsed after a lightning strike destroyed its basement insulator.
December 24th, 1971: LANSA Flight 508 crashed as a result of lightning in Peru, with 91 people killed.[93]
August 2004: Lightning strike killed 31 jersey cows sheltering under a tree in Denmark.[94]
Triggered lightning
[edit] Rocket-triggered
Volcanic material thrust high into the atmosphere can trigger lightning.Lightning has been triggered directly by human activity in several instances. Lightning struck the Apollo 12 soon after takeoff, and has struck soon after thermonuclear explosions.[52] It has also been triggered by launching lightning rockets carrying spools of wire into thunderstorms. The wire unwinds as the rocket ascends, providing a path for lightning. These bolts are typically very straight due to the path created by the wire.[53]
Flying aircraft can trigger lightning.[54]
[edit] Volcanically-triggered
Extremely large volcanic eruptions, which eject gases and material high into the atmosphere, can trigger lightning. This phenomenon was documented by Pliny The Elder during the AD79 eruption of Vesuvius, in which he perished.[55]
[edit] Laser-triggered
Since at least the 1970s, researchers have attempted to trigger lightning strikes by means of ultra-violet lasers, which create a channel of ionized gas through which the lightning would be conducted to ground. Such triggered lightning is intended to protect rocket launching pads, electric power facilities, and other sensitive targets.[56][57][58][59][60][61]
In New Mexico, U.S., scientists tested a new terawatt laser which provoked lightning. Scientists fired ultra-fast pulses from an extremely powerful laser thus sending several terawatts into the clouds to call down electrical discharges in storm clouds over the region.
The beams sent from the laser make channels of ionized molecules known as "filaments". Before the lightning strikes earth, the filaments lead electricity through the clouds, playing the role of lightning rods.
Researchers generated filaments that lived too short a period to trigger a real lightning strike. Nevertheless, a boost in electrical activity within the clouds was registered. According to the French and German scientists, who ran the experiment, the fast pulses sent from the laser will be able to provoke lightning strikes on demand.[62]
[edit] Extraterrestrial lightning
Lightning requires the electrical breakdown of a gas, so it cannot exist in a visual form in the vacuum of space. However, lightning has been observed within the atmospheres of other planets, such as Venus, Jupiter and Saturn. Lightning on Venus is still a controversial subject after decades of study. During the Soviet Venera and U.S. Pioneer missions of the 1970s and '80s, signals suggesting lightning may be present in the upper atmosphere were detected.[63] However, recently the Cassini-Huygens mission fly-by of Venus detected no signs of lightning at all. Despite this, in 2007, radio pulses recorded by the spacecraft Venus Express confirmed lightning on Venus.(S&T, Mar. 2008)
[edit] Trees and lightning
Lightning damage to tree in Maplewood, NJ
Eucalyptus tree that was blown apart by a lightning strikeTrees are frequent conductors of lightning to the ground.[64] Since sap is a poor conductor, its electrical resistance causes it to be heated explosively into steam, which blows off the bark outside the lightning's path. In following seasons trees overgrow the damaged area and may cover it completely, leaving only a vertical scar. If the damage is severe, the tree may not be able to recover, and decay sets in, eventually killing the tree. It is commonly thought that a tree standing alone is more frequently struck, though in some forested areas, lightning scars can be seen on almost every tree.
A eucalyptus tree that was struck by lightning but 2 pine trees next to the tree are untouched, Darwin, Northern TerritoryAfter the two most frequently struck tree types, the Oak and the Elm,[65] the Pine tree is also quite often hit by lightning. Unlike the Oak, which has a relatively shallow root structure, pine trees have a deep central root system that goes down into the water table.[66] Pine trees usually stand taller than other species, which also makes them a likely target. Factors which lead to its being targeted are a high resin content, loftiness, and its needles which lend themselves to a high electrical discharge during a thunderstorm.
Trees are natural lightning conductors, and are known to provide protection against lightning damages to the nearby buildings. Tall trees with high biomass for the root system provide good lightning protection. An example is the teak tree (Tectona grandis), which grows to a height of 45 metres (147.6 ft). It has a spread root system with a spread of 5 m and a biomass of 4 times that of the trunk; its penetration into the soil is 1.25 metres (4.10 ft) and has no tap root. When planted near a building, its height helps in catching the oncoming lightning leader, and the high biomass of the root system helps in dissipation of the lightning charges.[67]
Lightning currents have a very fast risetime, on the order of 40 kA per microsecond. Hence, conductors of such currents exhibit marked skin effect, causing most of the currents to flow through the conductor skin.[68] The effective resistance of the conductor is consequently very high and therefore, the conductor skin gets heated up much more than the conductor core. When a tree acts as a natural lightning conductor, due to skin effect most of the lightning currents flow through the skin of the tree and the sap wood. As a result, the skin gets burnt and may even peel off. The moisture in the skin and the sap wood evaporates instantaneously and may get split. If the tree struck by lightning is a teak tree (single stemmed with branches) it may not be completely destroyed since only the tree skin and a branch may be affected; the major parts of the tree may be saved from complete destruction due to lightning currents. But if the tree involved is a coconut tree it may be completely destroyed by the lightning currents.[citation needed]
[edit] Fulgurites
Main article: Fulgurite
Lightning strikes on sandy soil can produce fulgurites. These root-shaped tubes of melted and fused sand grains are sometimes called petrified lightning.
[edit] X-rays and lightning
The production of X-rays by a bolt of lightning was theoretically predicted as early as 1925 but no evidence was found until 2001/2002, when researchers at the New Mexico Institute of Mining and Technology, detected x-ray emissions from an induced lightning strike along a wire trailed behind a rocket shot into a storm cloud. In the same year University of Florida and Florida Tech researchers used an array of electric field and X-ray detectors at a lightning research facility in North Florida to confirm that natural lightning makes X-rays in large quantities. The cause of the X-ray emissions is still a matter for research. The temperature of lightning is too cold to account for the X-rays observed. [69]
[edit] Rocket-triggered
Volcanic material thrust high into the atmosphere can trigger lightning.Lightning has been triggered directly by human activity in several instances. Lightning struck the Apollo 12 soon after takeoff, and has struck soon after thermonuclear explosions.[52] It has also been triggered by launching lightning rockets carrying spools of wire into thunderstorms. The wire unwinds as the rocket ascends, providing a path for lightning. These bolts are typically very straight due to the path created by the wire.[53]
Flying aircraft can trigger lightning.[54]
[edit] Volcanically-triggered
Extremely large volcanic eruptions, which eject gases and material high into the atmosphere, can trigger lightning. This phenomenon was documented by Pliny The Elder during the AD79 eruption of Vesuvius, in which he perished.[55]
[edit] Laser-triggered
Since at least the 1970s, researchers have attempted to trigger lightning strikes by means of ultra-violet lasers, which create a channel of ionized gas through which the lightning would be conducted to ground. Such triggered lightning is intended to protect rocket launching pads, electric power facilities, and other sensitive targets.[56][57][58][59][60][61]
In New Mexico, U.S., scientists tested a new terawatt laser which provoked lightning. Scientists fired ultra-fast pulses from an extremely powerful laser thus sending several terawatts into the clouds to call down electrical discharges in storm clouds over the region.
The beams sent from the laser make channels of ionized molecules known as "filaments". Before the lightning strikes earth, the filaments lead electricity through the clouds, playing the role of lightning rods.
Researchers generated filaments that lived too short a period to trigger a real lightning strike. Nevertheless, a boost in electrical activity within the clouds was registered. According to the French and German scientists, who ran the experiment, the fast pulses sent from the laser will be able to provoke lightning strikes on demand.[62]
[edit] Extraterrestrial lightning
Lightning requires the electrical breakdown of a gas, so it cannot exist in a visual form in the vacuum of space. However, lightning has been observed within the atmospheres of other planets, such as Venus, Jupiter and Saturn. Lightning on Venus is still a controversial subject after decades of study. During the Soviet Venera and U.S. Pioneer missions of the 1970s and '80s, signals suggesting lightning may be present in the upper atmosphere were detected.[63] However, recently the Cassini-Huygens mission fly-by of Venus detected no signs of lightning at all. Despite this, in 2007, radio pulses recorded by the spacecraft Venus Express confirmed lightning on Venus.(S&T, Mar. 2008)
[edit] Trees and lightning
Lightning damage to tree in Maplewood, NJ
Eucalyptus tree that was blown apart by a lightning strikeTrees are frequent conductors of lightning to the ground.[64] Since sap is a poor conductor, its electrical resistance causes it to be heated explosively into steam, which blows off the bark outside the lightning's path. In following seasons trees overgrow the damaged area and may cover it completely, leaving only a vertical scar. If the damage is severe, the tree may not be able to recover, and decay sets in, eventually killing the tree. It is commonly thought that a tree standing alone is more frequently struck, though in some forested areas, lightning scars can be seen on almost every tree.
A eucalyptus tree that was struck by lightning but 2 pine trees next to the tree are untouched, Darwin, Northern TerritoryAfter the two most frequently struck tree types, the Oak and the Elm,[65] the Pine tree is also quite often hit by lightning. Unlike the Oak, which has a relatively shallow root structure, pine trees have a deep central root system that goes down into the water table.[66] Pine trees usually stand taller than other species, which also makes them a likely target. Factors which lead to its being targeted are a high resin content, loftiness, and its needles which lend themselves to a high electrical discharge during a thunderstorm.
Trees are natural lightning conductors, and are known to provide protection against lightning damages to the nearby buildings. Tall trees with high biomass for the root system provide good lightning protection. An example is the teak tree (Tectona grandis), which grows to a height of 45 metres (147.6 ft). It has a spread root system with a spread of 5 m and a biomass of 4 times that of the trunk; its penetration into the soil is 1.25 metres (4.10 ft) and has no tap root. When planted near a building, its height helps in catching the oncoming lightning leader, and the high biomass of the root system helps in dissipation of the lightning charges.[67]
Lightning currents have a very fast risetime, on the order of 40 kA per microsecond. Hence, conductors of such currents exhibit marked skin effect, causing most of the currents to flow through the conductor skin.[68] The effective resistance of the conductor is consequently very high and therefore, the conductor skin gets heated up much more than the conductor core. When a tree acts as a natural lightning conductor, due to skin effect most of the lightning currents flow through the skin of the tree and the sap wood. As a result, the skin gets burnt and may even peel off. The moisture in the skin and the sap wood evaporates instantaneously and may get split. If the tree struck by lightning is a teak tree (single stemmed with branches) it may not be completely destroyed since only the tree skin and a branch may be affected; the major parts of the tree may be saved from complete destruction due to lightning currents. But if the tree involved is a coconut tree it may be completely destroyed by the lightning currents.[citation needed]
[edit] Fulgurites
Main article: Fulgurite
Lightning strikes on sandy soil can produce fulgurites. These root-shaped tubes of melted and fused sand grains are sometimes called petrified lightning.
[edit] X-rays and lightning
The production of X-rays by a bolt of lightning was theoretically predicted as early as 1925 but no evidence was found until 2001/2002, when researchers at the New Mexico Institute of Mining and Technology, detected x-ray emissions from an induced lightning strike along a wire trailed behind a rocket shot into a storm cloud. In the same year University of Florida and Florida Tech researchers used an array of electric field and X-ray detectors at a lightning research facility in North Florida to confirm that natural lightning makes X-rays in large quantities. The cause of the X-ray emissions is still a matter for research. The temperature of lightning is too cold to account for the X-rays observed. [69]
Dry lightning
Dry lightning is a term in the United States for thunderstorms which produce no precipitation at the surface. This type of lightning is the most common natural cause of wildfires. Dry lightning may also be referred to as heat lightning.
[edit] Rocket lightning
It is a form of cloud discharge, generally horizontal and at cloud base, with a luminous channel appearing to advance through the air with visually resolvable speed, often intermittently.[33]
The movement has been compared to that of a skyrocket, hence its name. It is also one of the rarest of cloud discharges.[34]
[edit] Cloud-to-ground
Cloud-to-ground lightning is a great lightning discharge between a cumulonimbus cloud and the ground initiated by the downward-moving leader stroke. This is the second most common type of lightning, and poses the greatest threat to life and property of all known types.
[edit] Bead lightning
Bead lightning is a type of cloud-to-ground lightning which appears to break up into a string of short, bright sections, which last longer than the usual discharge channel. It is fairly rare. Several theories have been proposed to explain it; one is that the observer sees portions of the lightning channel end on, and that these portions appear especially bright. Another is that, in bead lightning, the width of the lightning channel varies; as the lightning channel cools and fades, the wider sections cool more slowly and remain visible longer, appearing as a string of beads.[35][36]
[edit] Ribbon lightning
Ribbon lightning occurs in thunderstorms with high cross winds and multiple return strokes. The wind will blow each successive return stroke slightly to one side of the previous return stroke, causing a ribbon effect.
[edit] Staccato lightning
Staccato lightning is a cloud to ground lightning strike which is a short-duration stroke that appears as a single very bright flash and often has considerable branching.[37]
[edit] Ground-to-cloud lightning
Ground-to-cloud lightning is a lightning discharge between the ground and a cumulonimbus cloud from an upward-moving leader stroke.
[edit] Ball lightning
Main article: Ball lightning
Ball lightning is described as a floating, illuminated ball that occurs during thunderstorms. They can be fast moving, slow moving or nearly stationary. Some make hissing or crackling noises or no noise at all. Some have been known to pass through windows and even dissipate with a bang. Ball lightning has been described by eyewitnesses but rarely recorded by meteorologists.[38]
The engineer Nikola Tesla wrote, "I have succeeded in determining the mode of their formation and producing them artificially".[39] There is some speculation that electrical breakdown and arcing of cotton and gutta-percha wire insulation used by Tesla may have been a contributing factor, since some theories of ball lightning require the involvement of carbonaceous materials. Some later experimenters have been able to briefly produce small luminous balls by igniting carbon-containing materials atop sparking Tesla Coils.
Several theories have been advanced to describe ball lightning, with none being universally accepted. Any complete theory of ball lightning must be able to describe the wide range of reported properties, such as those described in Singer's book "The Nature of Ball Lightning" and also more contemporary research. Japanese research shows that several instances have been reported of ball lightning without any connection to stormy weather or lightning.
Ball lightning is typically 20 – 30 cm (8-12 inches) in diameter, but ball lightning several meters in diameter has been reported.[40] Ball lightning has been seen in tornadoes, and has also been seen to split apart into two or more separate balls and recombine, and vertically-linked fireballs have been reported.[citation needed] Ball lightning has carved trenches in the peat swamps in Ireland.[citation needed] Because of its strange behavior, ball lightning has been mistaken for alien spacecraft by many witnesses, which often spawns UFO reports. One theory that may account for this wider spectrum of observational evidence is the idea of combustion inside the low-velocity region of axisymmetric (spherical) vortex breakdown of a natural vortex (e.g., the 'Hill's spherical vortex').[41]
Ball lightning apparently is created when lightning strikes silicon in soil, and has been created in a lab in this manner.[42]
[edit] Upper-atmospheric
Main article: Upper-atmospheric lightning
Reports by scientists of strange lightning phenomena above storms date back to at least 1886. However, it is only in recent years that fuller investigations have been made. This has sometimes been called megalightning.[43][44]
[edit] Sprites
Sprites are now well-documented electrical discharges that occur high above some types of thunderstorms. They appear as luminous reddish-orange or greenish-blue, plasma-like flashes, last longer than normal lower stratospheric discharges (typically around 17 milliseconds), and are triggered by the discharges of positive lightning between the thundercloud and the ground.[28] Sprites often occur in clusters of two or more, and typically span the distance from 50 miles (80 km) to 90 miles (145 km) above the earth, with what appear to be tendrils hanging below, and branches reaching above. A 2007 paper reports that the apparent tendrils and branches of sprites are actually formed by bright streamer heads of less than 140 m diameter moving up or down at 1 to 10 percent of the speed of light.[45] The abstract is publicly accessible.[46][47][44]
Sprites may be horizontally displaced by up to 30 miles (48 km) from the location of the underlying lightning strike, with a time delay following the lightning that is typically a few milliseconds, but on rare occasions may be up to 100 milliseconds. Sprites are sometimes, but not always, preceded by a sprite halo, a broad, pancake-like region of transient optical emission centered at an altitude of about 47 miles (76 km) above lightning.[43] Sprite halos are produced by weak ionization from transient electric fields of the same type that causes sprites, but which are insufficiently intense to exceed the threshold needed for sprites. Sprites were first photographed on July 6, 1989 by scientists from the University of Minnesota. Several years after their discovery they were named after the mischievous sprite (air spirit) Puck in Shakespeare's Midsummer Night's Dream.
Recent research carried out at the University of Houston in 2002 indicates that some normal (negative) lightning discharges produce a sprite halo, the precursor of a sprite, and that every lightning bolt between cloud and ground attempts to produce a sprite or a sprite halo.[citation needed] Research in 2004 by scientists from Tohoku University found that very low frequency emissions occur at the same time as the sprite, indicating that a discharge within the cloud may generate the sprites.[46]
[edit] Blue jets
Blue jets differ from sprites in that they project from the top of the cumulonimbus above a thunderstorm, typically in a narrow cone, to the lowest levels of the ionosphere 25 miles (40 km) to 30 miles (48 km) above the earth.[citation needed] They are also brighter than sprites and, as implied by their name, are blue in color. They were first recorded on October 21, 1989, on a video taken from the space shuttle as it passed over Australia, and subsequently extensively documented in 1994 during aircraft research flights by the University of Alaska.[48][49][44]
On September 14, 2001, scientists at the Arecibo Observatory photographed a huge jet double the height of those previously observed, reaching around 50 miles (80 km) into the atmosphere. The jet was located above a thunderstorm over the ocean, and lasted under a second. Lightning was initially observed traveling up at around 50,000 m/s in a similar way to a typical blue jet, but then divided in two and sped at 250,000 m/s to the ionosphere, where they spread out in a bright burst of light.[50] On July 22, 2002, five gigantic jets between 60 and 70 km (35 to 45 miles) in length were observed over the South China Sea from Taiwan, reported in Nature.[48] The jets lasted under a second, with shapes likened by the researchers to giant trees and carrots.[citation needed]
In 2001, the Arecibo scientists modeled the blue-jet phenomenon to better understand how it works. It is like an electron avalanche that can flood up toward the ionosphere or slide earthward, depending on the electric field direction. Intense hail may trigger the avalanche. The field accelerates the electrons and slams them into air molecules. The molecules break down into ions and free electrons and emit light. The newly generated electrons also accelerate.[49]
[edit] Elves
Elves often appear as dim, flattened, expanding glows around 250 miles (402 km) in diameter that last for, typically, just one millisecond.[51] They occur in the ionosphere 60 miles (97 km) above the ground over thunderstorms. Their color was a puzzle for some time, but is now believed to be a red hue. Elves were first recorded on another shuttle mission, this time recorded off French Guiana on October 7, 1990. Elves is a frivolous acronym for Emissions of Light and Very Low Frequency Perturbations From Electromagnetic Pulse Sources. This refers to the process by which the light is generated; the excitation of nitrogen molecules due to electron collisions (the electrons possibly having been energized by the electromagnetic pulse caused by a discharge from the Ionosphere).[44]
Dry lightning is a term in the United States for thunderstorms which produce no precipitation at the surface. This type of lightning is the most common natural cause of wildfires. Dry lightning may also be referred to as heat lightning.
[edit] Rocket lightning
It is a form of cloud discharge, generally horizontal and at cloud base, with a luminous channel appearing to advance through the air with visually resolvable speed, often intermittently.[33]
The movement has been compared to that of a skyrocket, hence its name. It is also one of the rarest of cloud discharges.[34]
[edit] Cloud-to-ground
Cloud-to-ground lightning is a great lightning discharge between a cumulonimbus cloud and the ground initiated by the downward-moving leader stroke. This is the second most common type of lightning, and poses the greatest threat to life and property of all known types.
[edit] Bead lightning
Bead lightning is a type of cloud-to-ground lightning which appears to break up into a string of short, bright sections, which last longer than the usual discharge channel. It is fairly rare. Several theories have been proposed to explain it; one is that the observer sees portions of the lightning channel end on, and that these portions appear especially bright. Another is that, in bead lightning, the width of the lightning channel varies; as the lightning channel cools and fades, the wider sections cool more slowly and remain visible longer, appearing as a string of beads.[35][36]
[edit] Ribbon lightning
Ribbon lightning occurs in thunderstorms with high cross winds and multiple return strokes. The wind will blow each successive return stroke slightly to one side of the previous return stroke, causing a ribbon effect.
[edit] Staccato lightning
Staccato lightning is a cloud to ground lightning strike which is a short-duration stroke that appears as a single very bright flash and often has considerable branching.[37]
[edit] Ground-to-cloud lightning
Ground-to-cloud lightning is a lightning discharge between the ground and a cumulonimbus cloud from an upward-moving leader stroke.
[edit] Ball lightning
Main article: Ball lightning
Ball lightning is described as a floating, illuminated ball that occurs during thunderstorms. They can be fast moving, slow moving or nearly stationary. Some make hissing or crackling noises or no noise at all. Some have been known to pass through windows and even dissipate with a bang. Ball lightning has been described by eyewitnesses but rarely recorded by meteorologists.[38]
The engineer Nikola Tesla wrote, "I have succeeded in determining the mode of their formation and producing them artificially".[39] There is some speculation that electrical breakdown and arcing of cotton and gutta-percha wire insulation used by Tesla may have been a contributing factor, since some theories of ball lightning require the involvement of carbonaceous materials. Some later experimenters have been able to briefly produce small luminous balls by igniting carbon-containing materials atop sparking Tesla Coils.
Several theories have been advanced to describe ball lightning, with none being universally accepted. Any complete theory of ball lightning must be able to describe the wide range of reported properties, such as those described in Singer's book "The Nature of Ball Lightning" and also more contemporary research. Japanese research shows that several instances have been reported of ball lightning without any connection to stormy weather or lightning.
Ball lightning is typically 20 – 30 cm (8-12 inches) in diameter, but ball lightning several meters in diameter has been reported.[40] Ball lightning has been seen in tornadoes, and has also been seen to split apart into two or more separate balls and recombine, and vertically-linked fireballs have been reported.[citation needed] Ball lightning has carved trenches in the peat swamps in Ireland.[citation needed] Because of its strange behavior, ball lightning has been mistaken for alien spacecraft by many witnesses, which often spawns UFO reports. One theory that may account for this wider spectrum of observational evidence is the idea of combustion inside the low-velocity region of axisymmetric (spherical) vortex breakdown of a natural vortex (e.g., the 'Hill's spherical vortex').[41]
Ball lightning apparently is created when lightning strikes silicon in soil, and has been created in a lab in this manner.[42]
[edit] Upper-atmospheric
Main article: Upper-atmospheric lightning
Reports by scientists of strange lightning phenomena above storms date back to at least 1886. However, it is only in recent years that fuller investigations have been made. This has sometimes been called megalightning.[43][44]
[edit] Sprites
Sprites are now well-documented electrical discharges that occur high above some types of thunderstorms. They appear as luminous reddish-orange or greenish-blue, plasma-like flashes, last longer than normal lower stratospheric discharges (typically around 17 milliseconds), and are triggered by the discharges of positive lightning between the thundercloud and the ground.[28] Sprites often occur in clusters of two or more, and typically span the distance from 50 miles (80 km) to 90 miles (145 km) above the earth, with what appear to be tendrils hanging below, and branches reaching above. A 2007 paper reports that the apparent tendrils and branches of sprites are actually formed by bright streamer heads of less than 140 m diameter moving up or down at 1 to 10 percent of the speed of light.[45] The abstract is publicly accessible.[46][47][44]
Sprites may be horizontally displaced by up to 30 miles (48 km) from the location of the underlying lightning strike, with a time delay following the lightning that is typically a few milliseconds, but on rare occasions may be up to 100 milliseconds. Sprites are sometimes, but not always, preceded by a sprite halo, a broad, pancake-like region of transient optical emission centered at an altitude of about 47 miles (76 km) above lightning.[43] Sprite halos are produced by weak ionization from transient electric fields of the same type that causes sprites, but which are insufficiently intense to exceed the threshold needed for sprites. Sprites were first photographed on July 6, 1989 by scientists from the University of Minnesota. Several years after their discovery they were named after the mischievous sprite (air spirit) Puck in Shakespeare's Midsummer Night's Dream.
Recent research carried out at the University of Houston in 2002 indicates that some normal (negative) lightning discharges produce a sprite halo, the precursor of a sprite, and that every lightning bolt between cloud and ground attempts to produce a sprite or a sprite halo.[citation needed] Research in 2004 by scientists from Tohoku University found that very low frequency emissions occur at the same time as the sprite, indicating that a discharge within the cloud may generate the sprites.[46]
[edit] Blue jets
Blue jets differ from sprites in that they project from the top of the cumulonimbus above a thunderstorm, typically in a narrow cone, to the lowest levels of the ionosphere 25 miles (40 km) to 30 miles (48 km) above the earth.[citation needed] They are also brighter than sprites and, as implied by their name, are blue in color. They were first recorded on October 21, 1989, on a video taken from the space shuttle as it passed over Australia, and subsequently extensively documented in 1994 during aircraft research flights by the University of Alaska.[48][49][44]
On September 14, 2001, scientists at the Arecibo Observatory photographed a huge jet double the height of those previously observed, reaching around 50 miles (80 km) into the atmosphere. The jet was located above a thunderstorm over the ocean, and lasted under a second. Lightning was initially observed traveling up at around 50,000 m/s in a similar way to a typical blue jet, but then divided in two and sped at 250,000 m/s to the ionosphere, where they spread out in a bright burst of light.[50] On July 22, 2002, five gigantic jets between 60 and 70 km (35 to 45 miles) in length were observed over the South China Sea from Taiwan, reported in Nature.[48] The jets lasted under a second, with shapes likened by the researchers to giant trees and carrots.[citation needed]
In 2001, the Arecibo scientists modeled the blue-jet phenomenon to better understand how it works. It is like an electron avalanche that can flood up toward the ionosphere or slide earthward, depending on the electric field direction. Intense hail may trigger the avalanche. The field accelerates the electrons and slams them into air molecules. The molecules break down into ions and free electrons and emit light. The newly generated electrons also accelerate.[49]
[edit] Elves
Elves often appear as dim, flattened, expanding glows around 250 miles (402 km) in diameter that last for, typically, just one millisecond.[51] They occur in the ionosphere 60 miles (97 km) above the ground over thunderstorms. Their color was a puzzle for some time, but is now believed to be a red hue. Elves were first recorded on another shuttle mission, this time recorded off French Guiana on October 7, 1990. Elves is a frivolous acronym for Emissions of Light and Very Low Frequency Perturbations From Electromagnetic Pulse Sources. This refers to the process by which the light is generated; the excitation of nitrogen molecules due to electron collisions (the electrons possibly having been energized by the electromagnetic pulse caused by a discharge from the Ionosphere).[44]
Gamma rays and the runaway breakdown theory
Double lightningIt has been discovered in the past 15 years that among the processes of lightning is some mechanism capable of generating gamma rays, which escape the atmosphere and are observed by orbiting spacecraft. Brought to light by NASA's Gerald Fishman in 1994 in an article in Nature, these so-called Terrestrial Gamma-Ray Flashes (TGFs) were observed by accident, while he was documenting instances of extraterrestrial gamma ray bursts observed by the Compton Gamma Ray Observatory (CGRO). TGFs are much shorter in duration, however, lasting only ~1 ms.
Professor Umran Inan of Stanford University linked a TGF to an individual lightning stroke occurring within 1.5 ms of the TGF event,[25] proving for the first time that the TGF was of atmospheric origin and associated with lightning strikes.
CGRO recorded only about 77 events in 10 years; however, more recently the RHESSI spacecraft, as reported by David Smith of UC Santa Cruz, has been observing TGFs at a much higher rate, indicating that these occur ~50 times per day globally (still a very small fraction of the total lightning on the planet). The energy levels recorded exceed 20 MeV.
Scientists from Duke University have also been studying the link between certain lightning events and the mysterious gamma ray emissions that emanate from the Earth's own atmosphere, in light of newer observations of TGFs made by RHESSI. Their study suggests that this gamma radiation fountains upward from starting points at surprisingly low altitudes in thunderclouds.
Steven Cummer, from Duke University's Pratt School of Engineering, said, "These are higher energy gamma rays than come from the sun. And yet here they are coming from the kind of terrestrial thunderstorm that we see here all the time."
Early hypotheses of this pointed to lightning generating high electric fields at altitudes well above the cloud, where the thin atmosphere allows gamma rays to easily escape into space, known as "relativistic runaway breakdown", similar to the way sprites are generated. Subsequent evidence has cast doubt, though, and suggested instead that TGFs may be produced at the tops of high thunderclouds. Though hindered by atmospheric absorption of the escaping gamma rays, these theories do not require the exceptionally high electric fields that high altitude theories of TGF generation rely on.
The role of TGFs and their relationship to lightning remains a subject of ongoing scientific study.
[edit] Re-strike
Lightning is a highly visible form of energy transfer.High speed videos (examined frame-by frame) show that most lightning strikes are made up of multiple individual strokes. A typical strike is made of 3 to 4 strokes. There may be more.[21]
Each re-strike is separated by a relatively large amount of time, typically 40 to 50 milliseconds. Re-strikes can cause a noticeable "strobe light" effect.[21]
Each successive stroke is preceded by intermediate dart leader strokes again to, but weaker than, the initial stepped leader. The stroke usually re-uses the discharge channel taken by the previous stroke.[21]
The variations in successive discharges are the result of smaller regions of charge within the cloud being depleted by successive strokes.[citation needed]
The sound of thunder from a lightning strike is prolonged by successive strokes.
[edit] Types of lightning
Cloud-to-cloud lightning, Steinenbronn, GermanySome lightning strikes take on particular characteristics; scientists and the public have given names to these various types of lightning. Most lightning is streak lightning. This is nothing more than the return stroke, the visible part of the lightning stroke. Because most of these strokes occur inside a cloud, we do not see many of the individual return strokes in a thunderstorm.
The return stroke of a lightning bolt, which is the visible bolt itself, follows a charge channel only about a half-inch (1.3 cm) wide. Most lightning bolts are about a mile (1.6 km) long.[14]
[edit] Positive lightning
Anvil to ground (Bolt from the blue) lightning strikeSee also: High_voltage#Lightning
Positive lightning, also known colloquially as a "bolt from the blue" makes up less than 5% of all lightning.[26] It occurs when the leader forms at the positively charged cloud tops, with the consequence that a negatively charged streamer issues from the ground. The overall effect is a discharge of positive charges to the ground. Research carried out after the discovery of positive lightning in the 1970s showed that positive lightning bolts are typically six to ten times more powerful than negative bolts, last around ten times longer, and can strike tens of kilometres/miles from the clouds.[27] The voltage difference for positive lightning must be considerably higher, due to the tens of thousands of additional metres/feet the strike must travel. During a positive lightning strike, huge quantities of ELF and VLF radio waves are generated.[28]
As a result of their greater power, positive lightning strikes are considerably more dangerous. At the present time, aircraft are not designed to withstand such strikes, since their existence was unknown at the time standards were set, and the dangers unappreciated until the destruction of a glider in 1999.[29]
One type of positive lightning is Anvil to Ground, since it emanates from the anvil top of a cumulonimbus cloud where the ice crystals are positively charged. The leader stroke of lightning issues forth in a nearly horizontal direction until it veers toward the ground. These usually occur kilometers/miles from (and often ahead of) the main storm and will sometimes strike without warning on a sunny day. An anvil-to-ground lightning bolt is a sign of an approaching storm, and if one occurs in a largely clear sky, it is known colloquially as a "Bolt from the blue."[30]
Positive lightning is also now believed to have been responsible for the 1963 in-flight explosion and subsequent crash of Pan Am Flight 214, a Boeing 707.[31] Subsequently, aircraft operating in U.S. airspace have been required to have lightning discharge wicks to reduce the chances of a similar occurrence.
Positive lightning has also been shown to trigger the occurrence of upper atmosphere lightning. It tends to occur more frequently in winter storms and at the end of a thunderstorm.[32]
An average bolt of positive lightning carries a current of up to 300 kA (kiloamperes) (about ten times as much current as a bolt of negative lightning), transfers a charge of up to 300 coulombs, has a potential difference up to 1 gigavolt (one billion volts), and lasts for hundreds of milliseconds, with a discharge energy of up to 300 GJ (gigajoules) (a billion joules).[citation needed]
Double lightningIt has been discovered in the past 15 years that among the processes of lightning is some mechanism capable of generating gamma rays, which escape the atmosphere and are observed by orbiting spacecraft. Brought to light by NASA's Gerald Fishman in 1994 in an article in Nature, these so-called Terrestrial Gamma-Ray Flashes (TGFs) were observed by accident, while he was documenting instances of extraterrestrial gamma ray bursts observed by the Compton Gamma Ray Observatory (CGRO). TGFs are much shorter in duration, however, lasting only ~1 ms.
Professor Umran Inan of Stanford University linked a TGF to an individual lightning stroke occurring within 1.5 ms of the TGF event,[25] proving for the first time that the TGF was of atmospheric origin and associated with lightning strikes.
CGRO recorded only about 77 events in 10 years; however, more recently the RHESSI spacecraft, as reported by David Smith of UC Santa Cruz, has been observing TGFs at a much higher rate, indicating that these occur ~50 times per day globally (still a very small fraction of the total lightning on the planet). The energy levels recorded exceed 20 MeV.
Scientists from Duke University have also been studying the link between certain lightning events and the mysterious gamma ray emissions that emanate from the Earth's own atmosphere, in light of newer observations of TGFs made by RHESSI. Their study suggests that this gamma radiation fountains upward from starting points at surprisingly low altitudes in thunderclouds.
Steven Cummer, from Duke University's Pratt School of Engineering, said, "These are higher energy gamma rays than come from the sun. And yet here they are coming from the kind of terrestrial thunderstorm that we see here all the time."
Early hypotheses of this pointed to lightning generating high electric fields at altitudes well above the cloud, where the thin atmosphere allows gamma rays to easily escape into space, known as "relativistic runaway breakdown", similar to the way sprites are generated. Subsequent evidence has cast doubt, though, and suggested instead that TGFs may be produced at the tops of high thunderclouds. Though hindered by atmospheric absorption of the escaping gamma rays, these theories do not require the exceptionally high electric fields that high altitude theories of TGF generation rely on.
The role of TGFs and their relationship to lightning remains a subject of ongoing scientific study.
[edit] Re-strike
Lightning is a highly visible form of energy transfer.High speed videos (examined frame-by frame) show that most lightning strikes are made up of multiple individual strokes. A typical strike is made of 3 to 4 strokes. There may be more.[21]
Each re-strike is separated by a relatively large amount of time, typically 40 to 50 milliseconds. Re-strikes can cause a noticeable "strobe light" effect.[21]
Each successive stroke is preceded by intermediate dart leader strokes again to, but weaker than, the initial stepped leader. The stroke usually re-uses the discharge channel taken by the previous stroke.[21]
The variations in successive discharges are the result of smaller regions of charge within the cloud being depleted by successive strokes.[citation needed]
The sound of thunder from a lightning strike is prolonged by successive strokes.
[edit] Types of lightning
Cloud-to-cloud lightning, Steinenbronn, GermanySome lightning strikes take on particular characteristics; scientists and the public have given names to these various types of lightning. Most lightning is streak lightning. This is nothing more than the return stroke, the visible part of the lightning stroke. Because most of these strokes occur inside a cloud, we do not see many of the individual return strokes in a thunderstorm.
The return stroke of a lightning bolt, which is the visible bolt itself, follows a charge channel only about a half-inch (1.3 cm) wide. Most lightning bolts are about a mile (1.6 km) long.[14]
[edit] Positive lightning
Anvil to ground (Bolt from the blue) lightning strikeSee also: High_voltage#Lightning
Positive lightning, also known colloquially as a "bolt from the blue" makes up less than 5% of all lightning.[26] It occurs when the leader forms at the positively charged cloud tops, with the consequence that a negatively charged streamer issues from the ground. The overall effect is a discharge of positive charges to the ground. Research carried out after the discovery of positive lightning in the 1970s showed that positive lightning bolts are typically six to ten times more powerful than negative bolts, last around ten times longer, and can strike tens of kilometres/miles from the clouds.[27] The voltage difference for positive lightning must be considerably higher, due to the tens of thousands of additional metres/feet the strike must travel. During a positive lightning strike, huge quantities of ELF and VLF radio waves are generated.[28]
As a result of their greater power, positive lightning strikes are considerably more dangerous. At the present time, aircraft are not designed to withstand such strikes, since their existence was unknown at the time standards were set, and the dangers unappreciated until the destruction of a glider in 1999.[29]
One type of positive lightning is Anvil to Ground, since it emanates from the anvil top of a cumulonimbus cloud where the ice crystals are positively charged. The leader stroke of lightning issues forth in a nearly horizontal direction until it veers toward the ground. These usually occur kilometers/miles from (and often ahead of) the main storm and will sometimes strike without warning on a sunny day. An anvil-to-ground lightning bolt is a sign of an approaching storm, and if one occurs in a largely clear sky, it is known colloquially as a "Bolt from the blue."[30]
Positive lightning is also now believed to have been responsible for the 1963 in-flight explosion and subsequent crash of Pan Am Flight 214, a Boeing 707.[31] Subsequently, aircraft operating in U.S. airspace have been required to have lightning discharge wicks to reduce the chances of a similar occurrence.
Positive lightning has also been shown to trigger the occurrence of upper atmosphere lightning. It tends to occur more frequently in winter storms and at the end of a thunderstorm.[32]
An average bolt of positive lightning carries a current of up to 300 kA (kiloamperes) (about ten times as much current as a bolt of negative lightning), transfers a charge of up to 300 coulombs, has a potential difference up to 1 gigavolt (one billion volts), and lasts for hundreds of milliseconds, with a discharge energy of up to 300 GJ (gigajoules) (a billion joules).[citation needed]
Lightning is an atmospheric discharge of electricity, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms.[1] In the atmospheric electrical discharge, a leader of a bolt of lightning can travel at speeds of 60,000 m/s, and can reach temperatures approaching 30,000 °C (54,000 °F), hot enough to fuse silica sand into petrified lightning, known scientifically as glass channels or fulgurites which are normally hollow and can extend some distance into the ground .[2][3] There are over 16 million lightning storms every year.[1]
Lightning can also occur within the ash clouds from volcanic eruptions, or can be caused by violent forest fires which generate sufficient dust to create a static charge.[1][4]
How lightning initially forms is still a matter of debate:[5] Scientists have studied root causes ranging from atmospheric perturbations (wind, humidity, friction, and atmospheric pressure) to the impact of solar wind and accumulation of charged solar particles.[6] Ice inside a cloud is thought to be a key element in lightning development, and may cause a forcible separation of positive and negative charges within the cloud, thus assisting in the formation of lightning.[6]
Properties of lightning
World map showing frequency of lightning strikes, in flashes per km² per year (equal-area projection). Lightning strikes most frequently in the Democratic Republic of the Congo. Combined 1995–2003 data from the Optical Transient Detector and 1998–2003 data from the Lightning Imaging Sensor.An average bolt of lightning carries an electric current of 40 kiloamperes (kA) (although some bolts can be up to 120 kA), and transfers a charge of five coulombs and 500 MJ. The voltage depends on the length of the bolt, with the dielectric breakdown of air being three million volts per meter; this works out to approximately one gigavolt (one billion volts) for a 300 m (1000 ft) lightning bolt. With an electric current of 100 kA, this gives a power of 100 terawatts. However, lightning leader development is not a simple matter of dielectric breakdown, and the ambient electric fields required for lightning leader propagation can be a few orders of magnitude less than dielectric breakdown strength. Further, the potential gradient inside a well-developed return-stroke channel is on the order of hundreds of volts per meter or less due to intense channel ionization, resulting in a true power output on the order of megawatts per meter for a vigorous return-stroke current of 100 kA [12].
Lightning heats nearby air to about 10,000 °C (18,000 °F) nearly instantly, which is almost twice the temperature of the Sun’s surface. The heating creates a shock wave that is heard as thunder.[13]
The return stroke of a lightning bolt follows a charge channel only about a centimeter (0.4-in) wide. Most lightning bolts are about 1.6 kilometers (1 mi) long. The longest recorded length was 190 kilometers (118 mi), sighted near Dallas, Texas.[14]
Different locations have different potentials (voltages) and currents for an average lightning strike. For example, Florida, with the United States' largest number of recorded strikes in a given period during the summer season, has very sandy ground in some areas and conductive saturated mucky soil in others. As much of Florida lies on a peninsula, it is bordered by the ocean on three sides. The result is the daily development of sea and lake breeze boundaries that collide and produce thunderstorms. Arizona, which has very dry, sandy soil and a very dry air, has cloud bases as high as 1800-2100 m (6,000-7,000 ft) above ground level, and gets very long and thin purplish discharges which crackle; while Oklahoma, with cloud bases about 450-600 m (1,500-2,000 ft) above ground level and fairly soft, clay-rich soil, has big, blue-white explosive lightning strikes that are very hot (high current) and cause sudden, explosive noise when the discharge comes. The difference in each case may consist of differences in voltage levels between clouds and ground. Research on this is still ongoing.[citation needed]
NASA scientists have found the radio waves created by lightning clear a safe zone in the radiation belt surrounding the earth. This zone, known as the Van Allen Belt slot, can potentially be a safe haven for satellites, offering them protection from the Sun's radiation.[15][16][17]
[edit] Formation
Note
Positive lightning (a rarer form of lightning that originates from positively charged regions of the thundercloud) does not generally fit the following pattern.
[edit] Charge separation
The first process in the generation of lightning is charge separation.
[edit] Polarization mechanism hypothesis
The mechanism by which charge separation happens is still the subject of research, but one hypothesis is the polarization mechanism, which has two components:[18]
Falling droplets of ice and rain become electrically polarized as they fall through the atmosphere's natural electric field;
Colliding ice particles become charged by electrostatic induction.
Ice and supercooled water are the keys to the process. Violent winds buffet tiny hailstones as they form, causing them to collide. When the hailstones hit ice crystals, some negative ions transfer from one particle to another. The smaller particles lose negative ions and become positive and the larger more massive particles gain negative ions and become negative.[19]
Lightning can also occur within the ash clouds from volcanic eruptions, or can be caused by violent forest fires which generate sufficient dust to create a static charge.[1][4]
How lightning initially forms is still a matter of debate:[5] Scientists have studied root causes ranging from atmospheric perturbations (wind, humidity, friction, and atmospheric pressure) to the impact of solar wind and accumulation of charged solar particles.[6] Ice inside a cloud is thought to be a key element in lightning development, and may cause a forcible separation of positive and negative charges within the cloud, thus assisting in the formation of lightning.[6]
Properties of lightning
World map showing frequency of lightning strikes, in flashes per km² per year (equal-area projection). Lightning strikes most frequently in the Democratic Republic of the Congo. Combined 1995–2003 data from the Optical Transient Detector and 1998–2003 data from the Lightning Imaging Sensor.An average bolt of lightning carries an electric current of 40 kiloamperes (kA) (although some bolts can be up to 120 kA), and transfers a charge of five coulombs and 500 MJ. The voltage depends on the length of the bolt, with the dielectric breakdown of air being three million volts per meter; this works out to approximately one gigavolt (one billion volts) for a 300 m (1000 ft) lightning bolt. With an electric current of 100 kA, this gives a power of 100 terawatts. However, lightning leader development is not a simple matter of dielectric breakdown, and the ambient electric fields required for lightning leader propagation can be a few orders of magnitude less than dielectric breakdown strength. Further, the potential gradient inside a well-developed return-stroke channel is on the order of hundreds of volts per meter or less due to intense channel ionization, resulting in a true power output on the order of megawatts per meter for a vigorous return-stroke current of 100 kA [12].
Lightning heats nearby air to about 10,000 °C (18,000 °F) nearly instantly, which is almost twice the temperature of the Sun’s surface. The heating creates a shock wave that is heard as thunder.[13]
The return stroke of a lightning bolt follows a charge channel only about a centimeter (0.4-in) wide. Most lightning bolts are about 1.6 kilometers (1 mi) long. The longest recorded length was 190 kilometers (118 mi), sighted near Dallas, Texas.[14]
Different locations have different potentials (voltages) and currents for an average lightning strike. For example, Florida, with the United States' largest number of recorded strikes in a given period during the summer season, has very sandy ground in some areas and conductive saturated mucky soil in others. As much of Florida lies on a peninsula, it is bordered by the ocean on three sides. The result is the daily development of sea and lake breeze boundaries that collide and produce thunderstorms. Arizona, which has very dry, sandy soil and a very dry air, has cloud bases as high as 1800-2100 m (6,000-7,000 ft) above ground level, and gets very long and thin purplish discharges which crackle; while Oklahoma, with cloud bases about 450-600 m (1,500-2,000 ft) above ground level and fairly soft, clay-rich soil, has big, blue-white explosive lightning strikes that are very hot (high current) and cause sudden, explosive noise when the discharge comes. The difference in each case may consist of differences in voltage levels between clouds and ground. Research on this is still ongoing.[citation needed]
NASA scientists have found the radio waves created by lightning clear a safe zone in the radiation belt surrounding the earth. This zone, known as the Van Allen Belt slot, can potentially be a safe haven for satellites, offering them protection from the Sun's radiation.[15][16][17]
[edit] Formation
Note
Positive lightning (a rarer form of lightning that originates from positively charged regions of the thundercloud) does not generally fit the following pattern.
[edit] Charge separation
The first process in the generation of lightning is charge separation.
[edit] Polarization mechanism hypothesis
The mechanism by which charge separation happens is still the subject of research, but one hypothesis is the polarization mechanism, which has two components:[18]
Falling droplets of ice and rain become electrically polarized as they fall through the atmosphere's natural electric field;
Colliding ice particles become charged by electrostatic induction.
Ice and supercooled water are the keys to the process. Violent winds buffet tiny hailstones as they form, causing them to collide. When the hailstones hit ice crystals, some negative ions transfer from one particle to another. The smaller particles lose negative ions and become positive and the larger more massive particles gain negative ions and become negative.[19]
Lightning
Natural static dischargeMain article: Lightning
Lightning is a dramatic natural example of static discharge. While the details are unclear and remain the subject of debate, the initial charge separation is thought to be associated with contact between ice particles within storm clouds. Whatever the cause may be, the resulting lightning bolt is simply a scaled up version of the sparks seen in more domestic occurrences of static discharge. The flash occurs because the air in the discharge channel is heated to such a high temperature that it emits light by incandescence. The clap of thunder is the result of the shockwave created as the superheated air rapidly expands.
Natural static dischargeMain article: Lightning
Lightning is a dramatic natural example of static discharge. While the details are unclear and remain the subject of debate, the initial charge separation is thought to be associated with contact between ice particles within storm clouds. Whatever the cause may be, the resulting lightning bolt is simply a scaled up version of the sparks seen in more domestic occurrences of static discharge. The flash occurs because the air in the discharge channel is heated to such a high temperature that it emits light by incandescence. The clap of thunder is the result of the shockwave created as the superheated air rapidly expands.
Static electricity refers to the accumulation of excess electric charge in a region with poor electrical conductivity (an insulator), such that the charge accumulation persists. The effects of static electricity are familiar to most people because we can see, feel and even hear the spark as the excess charge is neutralized when brought close to a large electrical conductor (for example a path to ground), or a region with an excess charge of the opposite polarity (positive or negative).
Causes of static electricity
The materials we observe and interact with from day-to-day are formed from atoms and molecules that are electrically neutral, having an equal number of positive charges (protons, in the nucleus) and negative charges (electrons, in shells surrounding the nucleus). The phenomenon of static electricity requires a sustained separation of positive and negative charges.
[edit] Contact induced charge separation
Main article: Triboelectric effect
Electrons can be exchanged between materials on contact; materials with weakly bound electrons tend to lose them, while materials with sparsely filled outer shells tend to gain them. This is known as the triboelectric effect and results in one material becoming positively charged and the other negatively charged. The polarity and strength of the charge on a material once they are separated depends on their relative positions in the triboelectric series. The tribo electric effect is the main cause of static electricity as observed in everyday life, and in common high-school science demonstrations involving rubbing different materials together (e.g. fur and an acrylic rod).
[edit] Pressure induced charge separation
Main article: Piezoelectric effect
Certain types of crystals and ceramics generate a separation of charge in response to applied mechanical stress.
[edit] Heat induced charge separation
Main article: Pyroelectric effect
Certain materials generate a separation of charge in response to heating. All pyroelectric materials are also piezoelectric, the two properties being closely related.
[edit] Charge induced charge separation
Main article: Electrostatic induction
A charged object brought into the vicinity of an electrically neutral object will cause a separation of charge within the conductor as charges of the same polarity are repelled and charges of the opposite polarity are attracted. As the force due to the interaction of electric charges falls off rapidly with increasing distance, the effect of the closer (opposite polarity) charges is greater and the two objects feel a force of attraction. The effect is most pronounced when the neutral object is an electrical conductor as the charges are more free to move around.
Careful grounding of part of an object with a charge induced charge separation can permanently add or remove electrons, leaving the object with a global, permanent charge. This process is integral to the workings of the Van de Graaf Generator, a device commonly used to demonstrate the effects of static electricity.
Causes of static electricity
The materials we observe and interact with from day-to-day are formed from atoms and molecules that are electrically neutral, having an equal number of positive charges (protons, in the nucleus) and negative charges (electrons, in shells surrounding the nucleus). The phenomenon of static electricity requires a sustained separation of positive and negative charges.
[edit] Contact induced charge separation
Main article: Triboelectric effect
Electrons can be exchanged between materials on contact; materials with weakly bound electrons tend to lose them, while materials with sparsely filled outer shells tend to gain them. This is known as the triboelectric effect and results in one material becoming positively charged and the other negatively charged. The polarity and strength of the charge on a material once they are separated depends on their relative positions in the triboelectric series. The tribo electric effect is the main cause of static electricity as observed in everyday life, and in common high-school science demonstrations involving rubbing different materials together (e.g. fur and an acrylic rod).
[edit] Pressure induced charge separation
Main article: Piezoelectric effect
Certain types of crystals and ceramics generate a separation of charge in response to applied mechanical stress.
[edit] Heat induced charge separation
Main article: Pyroelectric effect
Certain materials generate a separation of charge in response to heating. All pyroelectric materials are also piezoelectric, the two properties being closely related.
[edit] Charge induced charge separation
Main article: Electrostatic induction
A charged object brought into the vicinity of an electrically neutral object will cause a separation of charge within the conductor as charges of the same polarity are repelled and charges of the opposite polarity are attracted. As the force due to the interaction of electric charges falls off rapidly with increasing distance, the effect of the closer (opposite polarity) charges is greater and the two objects feel a force of attraction. The effect is most pronounced when the neutral object is an electrical conductor as the charges are more free to move around.
Careful grounding of part of an object with a charge induced charge separation can permanently add or remove electrons, leaving the object with a global, permanent charge. This process is integral to the workings of the Van de Graaf Generator, a device commonly used to demonstrate the effects of static electricity.
Saturday, July 26, 2008
Thursday, July 24, 2008
Electricity is the flow of electrical power or charge. It is a secondary energy source which means that we get it from the conversion of other sources of energy, like coal, natural gas, oil, nuclear power and other natural sources, which are called primary sources. The energy sources we use to make electricity can be renewable or non-renewable, but electricity itself is neither renewable or non-renewable.
Electricity is a basic part of nature and it is one of our most widely used forms of energy. Many cities and towns were built alongside waterfalls (a primary source of mechanical energy) that turned water wheels to perform work. Before electricity generation began over 100 years ago, houses were lit with kerosene lamps, food was cooled in iceboxes, and rooms were warmed by wood-burning or coal-burning stoves. Beginning with Benjamin Franklin's experiment with a kite one stormy night in Philadelphia, the principles of electricity gradually became understood. Thomas Edison helped change everyone's life -- he perfected his invention -- the electric light bulb. Prior to 1879, direct current (DC) electricity had been used in arc lights for outdoor lighting. In the late-1800s, Nikola Tesla pioneered the generation, transmission, and use of alternating current (AC) electricity, which can be transmitted over much greater distances than direct current. Tesla's inventions used electricity to bring indoor lighting to our homes and to power industrial machines.
Despite its great importance in our daily lives, most of us rarely stop to think what life would be like without electricity. Yet like air and water, we tend to take electricity for granted. Everyday, we use electricity to do many jobs for us -- from lighting and heating/cooling our homes, to powering our televisions and computers. Electricity is a controllable and convenient form of energy used in the applications of heat, light and power.
Electricity is a basic part of nature and it is one of our most widely used forms of energy. Many cities and towns were built alongside waterfalls (a primary source of mechanical energy) that turned water wheels to perform work. Before electricity generation began over 100 years ago, houses were lit with kerosene lamps, food was cooled in iceboxes, and rooms were warmed by wood-burning or coal-burning stoves. Beginning with Benjamin Franklin's experiment with a kite one stormy night in Philadelphia, the principles of electricity gradually became understood. Thomas Edison helped change everyone's life -- he perfected his invention -- the electric light bulb. Prior to 1879, direct current (DC) electricity had been used in arc lights for outdoor lighting. In the late-1800s, Nikola Tesla pioneered the generation, transmission, and use of alternating current (AC) electricity, which can be transmitted over much greater distances than direct current. Tesla's inventions used electricity to bring indoor lighting to our homes and to power industrial machines.
Despite its great importance in our daily lives, most of us rarely stop to think what life would be like without electricity. Yet like air and water, we tend to take electricity for granted. Everyday, we use electricity to do many jobs for us -- from lighting and heating/cooling our homes, to powering our televisions and computers. Electricity is a controllable and convenient form of energy used in the applications of heat, light and power.
The electricity that we get from power outlets and batteries can power all different kinds of devices. The fact is that electricity can be used in a thousand different ways. For example:
Electric motors turn electricity into motion.
Light bulbs, fluorescent lamps and LEDs turn electricity into light.
Computers turn electricity into information.
Telephones turn electricity into communication.
TVs turn electricity into moving pictures.
Speakers turn electricity into sound waves.
Stun guns turn electricity into pain.
Toasters, hair dryers and space heaters turn electricity into heat.
Radios turn electricity into electromagnetic waves that can travel millions of miles.
X-ray machines turn electricity into X-rays.
It is hard to imagine modern people living without electricity.
Electric motors turn electricity into motion.
Light bulbs, fluorescent lamps and LEDs turn electricity into light.
Computers turn electricity into information.
Telephones turn electricity into communication.
TVs turn electricity into moving pictures.
Speakers turn electricity into sound waves.
Stun guns turn electricity into pain.
Toasters, hair dryers and space heaters turn electricity into heat.
Radios turn electricity into electromagnetic waves that can travel millions of miles.
X-ray machines turn electricity into X-rays.
It is hard to imagine modern people living without electricity.
Tuesday, July 15, 2008
Electricity (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 electric field and the presence and motion of electric charge.
Electricity generation is the process of converting non-electrical energy to electricity. For electric utilities, it is the first process in the delivery of electricity to consumers. The other processes, electric power transmission and electricity distribution, are normally carried out by the electrical power industry. Electricity is most often generated at a power station by electromechanical generators, primarily driven by heat engines fueled by chemical combustion or nuclear fission but also by other means such as the kinetic energy of flowing water and wind. There are many other technologies that can be and are used to generate electricity such as solar photovoltaics.
Contents[hide] |
History
Centralized power generation became possible when it was recognized that alternating current power lines can transport electricity at very low costs across great distances by taking advantage of the ability to raise and lower the voltage using power transformers.
Electricity has been generated at central stations since 1881. The first power plants were run on water power or coal,[3] and today we rely mainly on coal, nuclear, natural gas, hydroelectric, and petroleum with a small amount from solar energy, tidal harnesses, wind generators, and
Electricity demand
![Coal fired power plants provide 49% of consumed electricity in the United States. Cleaner burning technologies continue to make coal fired power one of the cheapest ways to generate electricity[1]. This is the Castle Gate Plant near Helper, Utah.](http://en.wikipedia.org/wiki/Image:Grand_Junction_Trip_92007_098.JPG)
The demand for electricity is met in several ways. Large centralized generators have been the primary method thus far.
Distributed generation uses a larger number of smaller generators throughout the electricity network. Some use waste heat from industrial processes, others use fuels that would otherwise be wasted, such as landfill gas. Wind and solar generation tend to be distributed because of the low density of the natural energy they collect.
Methods of generating electricity
Most electric generation is driven by heat engines. The combustion of fossil fuels supplies most of the heat to these engines, with a significant fraction from nuclear fission.
Turbines
Virtually all of the heat engines just mentioned are turbines. Other types of turbines can be driven by wind or falling water. All turbines are driven by a fluid acting as an intermediate energy carrier. These fluids can be:
- Steam - Water is boiled by nuclear fission, the burning of fossil fuels (coal, natural gas, or petroleum) or biomass. Some power plants use the sun as the heat source: solar parabolic troughs and solar power towers concentrate sunlight to heat a heat transfer fluid, which is then used to produce steam. Another renewable source of heat used to drive a turbine is Geothermal power. Either steam under pressure emerges from the ground and drives a turbine or hot water evaporates a low boiling liquid to create vapour to drive a turbine.
- Water (hydroelectric) - Turbine blades are acted upon by flowing water, produced by hydroelectric dams or tidal forces.
- Wind - Most wind turbines generate electricity from naturally occurring wind. Solar updraft towers use wind that is artificially produced inside the chimney by heating it with sunlight, and are more properly seen as forms of solar thermal energy.
- Hot gas (gas turbine) - Turbines are driven directly by gases produced by the combustion of natural gas or oil.
Combined cycle gas turbine plants are driven by both steam and gas. They generate power by burning natural gas in a gas turbine and use residual heat to generate additional electricity from steam. These plants offer efficiencies of up to 60%.
Reciprocating engines
Small electricity generators are often powered by reciprocating engines burning diesel, biogas or natural gas. Diesel engines are often used for back up generation, usually at low voltages. Biogas is often combusted where it is produced, such as a landfill or wastewater treatment plant, with a reciprocating engine or a microturbine, which is a small gas turbine.
Photovoltaic panels
Unlike the solar heat concentrators mentioned above, photovoltaic panels convert sunlight directly to electricity. Although sunlight is free and abundant, solar electricity is still usually more expensive to produce than large-scale mechanically generated power due to the cost of the panels. Low-efficiency silicon solar cells have been decreasing in cost though, and multijunction cells with close to 30% conversion efficiency are now commercially available. Over 40% efficiency has been demonstrated in experimental systems.[5] Until recently, photovoltaics were most commonly used in remote sites where there is no access to a commercial power grid, or as a supplemental electricity source for individual homes and businesses. Recent advances in manufacturing efficiency and photovoltaic technology, combined with subsidies driven by environmental concerns, have dramatically accelerated the deployment of solar panels. Installed capacity is growing by 40% per year led by increases in Germany, Japan, California and New Jersey.
Other generation methods
Various other technologies have been studied and developed for power generation. Solid-state generation (without moving parts) is of particular interest in portable applications. This area is largely dominated by thermoelectric (TE) devices, though thermionic (TI) and thermophotovoltaic (TPV) systems have been developed as well. Typically, TE devices are used at lower temperatures than TI and TPV systems. Piezoelectric devices are used for power generation from mechanical strain, particularly in power harvesting. Betavoltaics are another type of solid-state power generator which produces electricity from radioactive decay. Fluid-based magnetohydrodynamic (MHD) power generation has been studied as a method for extracting electrical power from nuclear reactors and also from more conventional fuel combustion systems.
Electrochemical electricity generation is also important in portable and mobile applications. Currently, most electrochemical power comes from closed electrochemical cells ("batteries") [6], which are arguably utilized more as storage systems than generation systems, but open electrochemical systems, known as fuel cells, have been undergoing a great deal of research and development in the last few years. Fuel cells can be used to extract power either from natural fuels or from synthesized fuels (mainly electrolytic hydrogen) and so can be viewed as either generation systems or storage systems depending on their use.
Generation
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.[46] 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.[46] 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.[47] 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.[48][49] 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.[48] 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,[50] a rate of growth that is now being experienced by emerging economies such as those of India or China.[51][52] Historically, the growth rate for electricity demand has outstripped that for other forms of energy, such as coal.[53]
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.[54]
Uses
Electricity is an extremely flexible form of energy, and has been adapted to a huge, and growing, number of uses.[55] 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.[56] 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.[57] A number of countries, such as Denmark, have issued legislation restricting or banning the use of electric heating in new buildings.[58] Electricity is however a highly practical energy source for refrigeration,[59] with air conditioning representing a growing sector for electricity demand, the effects of which electricity utilities are increasingly obliged to accommodate.[60]
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,[61] 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.[62]
Electricity and the natural world
Physiological effects
A voltage applied to a human body causes an electric current to flow through the tissues, and although the relationship is non-linear, the greater the voltage, the greater the current.[63] 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.[64] If the current is sufficiently high, it will cause muscle contraction, fibrillation of the heart, and tissue burns.[63] 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.[65]
Electrical phenomena in nature
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.[66] Certain crystals, such as quartz, or even sugarcane, generate a potential difference across their faces when subjected to external pressure.[67] 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.[67]
Some organisms, such as sharks, are able to detect and respond to changes in electric fields, an ability known as electroreception,[68] while others, termed electrogenic, are able to generate voltages themselves to serve as a predatory or defensive weapon.[5] 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.[6][5] 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.[69] (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.[69]
See also
- Ampere's rule, connects the direction of an electric current and its associated magnetic currents.
- Electrical energy, the potential energy of a system of charges
- Electricity market, the sale of electrical energy
- Electrical phenomena, observable events which illuminate the physical principles of electricity
- Electric power, the rate at which electrical energy is transferred
- Electronics, the study of the movement of charge through certain materials and devices
- Hydraulic analogy, an analogy between the flow of water and electric current
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