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Acid rain is rain or any other form of precipitation that is unusually acidic, meaning that it has elevated levels of hydrogen ions (low pH). Most water, including drinking water, has a neutral pH that exists between 6.5 and 8.5, but acid rain has a pH level lower than this and ranges from 4–5 on average. The more acidic the acid rain is, the lower its pH is. Acid rain can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of sulfur dioxide and nitrogen oxide, which react with the water molecules in the atmosphere to produce acids. Acid rain has been shown to have adverse impacts on forests, freshwaters, soils, microbes, insects and aquatic life-forms. In ecosystems, persistent acid rain reduces tree bark durability, leaving flora more susceptible to environmental stressors such as drought, heat/cold and pest infestation. Acid rain is also capable of detrimenting soil composition by stripping it of nutrients such as calcium and magnesium which play a role in plant growth and maintaining healthy soil. In terms of human infrastructure, acid rain also causes paint to peel, corrosion of steel structures such as bridges, and weathering of stone buildings and statues as well as having impacts on human health. Some governments, including those in Europe and North America, have made efforts since the 1970s to reduce the release of sulfur dioxide and nitrogen oxide into the atmosphere through air pollution regulations. These efforts have had positive results due to the widespread research on acid rain starting in the 1960s and the publicized information on its harmful effects. The main source of sulfur and nitrogen compounds that result in acid rain are anthropogenic, but nitrogen oxides can also be produced naturally by lightning strikes and sulfur dioxide is produced by volcanic
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eruptions. == Definition == "Acid rain" is rain with a pH less than 5. "Clean" or unpolluted rain has a pH greater than 5 but still less than pH = 7 owing to the acidity caused by carbon dioxide acid according to the following reactions: H2O + CO2 ⇌ H2CO3 H2O + H2CO3 ⇌ HCO−3 + H3O+ A variety of natural and human-made sources contribute to the acidity. For example nitric acid produced by electric discharge in the atmosphere such as lightning. The usual anthropogenic sources are sulfur dioxide and nitrogen oxide. They react with water (as does carbon dioxide) to give solutions with pH < 5. Occasional pH readings in rain and fog water of well below 2.4 have been reported in industrialized areas. == History == Acid rain was first systematically studied in Europe in the 1960s, in the United States and Canada in 1970s, and in India in the late 1980s. === In Europe === The corrosive effect of polluted, acidic city air on limestone and marble was noted in the 17th century by John Evelyn, who remarked upon the poor condition of the Arundel marbles. Since the Industrial Revolution, emissions of sulfur dioxide and nitrogen oxides into the atmosphere have increased. In 1852, Robert Angus Smith was the first to show the relationship between acid rain and atmospheric pollution in Manchester, England. Smith coined the term "acid rain" in 1872. In the late 1960s, scientists began widely observing and studying the phenomenon. At first, the main focus in this research lay on local effects of acid rain. Waldemar Christofer Brøgger was the first to acknowledge long-distance transportation of pollutants crossing borders from the United Kingdom to Norway – a problem systematically studied by Brynjulf Ottar in the 1970s. Ottar's work was strongly influenced by Swedish soil
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scientist Svante Odén, who had drawn widespread attention to Europe's acid rain problem in popular newspapers and wrote a landmark paper on the subject in 1968. === In the United States === The earliest report about acid rain in the United States came from chemical evidence gathered from Hubbard Brook Valley; public awareness of acid rain in the US increased in the 1970s after The New York Times reported on these findings. In 1972, a group of scientists, including Gene Likens, discovered the rain that was deposited at White Mountains of New Hampshire was acidic. The pH of the sample was measured to be 4.03 at Hubbard Brook. The Hubbard Brook Ecosystem Study followed up with a series of research studies that analyzed the environmental effects of acid rain. The alumina from soils neutralized acid rain that mixed with stream water at Hubbard Brook. The result of this research indicated that the chemical reaction between acid rain and aluminium leads to an increasing rate of soil weathering. Experimental research examined the effects of increased acidity in streams on ecological species. In 1980, scientists modified the acidity of Norris Brook, New Hampshire, and observed the change in species' behaviors. There was a decrease in species diversity, an increase in community dominants, and a reduction in the food web complexity. In 1980, the US Congress passed an Acid Deposition Act. This Act established an 18-year assessment and research program under the direction of the National Acidic Precipitation Assessment Program (NAPAP). NAPAP enlarged a network of monitoring sites to determine how acidic precipitation was, seeking to determine long-term trends, and established a network for dry deposition. Using a statistically based sampling design, NAPAP quantified the effects of acid rain on a regional basis by targeting research and surveys to identify and quantify the
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impact of acid precipitation on freshwater and terrestrial ecosystems. NAPAP also assessed the effects of acid rain on historical buildings, monuments, and building materials. It also funded extensive studies on atmospheric processes and potential control programs. From the start, policy advocates from all sides attempted to influence NAPAP activities to support their particular policy advocacy efforts, or to disparage those of their opponents. For the US Government's scientific enterprise, a significant impact of NAPAP were lessons learned in the assessment process and in environmental research management to a relatively large group of scientists, program managers, and the public. In 1981, the National Academy of Sciences was looking into research about the controversial issues regarding acid rain. President Ronald Reagan dismissed the issues of acid rain until his personal visit to Canada and confirmed that the Canadian border suffered from the drifting pollution from smokestacks originating in the US Midwest. Reagan honored the agreement to Canadian Prime Minister Pierre Trudeau's enforcement of anti-pollution regulation. In 1982, Reagan commissioned William Nierenberg to serve on the National Science Board. Nierenberg selected scientists including Gene Likens to serve on a panel to draft a report on acid rain. In 1983, the panel of scientists came up with a draft report, which concluded that acid rain is a real problem and solutions should be sought. White House Office of Science and Technology Policy reviewed the draft report and sent Fred Singer's suggestions of the report, which cast doubt on the cause of acid rain. The panelists revealed rejections against Singer's positions and submitted the report to Nierenberg in April. In May 1983, the House of Representatives voted against legislation controlling sulfur emissions. There was a debate about whether Nierenberg delayed the release of the report. Nierenberg denied the saying about his suppression of the report
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and stated that it was withheld after the House's vote because it was not ready to be published. In 1991, the US National Acid Precipitation Assessment Program (NAPAP) provided its first assessment of acid rain in the United States. It reported that 5% of New England Lakes were acidic, with sulfates being the most common problem. They noted that 2% of the lakes could no longer support Brook Trout, and 6% of the lakes were unsuitable for the survival of many minnow species. Subsequent Reports to Congress have documented chemical changes in soil and freshwater ecosystems, nitrogen saturation, soil nutrient decreases, episodic acidification, regional haze, and damage to historical monuments. Meanwhile, in 1990, the US Congress passed a series of amendments to the Clean Air Act. Title IV of these amendments established a cap and trade system designed to control emissions of sulfur dioxide and nitrogen oxides. Both these emissions proved to cause a significant problem for U.S. citizens and their access to healthy, clean air. Title IV called for a total reduction of about 10 million tons of SO2 emissions from power plants, close to a 50% reduction. It was implemented in two phases. Phase I began in 1995 and limited sulfur dioxide emissions from 110 of the largest power plants to 8.7 million tons of sulfur dioxide. One power plant in New England (Merrimack) was in Phase I. Four other plants (Newington, Mount Tom, Brayton Point, and Salem Harbor) were added under other program provisions. Phase II began in 2000 and affects most of the power plants in the country. During the 1990s, research continued. On March 10, 2005, the EPA issued the Clean Air Interstate Rule (CAIR). This rule provides states with a solution to the problem of power plant pollution that drifts from one state to
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another. CAIR will permanently cap emissions of SO2 and NOx in the eastern United States. When fully implemented, CAIR will reduce SO2 emissions in 28 eastern states and the District of Columbia by over 70% and NOx emissions by over 60% from 2003 levels. Overall, the program's cap and trade program has been successful in achieving its goals. Since the 1990s, SO2 emissions have dropped 40%, and according to the Pacific Research Institute, acid rain levels have dropped 65% since 1976. Conventional regulation was used in the European Union, which saw a decrease of over 70% in SO2 emissions during the same period. In 2007, total SO2 emissions were 8.9 million tons, achieving the program's long-term goal ahead of the 2010 statutory deadline. In 2007 the EPA estimated that by 2010, the overall costs of complying with the program for businesses and consumers would be $1 billion to $2 billion a year, only one-fourth of what was initially predicted. Forbes says: "In 2010, by which time the cap and trade system had been augmented by the George W. Bush administration's Clean Air Interstate Rule, SO2 emissions had fallen to 5.1 million tons." The term citizen science can be traced back as far as January 1989 to a campaign by the Audubon Society to measure acid rain. Scientist Muki Haklay cites in a policy report for the Wilson Center entitled 'Citizen Science and Policy: A European Perspective' a first use of the term 'citizen science' by R. Kerson in the magazine MIT Technology Review from January 1989. Quoting from the Wilson Center report: "The new form of engagement in science received the name "citizen science". The first recorded example of using the term is from 1989, describing how 225 volunteers across the US collected rain samples to assist the Audubon Society
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in an acid-rain awareness-raising campaign. The volunteers collected samples, checked for acidity, and reported to the organization. The information was then used to demonstrate the full extent of the phenomenon." === In Canada === Canadian Harold Harvey was among the first to research a "dead" lake. In 1971, he and R. J. Beamish published a report, "Acidification of the La Cloche Mountain Lakes", documenting the gradual deterioration of fish stocks in 60 lakes in Killarney Park in Ontario, which they had been studying systematically since 1966. In the 1970s and 80s, acid rain was a major topic of research at the Experimental Lakes Area (ELA) in Northwestern Ontario, Canada. Researchers added sulfuric acid to whole lakes in controlled ecosystem experiments to simulate the effects of acid rain. Because its remote conditions allowed for whole-ecosystem experiments, research at the ELA showed that the effect of acid rain on fish populations started at concentrations much lower than those observed in laboratory experiments. In the context of a food web, fish populations crashed earlier than when acid rain had direct toxic effects to the fish because the acidity led to crashes in prey populations (e.g. mysids). As experimental acid inputs were reduced, fish populations and lake ecosystems recovered at least partially, although invertebrate populations have still not completely returned to the baseline conditions. This research showed both that acidification was linked to declining fish populations and that the effects could be reversed if sulfuric acid emissions decreased, and influenced policy in Canada and the United States. In 1985, seven Canadian provinces (all except British Columbia, Alberta, and Saskatchewan) and the federal government signed the Eastern Canada Acid Rain Program. The provinces agreed to limit their combined sulfur dioxide emissions to 2.3 million tonnes by 1994. The Canada-US Air Quality Agreement was signed in
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1991. In 1998, all federal, provincial, and territorial Ministers of Energy and Environment signed The Canada-Wide Acid Rain Strategy for Post-2000, which was designed to protect lakes that are more sensitive than those protected by earlier policies. === In India === Acid rain was first reported in Mumbai (then Bombay) in 1974. Acid rain has been a reported cause of decrease in soil pH, especially in the areas of northeast, coastal regions of Karnataka, Kerala, Odisha, Bihar, and West Bengal. The spread of acid rain over India was first studied by a team of researchers in 1989. Increased risk might be posed by the expected rise in total sulphur emissions from 4,400 kilotonnes (kt) in 1990 to 6,500 kt in 2000, 10,900 kt in 2010 and 18,500 in 2020. Damage to Taj Mahal is a popular example of acid rain's corrosive effect in India. == Emissions of chemicals leading to acidification == The most important gas which leads to acidification is sulfur dioxide. Emissions of nitrogen oxides which are oxidized to form nitric acid are of increasing importance due to stricter controls on emissions of sulfur compounds. 70 Tg(S) per year in the form of SO2 comes from fossil fuel combustion and industry, 2.8 Tg(S) from wildfires, and 7–8 Tg(S) per year from volcanoes. === Natural phenomena === The principal natural phenomena that contribute acid-producing gases to the atmosphere are emissions from volcanoes. Thus, for example, fumaroles from the Laguna Caliente crater of Poás Volcano create extremely high amounts of acid rain and fog, with acidity as high as a pH of 2, clearing an area of any vegetation and frequently causing irritation to the eyes and lungs of inhabitants in nearby settlements. Acid-producing gasses are also created by biological processes that occur on the land, in wetlands, and in
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the oceans. The major biological source of sulfur compounds is dimethyl sulfide. Nitric acid in rainwater is an important source of fixed nitrogen for plant life, and is also produced by electrical activity in the atmosphere such as lightning. Acidic deposits have been detected in glacial ice thousands of years old in remote parts of the globe. === Human activity === The principal cause of acid rain is sulfur and nitrogen compounds from human sources, such as electricity generation, animal agriculture, factories, and motor vehicles. These also include power plants, which use electric power generators that account for a quarter of nitrogen oxides and two-thirds of sulfur dioxide within the atmosphere. Industrial acid rain is a substantial problem in China and Russia and areas downwind from them. These areas all burn sulfur-containing coal to generate heat and electricity. The problem of acid rain has not only increased with population and industrial growth, but has become more widespread. The use of tall smokestacks to reduce local pollution has contributed to the spread of acid rain by releasing gases into regional atmospheric circulation; dispersal from these taller stacks causes pollutants to be carried farther, causing widespread ecological damage. Often deposition occurs a considerable distance downwind of the emissions, with mountainous regions tending to receive the greatest deposition (because of their higher rainfall). An example of this effect is the low pH of rain which falls in Scandinavia. Regarding low pH and pH imbalances in correlation to acid rain, low levels, or those under the pH value of 7, are considered acidic. Acid rain falls at a pH value of roughly 4, making it harmful to consume for humans. When these low pH levels fall in specific regions, they not only affect the environment but also human health. With acidic pH levels in
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humans comes hair loss, low urinary pH, severe mineral imbalances, constipation, and many cases of chronic disorders like Fibromyalgia and Basal Carcinoma. == Chemical process == Combustion of fuels and smelting of some ores produce sulfur dioxide and nitric oxides. They are converted into sulfuric acid and nitric acid. In the gas phase sulfur dioxide is oxidized to sulfuric acid: SO2 + 0.5 O2 + H2O → H2SO4 Nitrogen dioxide reacts with hydroxyl radicals to form nitric acid: NO2 + OH· → HNO3 The detailed mechanisms depend on the presence water and traces of iron and manganese. A number of oxidants are capable of these reactions aside from O2, these include ozone, hydrogen peroxide, and oxygen. == Acid deposition == === Wet deposition === Wet deposition of acids occurs when any form of precipitation (rain, snow, and so on) removes acids from the atmosphere and delivers it to the Earth's surface. This can result from the deposition of acids produced in the raindrops (see aqueous phase chemistry above) or by the precipitation removing the acids either in clouds or below clouds. Wet removal of both gases and aerosols are both of importance for wet deposition. === Dry deposition === Acid deposition also occurs via dry deposition in the absence of precipitation. This can be responsible for as much as 20 to 60% of total acid deposition. This occurs when particles and gases stick to the ground, plants or other surfaces. == Adverse effects == Acid rain has been shown to have adverse impacts on forests, freshwaters and soils, killing insect and aquatic life-forms as well as causing damage to buildings and having impacts on human health. === Surface waters and aquatic animals === Sulfuric acid and nitric acid have multiple impacts on aquatic ecosystems, including acidification, increased nitrogen and aluminum
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content, and alteration of biogeochemical processes. Both the lower pH and higher aluminium concentrations in surface water that occur as a result of acid rain can cause damage to fish and other aquatic animals. At pH lower than 5 most fish eggs will not hatch and lower pH can kill adult fish. As lakes and rivers become more acidic, biodiversity is reduced. Acid rain has eliminated insect life and some fish species, including the brook trout in some lakes, streams, and creeks in geographically sensitive areas, such as the Adirondack Mountains of the United States. However, the extent to which acid rain contributes directly or indirectly via runoff from the catchment to lake and river acidity (i.e., depending on characteristics of the surrounding watershed) is variable. The United States Environmental Protection Agency's (EPA) website states: "Of the lakes and streams surveyed, acid rain caused acidity in 75% of the acidic lakes and about 50% of the acidic streams". Lakes hosted by silicate basement rocks are more acidic than lakes within limestone or other basement rocks with a carbonate composition (i.e. marble) due to buffering effects by carbonate minerals, even with the same amount of acid rain. === Soils === Soil biology and chemistry can be seriously damaged by acid rain. Some microbes are unable to tolerate changes to low pH and are killed. The enzymes of these microbes are denatured (changed in shape so they no longer function) by the acid. The hydronium ions of acid rain also mobilize toxins, such as aluminium, and leach away essential nutrients and minerals such as magnesium. 2 H+ (aq) + Mg2+ (clay) ⇌ 2 H+ (clay) + Mg2+ (aq) Soil chemistry can be dramatically changed when base cations, such as calcium and magnesium, are leached by acid rain, thereby affecting sensitive species, such
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as sugar maple (Acer saccharum). Soil acidification Impacts of acidic water and soil acidification on plants could be minor or in most cases major. Most minor cases which do not result in fatality of plant life can be attributed to the plants being less susceptible to acidic conditions and/or the acid rain being less potent. However, even in minor cases, the plant will eventually die due to the acidic water lowering the plant's natural pH. Acidic water enters the plant and causes important plant minerals to dissolve and get carried away; which ultimately causes the plant to die of lack of minerals for nutrition. In major cases, which are more extreme, the same process of damage occurs as in minor cases, which is removal of essential minerals, but at a much quicker rate. Likewise, acid rain that falls on soil and on plant leaves causes drying of the waxy leaf cuticle, which ultimately causes rapid water loss from the plant to the outside atmosphere and eventually results in death of the plant. Soil acidification can lead to a decline in soil microbes as a result of a change in pH, which would have an adverse effect on plants due to their dependence on soil microbes to access nutrients. To see if a plant is being affected by soil acidification, one can closely observe the plant leaves. If the leaves are green and look healthy, the soil pH is normal and acceptable for plant life. But if the plant leaves have yellowing between the veins on their leaves, that means the plant is suffering from acidification and is unhealthy. Moreover, a plant suffering from soil acidification cannot photosynthesize; the acid-water-induced process of drying out of the plant can destroy chloroplast organelles. Without being able to photosynthesize, a plant cannot create nutrients
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for its own survival or oxygen for the survival of aerobic organisms, which affects most species on Earth and ultimately ends the purpose of the plant's existence. === Forests and other vegetation === Adverse effects may be indirectly related to acid rain, like the acid's effects on soil (see above) or high concentration of gaseous precursors to acid rain. High altitude forests are especially vulnerable as they are often surrounded by clouds and fog which are more acidic than rain. Plants are capable of adapting to acid rain. On Jinyun Mountain, Chongqing, plant species were seen adapting to new environmental conditions. The affects on the species ranged from being beneficial to detrimental. With natural rainfall or mild acid rainfall, the biochemical and physiological characteristics of plant seedlings were enhanced. However, once the pH decreases below the threshold of 3.5, the acid rain can no longer be beneficial and begins to have negative affects. Acid rain can negatively impact photosynthesis in plant leaves, when leaves are exposed to a lower pH, photosynthesis is impacted due to the decline in chlorophyll. Acid rain also has the ability to cause deformation to leaves at a cellular level, examples include; tissue scaring and changes to the stomatal, epidermis and mesophyll cells. Additional impacts of acid rain includes a decline in cuticle thickness present on the leaf surface. Because acid rain damages leaves, this directly impacts a plants ability to have a strong canopy cover, a decline in canopy cover can lead plants to be more vulnerable to diseases. Dead or dying trees often appear in areas impacted by acid rain. Acid rain causes aluminum to leach from the soil, posing risks to both plant and animal life. Furthermore, it strips the soil of critical minerals and nutrients necessary for tree growth. At higher altitudes,
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acidic fog and clouds can deplete nutrients from tree foliage, leading to discolored or dead leaves and needles. This depletion compromises the trees' ability to absorb sunlight, weakening them and diminishing their capacity to endure cold conditions. Other plants can also be damaged by acid rain, but the effect on food crops is minimized by the application of lime and fertilizers to replace lost nutrients. In cultivated areas, limestone may also be added to increase the ability of the soil to keep the pH stable, but this tactic is largely unusable in the case of wilderness lands. When calcium is leached from the needles of red spruce, these trees become less cold tolerant and exhibit winter injury and even death. Acid rain may also affect crop productivity by necrosis or changes to soil nutrients, which ultimately prevent plants from reaching maturity. === Ocean acidification === Acid rain has a much less harmful effect on oceans on a global scale, but it creates an amplified impact in the shallower waters of coastal waters. Acid rain can cause the ocean's pH to fall, known as ocean acidification, making it more difficult for different coastal species to create their exoskeletons that they need to survive. These coastal species link together as part of the ocean's food chain, and without them being a source for other marine life to feed off of, more marine life will die. Coral's limestone skeleton is particularly sensitive to pH decreases, because the calcium carbonate, a core component of the limestone skeleton, dissolves in acidic (low pH) solutions. In addition to acidification, excess nitrogen inputs from the atmosphere promote increased growth of phytoplankton and other marine plants, which, in turn, may cause more frequent harmful algal blooms and eutrophication (the creation of oxygen-depleted "dead zones") in some parts of
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the ocean. === Human health effects === Acid rain can negatively impact human health, especially when people breathe in particles released from acid rain. The effects of acid rain on human health are complex and may be seen in several ways, such as respiratory issues for long-term exposure and indirect exposure through contaminated food and water sources. ==== Nitrogen Dioxide Effects ==== Exposure to air pollutants associated with acid rain, such as nitrogen dioxide (NO2), may have a negative impact on respiratory health. Water-soluble nitrogen dioxide accumulates in the tiny airways, where it is transformed into nitric and nitrous acids. Pneumonia caused by nitric acids directly damages the epithelial cells lining the airways, resulting in pulmonary edema. Exposure to nitrogen dioxide also reduces the immune response by inhibiting the generation of inflammatory cytokines by alveolar macrophages in response to bacterial infection. In animal studies, the pollutant further reduces respiratory immunity by decreasing mucociliary clearance in the lower respiratory tract, which results in a reduced ability to remove respiratory infections. ==== Sulfur Trioxide Effects ==== The effects of sulfur trioxide and sulfuric acid are similar because they both produce sulfuric acid when they come into touch with the wet surfaces of your skin or respiratory system. The amount of SO3 breath through the mouth is larger than the amount of SO3 breath through the nose. When humans breathe in sulfur trioxide, small droplets of sulfuric acid will form inside the body and enter the respiratory tract to the lungs depending on the particle size. The effects of SO3 on the respiratory system lead to breathing difficulty in people who have asthma symptoms. Sulfur trioxide also causes very corrosive and irritation on the skin, eye, and gastrointestinal tracts when there is direct exposure to a specific concentration or long-term exposure. Consuming concentrated
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sulfuric acid has been known to burn the mouth and throat, erode a hole in the stomach, burns when it comes into contact with skin, make your eyes weep if it gets into them, and mortality. ==== Federal Government's recommendation ==== ===== Nitrogen Dioxides ===== A 25 parts per million (ppm) maximum for nitric oxide in working air has been set by the Occupational Safety and Health Administration (OSHA) for an 8-hour workday and a 40-hour workweek. Additionally, OSHA has established a 5-ppm nitrogen dioxide exposure limit for 15 minutes in the workplace. ==== Sulfur Trioxide ==== The not-to-exceed limits in the air, water, soil, or food that are recommended by regulations are often based on levels that affect animals before being modified to assist in safeguarding people. Depending on whether they employ different animal studies, have different exposure lengths (e.g., an 8-hour workday versus a 24-hour day), or for other reasons, these not-to-exceed values can vary between federal bodies. The amount of sulfur dioxide that can be emitted into the atmosphere is capped by the EPA. This reduces the quantity of sulfur dioxide in the air that turns into sulfur trioxide and sulfuric acid. Sulfuric acid concentrations in workroom air are restricted by OSHA to 1 mg/m3. Moreover, NIOSH advises a time-weighted average limit of 1 mg/m3. When you are aware of NO2 and SO3 exposure, you should talk to your doctor and ask people who are around you, especially children. === Other adverse effects === Acid rain can damage buildings, historic monuments, and statues, especially those made of rocks, such as limestone and marble, that contain large amounts of calcium carbonate. Acids in the rain react with the calcium compounds in the stones to create gypsum, which then flakes off. CaCO3 (s) + H2SO4 (aq) ⇌ CaSO4 (s)
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+ CO2 (g) + H2O (l) The effects of this are commonly seen on old gravestones, where acid rain can cause the inscriptions to become completely illegible. Acid rain also increases the corrosion rate of metals, in particular iron, steel, copper and bronze. == Affected areas == Places significantly impacted by acid rain around the globe include most of eastern Europe from Poland northward into Scandinavia, the eastern third of the United States, and southeastern Canada. Other affected areas include the southeastern coast of China and Taiwan. == Prevention methods == === Technical solutions === Many coal-firing power stations use flue-gas desulfurization (FGD) to remove sulfur-containing gases from their stack gases. For a typical coal-fired power station, FGD will remove 95% or more of the SO2 in the flue gases. An example of FGD is the wet scrubber which is commonly used. A wet scrubber is basically a reaction tower equipped with a fan that extracts hot smoke stack gases from a power plant into the tower. Lime or limestone in slurry form is also injected into the tower to mix with the stack gases and combine with the sulfur dioxide present. The calcium carbonate of the limestone produces pH-neutral calcium sulfate that is physically removed from the scrubber. That is, the scrubber turns sulfur pollution into industrial sulfates. In some areas the sulfates are sold to chemical companies as gypsum when the purity of calcium sulfate is high. In others, they are placed in landfill. The effects of acid rain can last for generations, as the effects of pH level change can stimulate the continued leaching of undesirable chemicals into otherwise pristine water sources, killing off vulnerable insect and fish species and blocking efforts to restore native life. Fluidized bed combustion also reduces the amount of sulfur emitted by
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power production. Vehicle emissions control reduces emissions of nitrogen oxides from motor vehicles. === International treaties === International treaties on the long-range transport of atmospheric pollutants have been agreed upon by western countries for some time now. Beginning in 1979, European countries convened in order to ratify general principles discussed during the UNECE Convention. The purpose was to combat Long-Range Transboundary Air Pollution. The 1985 Helsinki Protocol on the Reduction of Sulfur Emissions under the Convention on Long-Range Transboundary Air Pollution furthered the results of the convention. Results of the treaty have already come to fruition, as evidenced by an approximate 40 percent drop in particulate matter in North America. The effectiveness of the Convention in combatting acid rain has inspired further acts of international commitment to prevent the proliferation of particulate matter. Canada and the US signed the Air Quality Agreement in 1991. Most European countries and Canada signed the treaties. Activity of the Long-Range Transboundary Air Pollution Convention remained dormant after 1999, when 27 countries convened to further reduce the effects of acid rain. In 2000, foreign cooperation to prevent acid rain was sparked in Asia for the first time. Ten diplomats from countries ranging throughout the continent convened to discuss ways to prevent acid rain. Following these discussions, the Acid Deposition Monitoring Network in East Asia (EANET) was established in 2001 as an intergovernmental initiative to provide science-based inputs for decision makers and promote international cooperation on acid deposition in East Asia. In 2023, the EANET member countries include Cambodia, China, Indonesia, Japan, Lao PDR, Malaysia, Mongolia, Myanmar, the Philippines, Republic of Korea, Russia, Thailand and Vietnam. === Emissions trading === In this regulatory scheme, every current polluting facility is given or may purchase on an open market an emissions allowance for each unit of a designated
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pollutant it emits. Operators can then install pollution control equipment, and sell portions of their emissions allowances they no longer need for their own operations, thereby recovering some of the capital cost of their investment in such equipment. The intention is to give operators economic incentives to install pollution controls. The first emissions trading market was established in the United States by enactment of the Clean Air Act Amendments of 1990. The overall goal of the Acid Rain Program established by the Act is to achieve significant environmental and public health benefits through reductions in emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx), the primary causes of acid rain. To achieve this goal at the lowest cost to society, the program employs both regulatory and market based approaches for controlling air pollution. == See also == Alkaline precipitation Citizen science – one of two 'first uses' of the term was in an acid rain campaign in 1989. List of environmental issues Lists of environmental topics Ocean acidification Rain dust (an alkaline rain) Soil retrogression and degradation == References == == Further reading == Ritchie, Hannah, "What We Learned from Acid Rain: By working together, the nations of the world can solve climate change", Scientific American, vol. 330, no. 1 (January 2024), pp. 75–76. "[C]ountries will act only if they know others are willing to do the same. With acid rain, they did act collectively.... We did something similar to restore Earth's protective ozone layer.... [T]he cost of technology really matters.... In the past decade the price of solar energy has fallen by more than 90 percent and that of wind energy by more than 70 percent. Battery costs have tumbled by 98 percent since 1990, bringing the price of electric cars down with them....[T]he stance of elected officials matters
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more than their party affiliation.... Change can happen – but not on its own. We need to drive it." (p. 76.) == External links == National Acid Precipitation Assessment Program Report – a 98-page report to Congress (2005) Acid rain for schools Acid rain for schools – Hubbard Brook United States Environmental Protection Agency – New England Acid Rain Program (superficial) Acid Rain (more depth than ref. above) U.S. Geological Survey – What is acid rain? Acid Rain: A Continuing National Tragedy – a report from The Adirondack Council on acid rain in the Adirondack region (1998) What Happens to Acid Rain? Acid Rain and how it affects fish and other aquatic organisms Fourth Report for Policy Makers (RPM4): Towards Clean Air for Sustainable Future in East Asia through Collaborative Activities- a report for policy-makers, Acid Deposition Monitoring Network in East Asia, EANET, (2019).
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Marcus Manuel Hartog (19 August 1851, London – 21 January 1924, Paris) was an English educator, natural historian, philosopher of biology and zoologist in Cork, Ireland. He contributed to multiple volumes of the Cambridge Natural History. == Life == Hartog was born in London 1851, the second son of the Professor Alphonse Hartog (died 1904) and Marion (née Moss, 1821–1907), younger brother of Numa Edward Hartog and elder brother of Sir Philip Joseph Hartog, Academic Registrar of London University and Vice-Chancellor of the University of Dacca. His two younger sisters were the pianist and composer Cécile Hartog and the portrait painter Héléna Arsène Darmesteter, Marcus Hartog was educated at the North London Collegiate School, University College, London, and Trinity College, Cambridge, where he took a first class in the National Science Tripos in 1874, and went out in the same year to Ceylon as assistant to the Director of the Royal Botanic Gardens — a post that he held for three years. On his return he became a demonstrator, and afterwards a lecturer in natural history at Owens College, Manchester. In 1882 he began an association of more than 40 years with the educational life of Cork. For 27 years he was Professor of Natural History at Queen's College, Cork (1882–1907), and in 1909 proceeded to the chair of Zoology in what had become University College Cork. When in 1921 he vacated the appointment, he was made Emeritus Professor. Hartog was a Lamarckian. He argued for the inheritance of acquired characteristics and identified as a vitalist. He supported the non-Darwinian evolutionary ideas of Samuel Butler and wrote a supportive introduction to his book Unconscious Memory. He argued that cell division occurs due to a new force he termed "mitokinetism". Hartog died in Paris on 21 January 1924. == Selected publications
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{
"page_id": 20516033,
"source": null,
"title": "Marcus Hartog"
}
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== Hartog contributed articles to the Dictionary of National Biography and the Encyclopædia Britannica, as well as writing many articles for scientific journals. Problems of Life and Reproduction (1913) The True Mechanism of Mitosis (1914) == Family == In 1874 in Paris, France, Hartog married Blanche Levy, daughter of R. Levy, of Paris, and had issue. == References == == External links == Works by or about Marcus Hartog at the Internet Archive
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{
"page_id": 20516033,
"source": null,
"title": "Marcus Hartog"
}
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Characteristic samples is a concept in the field of grammatical inference, related to passive learning. In passive learning, an inference algorithm I {\displaystyle I} is given a set of pairs of strings and labels S {\displaystyle S} , and returns a representation R {\displaystyle R} that is consistent with S {\displaystyle S} . Characteristic samples consider the scenario when the goal is not only finding a representation consistent with S {\displaystyle S} , but finding a representation that recognizes a specific target language. A characteristic sample of language L {\displaystyle L} is a set of pairs of the form ( s , l ( s ) ) {\displaystyle (s,l(s))} where: l ( s ) = 1 {\displaystyle l(s)=1} if and only if s ∈ L {\displaystyle s\in L} l ( s ) = − 1 {\displaystyle l(s)=-1} if and only if s ∉ L {\displaystyle s\notin L} Given the characteristic sample S {\displaystyle S} , I {\displaystyle I} 's output on it is a representation R {\displaystyle R} , e.g. an automaton, that recognizes L {\displaystyle L} . == Formal Definition == === The Learning Paradigm associated with Characteristic Samples === There are three entities in the learning paradigm connected to characteristic samples, the adversary, the teacher and the inference algorithm. Given a class of languages C {\displaystyle \mathbb {C} } and a class of representations for the languages R {\displaystyle \mathbb {R} } , the paradigm goes as follows: The adversary A {\displaystyle A} selects a language L ∈ C {\displaystyle L\in \mathbb {C} } and reports it to the teacher The teacher T {\displaystyle T} then computes a set of strings and label them correctly according to L {\displaystyle L} , trying to make sure that the inference algorithm will compute L {\displaystyle L} The adversary can add
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{
"page_id": 77139138,
"source": null,
"title": "Characteristic samples"
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|
correctly labeled words to the set in order to confuse the inference algorithm The inference algorithm I {\displaystyle I} gets the sample and computes a representation R ∈ R {\displaystyle R\in \mathbb {R} } consistent with the sample. The goal is that when the inference algorithm receives a characteristic sample for a language L {\displaystyle L} , or a sample that subsumes a characteristic sample for L {\displaystyle L} , it will return a representation that recognizes exactly the language L {\displaystyle L} . === Sample === Sample S {\displaystyle S} is a set of pairs of the form ( s , l ( s ) ) {\displaystyle (s,l(s))} such that l ( s ) ∈ { − 1 , 1 } {\displaystyle l(s)\in \{-1,1\}} ==== Sample consistent with a language ==== We say that a sample S {\displaystyle S} is consistent with language L {\displaystyle L} if for every pair ( s , l ( s ) ) {\displaystyle (s,l(s))} in S {\displaystyle S} : l ( s ) = 1 if and only if s ∈ L {\displaystyle l(s)=1{\text{ if and only if }}s\in L} l ( s ) = − 1 if and only if s ∉ L {\displaystyle l(s)=-1{\text{ if and only if }}s\notin L} === Characteristic sample === Given an inference algorithm I {\displaystyle I} and a language L {\displaystyle L} , a sample S {\displaystyle S} that is consistent with L {\displaystyle L} is called a characteristic sample of L {\displaystyle L} for I {\displaystyle I} if: I {\displaystyle I} 's output on S {\displaystyle S} is a representation R {\displaystyle R} that recognizes L {\displaystyle L} . For every sample D {\displaystyle D} that is consistent with L {\displaystyle L} and also fulfils S ⊆ D {\displaystyle S\subseteq D} , I {\displaystyle I}
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{
"page_id": 77139138,
"source": null,
"title": "Characteristic samples"
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's output on D {\displaystyle D} is a representation R {\displaystyle R} that recognizes L {\displaystyle L} . A Class of languages C {\displaystyle \mathbb {C} } is said to have charistaristic samples if every L ∈ C {\displaystyle L\in \mathbb {C} } has a characteristic sample. == Related Theorems == === Theorem === If equivalence is undecidable for a class C {\textstyle \mathbb {C} } over Σ {\textstyle \Sigma } of cardinality bigger than 1, then C {\textstyle \mathbb {C} } doesn't have characteristic samples. ==== Proof ==== Given a class of representations C {\textstyle \mathbb {C} } such that equivalence is undecidable, for every polynomial p ( x ) {\displaystyle p(x)} and every n ∈ N {\displaystyle n\in \mathbb {N} } , there exist two representations r 1 {\displaystyle r_{1}} and r 2 {\displaystyle r_{2}} of sizes bounded by n {\displaystyle n} , that recognize different languages but are inseparable by any string of size bounded by p ( n ) {\displaystyle p(n)} . Assuming this is not the case, we can decide if r 1 {\displaystyle r_{1}} and r 2 {\displaystyle r_{2}} are equivalent by simulating their run on all strings of size smaller than p ( n ) {\displaystyle p(n)} , contradicting the assumption that equivalence is undecidable. === Theorem === If S 1 {\displaystyle S_{1}} is a characteristic sample for L 1 {\displaystyle L_{1}} and is also consistent with L 2 {\displaystyle L_{2}} , then every characteristic sample of L 2 {\displaystyle L_{2}} , is inconsistent with L 1 {\displaystyle L_{1}} . ==== Proof ==== Given a class C {\textstyle \mathbb {C} } that has characteristic samples, let R 1 {\displaystyle R_{1}} and R 2 {\displaystyle R_{2}} be representations that recognize L 1 {\displaystyle L_{1}} and L 2 {\displaystyle L_{2}} respectively. Under the assumption that
|
{
"page_id": 77139138,
"source": null,
"title": "Characteristic samples"
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there is a characteristic sample for L 1 {\displaystyle L_{1}} , S 1 {\displaystyle S_{1}} that is also consistent with L 2 {\displaystyle L_{2}} , we'll assume falsely that there exist a characteristic sample for L 2 {\displaystyle L_{2}} , S 2 {\displaystyle S_{2}} that is consistent with L 1 {\displaystyle L_{1}} . By the definition of characteristic sample, the inference algorithm I {\displaystyle I} must return a representation which recognizes the language if given a sample that subsumes the characteristic sample itself. But for the sample S 1 ∪ S 2 {\displaystyle S_{1}\cup S_{2}} , the answer of the inferring algorithm needs to recognize both L 1 {\displaystyle L_{1}} and L 2 {\displaystyle L_{2}} , in contradiction. === Theorem === If a class is polynomially learnable by example based queries, it is learnable with characteristic samples. == Polynomialy characterizable classes == === Regular languages === The proof that DFA's are learnable using characteristic samples, relies on the fact that every regular language has a finite number of equivalence classes with respect to the right congruence relation, ∼ L {\displaystyle \sim _{L}} (where x ∼ L y {\displaystyle x\sim _{L}y} for x , y ∈ Σ ∗ {\displaystyle x,y\in \Sigma ^{*}} if and only if ∀ z ∈ Σ ∗ : x z ∈ L ↔ y z ∈ L {\displaystyle \forall z\in \Sigma ^{*}:xz\in L\leftrightarrow yz\in L} ). Note that if x {\displaystyle x} , y {\displaystyle y} are not congruent with respect to ∼ L {\displaystyle \sim _{L}} , there exists a string z {\displaystyle z} such that x z ∈ L {\displaystyle xz\in L} but y z ∉ L {\displaystyle yz\notin L} or vice versa, this string is called a separating suffix. ==== Constructing a characteristic sample ==== The construction of a characteristic sample for a language
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{
"page_id": 77139138,
"source": null,
"title": "Characteristic samples"
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L {\displaystyle L} by the teacher goes as follows. Firstly, by running a depth first search on a deterministic automaton A {\displaystyle A} recognizing L {\displaystyle L} , starting from its initial state, we get a suffix closed set of words, W {\displaystyle W} , ordered in shortlex order. From the fact above, we know that for every two states in the automaton, there exists a separating suffix that separates between every two strings that the run of A {\displaystyle A} on them ends in the respective states. We refer to the set of separating suffixes as S {\displaystyle S} . The labeled set (sample) of words the teacher gives the adversary is { ( w , l ( w ) ) | w ∈ W ⋅ S ∪ W ⋅ Σ ⋅ S } {\displaystyle \{(w,l(w))|w\in W\cdot S\cup W\cdot \Sigma \cdot S\}} where l ( w ) {\displaystyle l(w)} is the correct lable of w {\displaystyle w} (whether it is in L {\displaystyle L} or not). We may assume that ϵ ∈ S {\displaystyle \epsilon \in S} . ==== Constructing a deterministic automata ==== Given the sample from the adversary W {\displaystyle W} , the construction of the automaton by the inference algorithm I {\displaystyle I} starts with defining P = prefix ( W ) {\displaystyle P={\text{prefix}}(W)} and S = suffix ( W ) {\displaystyle S={\text{suffix}}(W)} , which are the set of prefixes and suffixes of W {\displaystyle W} respectively. Now the algorithm constructs a matrix M {\displaystyle M} where the elements of P {\displaystyle P} function as the rows, ordered by the shortlex order, and the elements of S {\displaystyle S} function as the columns, ordered by the shortlex order. Next, the cells in the matrix are filled in the following manner for prefix p i {\displaystyle p_{i}}
|
{
"page_id": 77139138,
"source": null,
"title": "Characteristic samples"
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and suffix s j {\displaystyle s_{j}} : If p i s j ∈ W → M i j = l ( p i s j ) {\displaystyle p_{i}s_{j}\in W\rightarrow M_{ij}=l(p_{i}s_{j})} else, M i j = 0 {\displaystyle M_{ij}=0} Now, we say row i {\displaystyle i} and t {\displaystyle t} are distinguishable if there exists an index j {\displaystyle j} such that M i j = − 1 × M t j {\displaystyle M_{ij}=-1\times M_{tj}} . The next stage of the inference algorithm is to construct the set Q {\displaystyle Q} of distinguishable rows in M {\displaystyle M} , by initializing Q {\displaystyle Q} with ϵ {\displaystyle \epsilon } and iterating from the first row of M {\displaystyle M} downwards and doing the following for row r i {\displaystyle r_{i}} : If r i {\displaystyle r_{i}} is distinguishable from all elements in Q {\displaystyle Q} , add it to Q {\displaystyle Q} else, pass on it to the next row From the way the teacher constructed the sample it passed to the adversary, we know that for every s ∈ Q {\displaystyle s\in Q} and every σ ∈ Σ {\displaystyle \sigma \in \Sigma } , the row s σ {\displaystyle s\sigma } exists in M {\displaystyle M} , and from the construction of Q {\displaystyle Q} , there exists a row s ′ ∈ Q {\displaystyle s'\in Q} such that s ′ {\displaystyle s'} and s σ {\displaystyle s\sigma } are indistinguishable. The output automaton will be defined as follows: The set of states is Q {\displaystyle Q} . The initial state is the state corresponding to row ϵ ∈ Q {\displaystyle \epsilon \in Q} . The accepting states is the set { s ∈ Q | l ( s ) = 1 } {\displaystyle \{s\in Q|{\text{ }}l(s)=1\}} . The transitions
|
{
"page_id": 77139138,
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"title": "Characteristic samples"
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function will be defined δ ( s , σ ) = s ′ {\displaystyle \delta (s,\sigma )=s'} , where s ′ {\displaystyle s'} is the element in Q {\displaystyle Q} that is indistinguishable from s σ {\displaystyle s\sigma } . === Other polynomially characterizable classes === Class of languages recognizable by multiplicity automatons Class of languages recognizable by tree automata Class of languages recognizable by multiplicity tree automata Class of languages recognizable by Fully-Ordered Lattice Automata Class of languages recognizable by Visibly One-Counter Automata Class of fully informative omega regular languages == Non polynomially characterizable classes == There are some classes that do not have polynomially sized characteristic samples. For example, from the first theorem in the Related theorems segment, it has been shown that the following classes of languages do not have polynomial sized characteristic samples: C F G {\displaystyle \mathbb {CFG} } - The class of context-free grammars Languages over Σ {\displaystyle \Sigma } of cardinality larger than 1 {\displaystyle 1} L I N G {\displaystyle \mathbb {LING} } - The class of linear grammar languages over Σ {\displaystyle \Sigma } of cardinality larger than 1 {\displaystyle 1} S D G {\displaystyle \mathbb {SDG} } - The class of simple deterministic grammars Languages N F A {\displaystyle \mathbb {NFA} } - The class of nondeterministic finite automata Languages == Relations to other learning paradigms == Classes of representations that has characteristic samples relates to the following learning paradigms: === Class of semi-poly teachable languages === A representation class C {\displaystyle \mathbb {C} } is semi-poly T / L {\displaystyle T/L} teachable if there exist 3 polynomials p , q , r {\displaystyle p,q,r} , a teacher T {\displaystyle T} and an inference algorithm I {\displaystyle I} , such that for any adversary A {\displaystyle A} the following holds:
|
{
"page_id": 77139138,
"source": null,
"title": "Characteristic samples"
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A {\displaystyle A} Selects a representation R {\displaystyle R} of size n {\displaystyle n} from C {\displaystyle \mathbb {C} } T {\displaystyle T} computes a sample that is consistent with the language that R {\displaystyle R} recognize, of size bounded by p ( n ) {\displaystyle p(n)} and the strings in the sample bounded by length q ( n ) {\displaystyle q(n)} A {\displaystyle A} adds correctly labeled strings to the sample computed by T {\displaystyle T} , making the new sample of size m {\displaystyle m} I {\displaystyle I} then computes a representation equivalent to R {\displaystyle R} in time bounded by r ( m ) {\displaystyle r(m)} The class of languages that there exists a polynomial algorithm that given a sample, returns a representation consistent with the sample is called consistency easy. === Polynomially characterizable languages === Given a representation class R {\displaystyle \mathbb {R} } , and I {\displaystyle {\mathcal {I}}} a set of identification algorithms for R {\displaystyle \mathbb {R} } , R {\displaystyle \mathbb {R} } is polynomially characterizable for I {\displaystyle {\mathcal {I}}} if any R ∈ R {\displaystyle R\in \mathbb {R} } has a characteristic sample of size polynomial of R {\displaystyle R} 's size, S {\displaystyle S} , that for every I ∈ I {\displaystyle I\in {\mathcal {I}}} , I {\displaystyle I} 's output on S {\displaystyle S} is R {\displaystyle R} . === Releations between the paradigms === ==== Theorem ==== A consistency-easy class C {\displaystyle \mathbb {C} } has characteristic samples if and only if it is semi-poly T / L {\displaystyle T/L} teachable. ===== Proof ===== Assuming C {\displaystyle \mathbb {C} } has characteristic samples, then for every representation R ∈ C {\displaystyle R\in \mathbb {C} } , its characteristic sample S {\displaystyle S} holds the conditions for the
|
{
"page_id": 77139138,
"source": null,
"title": "Characteristic samples"
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sample computaed by the teacher, and the output of I {\displaystyle I} on every sample S ′ {\displaystyle S'} such that S ⊆ S ′ {\displaystyle S\subseteq S'} is equivalent to R {\displaystyle R} from the definition of characteristic sample. Assuming that C {\displaystyle \mathbb {C} } is semi-poly T / L {\displaystyle T/L} teachable, then for every representation R ∈ C {\displaystyle R\in \mathbb {C} } , the computed sample by the teacher S {\displaystyle S} is a characteristic sample for R {\displaystyle R} . ==== Theorem ==== If C {\displaystyle \mathbb {C} } has characteristic sample, then C {\displaystyle \mathbb {C} } is polynomially characterizable. ===== Proof ===== Assuming falsely that C {\displaystyle \mathbb {C} } is not polynomially characterizable, there are two non equivalent representations R 1 , R 2 ∈ C {\displaystyle R_{1},R_{2}\in \mathbb {C} } , with characteristic samples S 1 {\displaystyle S_{1}} and S 2 {\displaystyle S_{2}} respectively. From the definition of characteristic samples, any inference algorithm I {\displaystyle I} need to infer from the sample S 1 ∪ S 2 {\displaystyle S_{1}\cup S_{2}} a representation compatible with R 1 {\displaystyle R_{1}} and R 2 {\displaystyle R_{2}} , in contradiction. == See also == Grammar induction Passive learning Induction of regular languages Deterministic finite automaton == References ==
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{
"page_id": 77139138,
"source": null,
"title": "Characteristic samples"
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Lysine iron agar or LIA is a differential media used to distinguish bacteria that are able to decarboxylate lysine and/or produce hydrogen sulfide from those that cannot. This test is particularly useful for distinguishing different Gram-negative bacilli—especially among the Enterobacteriaceae. == Composition == A liter of lysine iron agar contains 13.5g of the gelling agent agar, as well as the nutrients lysine (10 g), pancreatic digest of gelatin (5 g), yeast extract (3 g), glucose (1 g), ferric ammonium citrate (0.5 g), and sodium thiosulfate pentahydrate (40 mg). Additionally, 20 mg of the indicator bromcresol purple is added. == Use == Different types of bacteria can be differentiated on the agar by their color. Bacteria able to decarboxylate lysine will leave the media purple colored. Bacteria producing hydrogen sulfide will appear black. A frequent test done with LIA agar is the LIA slant. Here the LIA is solidified at an angle, then inoculated with bacteria by stabbing the agar to within 1/4 inch of the bottom of the tube and streaking the slant. The slant is then incubated at 35 °C for 18–24 hours. The results are scored as follows: purple slant/yellow butt = LSI Negative turbid, purple butt = LSI Positive black precipitate = H2S Positive == References ==
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{
"page_id": 33230017,
"source": null,
"title": "Lysine iron agar"
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Photoinduced electron transfer (PET) is an excited state electron transfer process by which an excited electron is transferred from donor to acceptor. Due to PET a charge separation is generated, i.e., redox reaction takes place in excited state (this phenomenon is not observed in Dexter electron transfer). == Breadth == Such materials include semiconductors that can be photoactivated like many solar cells, biological systems such as those used in photosynthesis, and small molecules with suitable absorptions and redox states. == Process == It is common to describe where electrons reside as electron bands in bulk materials and electron orbitals in molecules. For the sake of expedience the following description will be described in molecular terms. When a photon excites a molecule, an electron in a ground state orbital can be excited to a higher energy orbital. This excited state leaves a vacancy in a ground state orbital that can be filled by an electron donor. It produces an electron in a high energy orbital which can be donated to an electron acceptor. In these respects a photoexcited molecule can act as a good oxidizing agent or a good reducing agent. Photoinduced oxidation [MLn]2+ + hν → [MLn]2+* [MLn]2+* + donor → [MLn]+ + donor+ Photoinduced reduction [MLn]2+ + hν → [MLn]2+* [MLn]2+* + acceptor → [MLn]3+ + acceptor− The end result of both reactions is that an electron is delivered to an orbital that is higher in energy than where it previously resided. This is often described as a charge separated electron-hole pair when working with semiconductors. In the absence of a proper electron donor or acceptor it is possible for such molecules to undergo ordinary fluorescence emission. The electron transfer is one form of photoquenching. == Subsequent processes == In many photo-productive systems this charge separation is kinetically isolated
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{
"page_id": 12979395,
"source": null,
"title": "Photoinduced electron transfer"
}
|
by delivery of the electron to a lower energy conductor attached to the p/n junction or into an electron transport chain. In this case some of the energy can be captured to do work. If the electron is not kinetically isolated thermodynamics will take over and the products will react with each other to regenerate the ground state starting material. This process is called recombination and the photon's energy is released as heat. Recombination of photoinduced oxidation [MLn]+ + donor+ → [MLn]2+ + donor == Potential induced photon production == The reverse process to photoinduced electron transfer is displayed by light emitting diodes (LED) and chemiluminescence, where potential gradients are used to create excited states that decay by light emission. == References ==
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{
"page_id": 12979395,
"source": null,
"title": "Photoinduced electron transfer"
}
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Bolesatine is a glycoprotein isolated from the Rubroboletus satanas (Boletus satanas Lenz) mushroom which has a lectin function that is specific to the sugar binding site of D-galactose. It is a monomeric protein with a compact globular structure and is thermostable. One tryptophan can be found in its primary sequence along with one disulfide bridge. Bolesatine causes gastroenteritis in humans and, at high enough concentrations, inhibits protein synthesis. It does not inhibit protein synthesis directly. Instead, it acts as a phosphatase for nucleoside triphosphate, particularly for GTP. At lower concentrations, it is a mitogen to human and rat T lymphocytes. Studies have shown that at low concentrations, protein kinases C (PKC) are activated in vitro and in vero cells, leading to an increase in DNA synthesis activity. == Effects of bolesatine poisoning == Other than the accumulation of toxins in human liver and organs, Bolesatine poisoning causes agglutination in human red blood cells and platelets at threshold concentrations. The following symptoms of hypertension and dizziness would be expected when affected. In severe cases, death may result. == References ==
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{
"page_id": 60820678,
"source": null,
"title": "Bolesatine"
}
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Michael Osborne (born 1982) is an Australian academic and scientist who serves as a professor of machine learning at University of Oxford in the Machine Learning Research Group in the Department of Engineering Science. In 2016 he co-founded Mind Foundry, an artificial intelligence company, along with fellow professor Stephen Roberts. Osborne suffered from long COVID syndrome. He is an advocate for masking to limit the spread of SARS-CoV-2 and COVID disease. == Education == He has a BEng in Mechanical Engineering and a BSc in both Pure Mathematics and Physics from the University of Western Australia. He has a PhD in Machine Learning from the University of Oxford. == Career == Osborne has contributed to over 100 publications, and his work has received over 24,000 citations with an h-index of 46 according to Google Scholar. and has acted as principal or co-investigator for £10.6M of research funding. His career has focused in particular on Bayesian approaches to AI and machine learning, named after the famous British statistician Thomas Bayes. Osborne's work has contributed to Probabilistic numerics, with Osborne co-authoring the first textbook on the subject. In 2013, Osborne co-authored a paper alongside Swedish-German economist Carl Benedikt Frey called "The Future of Employment: How Susceptible are Jobs to Computerisation?". The paper has received over 13,000 citations and extensive media coverage. In 2023 Osborne gave oral evidence to the UK House of Commons Science and Technology Committee on the subject of the "Governance of Artificial Intelligence". His testimony received significant coverage around his warnings of the threat of "rogue AI". == Honors == He is also an Official Fellow of Exeter College, a Fellow of the ELLIS society, and a Faculty Member of the Oxford-Man Institute of Quantitative Finance. He joined the Oxford Martin School as Lead Researcher on the Oxford Martin
|
{
"page_id": 75238604,
"source": null,
"title": "Michael Osborne (academic)"
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Programme on Technology and Employment in 2015. He is a Director of the EPSRC Centre for Doctoral Training in Autonomous Intelligent Machines and Systems. == References == == External links == Official homepage
|
{
"page_id": 75238604,
"source": null,
"title": "Michael Osborne (academic)"
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An inverted pendulum is a pendulum that has its center of mass above its pivot point. It is unstable and falls over without additional help. It can be suspended stably in this inverted position by using a control system to monitor the angle of the pole and move the pivot point horizontally back under the center of mass when it starts to fall over, keeping it balanced. The inverted pendulum is a classic problem in dynamics and control theory and is used as a benchmark for testing control strategies. It is often implemented with the pivot point mounted on a cart that can move horizontally under control of an electronic servo system as shown in the photo; this is called a cart and pole apparatus. Most applications limit the pendulum to 1 degree of freedom by affixing the pole to an axis of rotation. Whereas a normal pendulum is stable when hanging downward, an inverted pendulum is inherently unstable, and must be actively balanced in order to remain upright; this can be done either by applying a torque at the pivot point, by moving the pivot point horizontally as part of a feedback system, changing the rate of rotation of a mass mounted on the pendulum on an axis parallel to the pivot axis and thereby generating a net torque on the pendulum, or by oscillating the pivot point vertically. A simple demonstration of moving the pivot point in a feedback system is achieved by balancing an upturned broomstick on the end of one's finger. A second type of inverted pendulum is a tiltmeter for tall structures, which consists of a wire anchored to the bottom of the foundation and attached to a float in a pool of oil at the top of the structure that has devices for measuring
|
{
"page_id": 265421,
"source": null,
"title": "Inverted pendulum"
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movement of the neutral position of the float away from its original position. == Overview == A pendulum with its bob hanging directly below the support pivot is at a stable equilibrium point, where it remains motionless because there is no torque on the pendulum. If displaced from this position, it experiences a restoring torque that returns it toward the equilibrium position. A pendulum with its bob in an inverted position, supported on a rigid rod directly above the pivot, 180° from its stable equilibrium position, is at an unstable equilibrium point. At this point again there is no torque on the pendulum, but the slightest displacement away from this position causes a gravitation torque on the pendulum that accelerates it away from equilibrium, causing it to fall over. In order to stabilize a pendulum in this inverted position, a feedback control system can be used, which monitors the pendulum's angle and moves the position of the pivot point sideways when the pendulum starts to fall over, to keep it balanced. The inverted pendulum is a classic problem in dynamics and control theory and is widely used as a benchmark for testing control algorithms (PID controllers, state-space representation, neural networks, fuzzy control, genetic algorithms, etc.). Variations on this problem include multiple links, allowing the motion of the cart to be commanded while maintaining the pendulum, and balancing the cart-pendulum system on a see-saw. The inverted pendulum is related to rocket or missile guidance, where the center of gravity is located behind the center of drag causing aerodynamic instability. The understanding of a similar problem can be shown by simple robotics in the form of a balancing cart. Balancing an upturned broomstick on the end of one's finger is a simple demonstration, and the problem is solved by self-balancing personal transporters
|
{
"page_id": 265421,
"source": null,
"title": "Inverted pendulum"
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such as the Segway PT, the self-balancing hoverboard and the self-balancing unicycle. Another way that an inverted pendulum may be stabilized, without any feedback or control mechanism, is by oscillating the pivot rapidly up and down. This is called Kapitza's pendulum. If the oscillation is sufficiently strong (in terms of its acceleration and amplitude) then the inverted pendulum can recover from perturbations in a strikingly counterintuitive manner. If the driving point moves in simple harmonic motion, the pendulum's motion is described by the Mathieu equation. == Equations of motion == The equations of motion of inverted pendulums are dependent on what constraints are placed on the motion of the pendulum. Inverted pendulums can be created in various configurations resulting in a number of Equations of Motion describing the behavior of the pendulum. === Stationary pivot point === In a configuration where the pivot point of the pendulum is fixed in space, the equation of motion is similar to that for an uninverted pendulum. The equation of motion below assumes no friction or any other resistance to movement, a rigid massless rod, and the restriction to 2-dimensional movement. θ ¨ − g ℓ sin θ = 0 {\displaystyle {\ddot {\theta }}-{g \over \ell }\sin \theta =0} Where θ ¨ {\displaystyle {\ddot {\theta }}} is the angular acceleration of the pendulum, g {\displaystyle g} is the standard gravity on the surface of the Earth, ℓ {\displaystyle \ell } is the length of the pendulum, and θ {\displaystyle \theta } is the angular displacement measured from the equilibrium position. When θ ¨ {\displaystyle {\ddot {\theta }}} added to both sides, it has the same sign as the angular acceleration term: θ ¨ = g ℓ sin θ {\displaystyle {\ddot {\theta }}={g \over \ell }\sin \theta } Thus, the inverted pendulum accelerates
|
{
"page_id": 265421,
"source": null,
"title": "Inverted pendulum"
}
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away from the vertical unstable equilibrium in the direction initially displaced, and the acceleration is inversely proportional to the length. Tall pendulums fall more slowly than short ones. Derivation using torque and moment of inertia: The pendulum is assumed to consist of a point mass, of mass m {\displaystyle m} , affixed to the end of a massless rigid rod, of length ℓ {\displaystyle \ell } , attached to a pivot point at the end opposite the point mass. The net torque of the system must equal the moment of inertia times the angular acceleration: τ n e t = I θ ¨ {\displaystyle {\boldsymbol {\tau }}_{\mathrm {net} }=I{\ddot {\theta }}} The torque due to gravity providing the net torque: τ n e t = m g ℓ sin θ {\displaystyle {\boldsymbol {\tau }}_{\mathrm {net} }=mg\ell \sin \theta \,\!} Where θ {\displaystyle \theta \ } is the angle measured from the inverted equilibrium position. The resulting equation: I θ ¨ = m g ℓ sin θ {\displaystyle I{\ddot {\theta }}=mg\ell \sin \theta \,\!} The moment of inertia for a point mass: I = m R 2 {\displaystyle I=mR^{2}} In the case of the inverted pendulum the radius is the length of the rod, ℓ {\displaystyle \ell } . Substituting in I = m ℓ 2 {\displaystyle I=m\ell ^{2}} m ℓ 2 θ ¨ = m g ℓ sin θ {\displaystyle m\ell ^{2}{\ddot {\theta }}=mg\ell \sin \theta \,\!} Mass and ℓ 2 {\displaystyle \ell ^{2}} is divided from each side resulting in: θ ¨ = g ℓ sin θ {\displaystyle {\ddot {\theta }}={g \over \ell }\sin \theta } === Inverted pendulum on a cart === An inverted pendulum on a cart consists of a mass m {\displaystyle m} at the top of a pole of length ℓ
|
{
"page_id": 265421,
"source": null,
"title": "Inverted pendulum"
}
|
{\displaystyle \ell } pivoted on a horizontally moving base as shown in the adjacent image. The cart is restricted to linear motion and is subject to forces resulting in or hindering motion. === Essentials of stabilization === The essentials of stabilizing the inverted pendulum can be summarized qualitatively in three steps. 1. If the tilt angle θ {\displaystyle \theta } is to the right, the cart must accelerate to the right and vice versa. 2. The position of the cart x {\displaystyle x} relative to track center is stabilized by slightly modulating the null angle (the angle error that the control system tries to null) by the position of the cart, that is, null angle = θ + k x {\displaystyle =\theta +kx} where k {\displaystyle k} is small. This makes the pole want to lean slightly toward track center and stabilize at track center where the tilt angle is exactly vertical. Any offset in the tilt sensor or track slope that would otherwise cause instability translates into a stable position offset. A further added offset gives position control. 3. A normal pendulum subject to a moving pivot point such as a load lifted by a crane, has a peaked response at the pendulum radian frequency of ω p = g / ℓ {\displaystyle \omega _{p}={\sqrt {g/\ell }}} . To prevent uncontrolled swinging, the frequency spectrum of the pivot motion should be suppressed near ω p {\displaystyle \omega _{p}} . The inverted pendulum requires the same suppression filter to achieve stability. As a consequence of the null angle modulation strategy, the position feedback is positive, that is, a sudden command to move right produces an initial cart motion to the left followed by a move right to rebalance the pendulum. The interaction of the pendulum instability and the positive position
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feedback instability to produce a stable system is a feature that makes the mathematical analysis an interesting and challenging problem. === From Lagrange's equations === The equations of motion can be derived using Lagrange's equations. We refer to the drawing to the right where θ ( t ) {\displaystyle \theta (t)} is the angle of the pendulum of length l {\displaystyle l} with respect to the vertical direction and the acting forces are gravity and an external force F in the x-direction. Define x ( t ) {\displaystyle x(t)} to be the position of the cart. The kinetic energy T {\displaystyle T} of the system is: T = 1 2 M v 1 2 + 1 2 m v 2 2 , {\displaystyle T={\frac {1}{2}}Mv_{1}^{2}+{\frac {1}{2}}mv_{2}^{2},} where v 1 {\displaystyle v_{1}} is the velocity of the cart and v 2 {\displaystyle v_{2}} is the velocity of the point mass m {\displaystyle m} . v 1 {\displaystyle v_{1}} and v 2 {\displaystyle v_{2}} can be expressed in terms of x and θ {\displaystyle \theta } by writing the velocity as the first derivative of the position; v 1 2 = x ˙ 2 , {\displaystyle v_{1}^{2}={\dot {x}}^{2},} v 2 2 = ( d d t ( x − ℓ sin θ ) ) 2 + ( d d t ( ℓ cos θ ) ) 2 . {\displaystyle v_{2}^{2}=\left({\frac {\rm {d}}{{\rm {d}}t}}{\left(x-\ell \sin \theta \right)}\right)^{2}+\left({\frac {\rm {d}}{{\rm {d}}t}}{\left(\ell \cos \theta \right)}\right)^{2}.} Simplifying the expression for v 2 {\displaystyle v_{2}} leads to: v 2 2 = x ˙ 2 − 2 ℓ x ˙ θ ˙ cos θ + ℓ 2 θ ˙ 2 . {\displaystyle v_{2}^{2}={\dot {x}}^{2}-2\ell {\dot {x}}{\dot {\theta }}\cos \theta +\ell ^{2}{\dot {\theta }}^{2}.} The kinetic energy is now given by: T = 1 2 (
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"page_id": 265421,
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M + m ) x ˙ 2 − m ℓ x ˙ θ ˙ cos θ + 1 2 m ℓ 2 θ ˙ 2 . {\displaystyle T={\frac {1}{2}}\left(M+m\right){\dot {x}}^{2}-m\ell {\dot {x}}{\dot {\theta }}\cos \theta +{\frac {1}{2}}m\ell ^{2}{\dot {\theta }}^{2}.} The generalized coordinates of the system are θ {\displaystyle \theta } and x {\displaystyle x} , each has a generalized force. On the x {\displaystyle x} axis, the generalized force Q x {\displaystyle Q_{x}} can be calculated through its virtual work Q x δ x = F δ x , Q x = F , {\displaystyle Q_{x}\delta x=F\delta x,\quad Q_{x}=F,} on the θ {\displaystyle \theta } axis, the generalized force Q θ {\displaystyle Q_{\theta }} can be also calculated through its virtual work Q θ δ θ = m g l sin θ δ θ , Q θ = m g l sin θ . {\displaystyle Q_{\theta }\delta \theta =mgl\sin \theta \delta \theta ,\quad Q_{\theta }=mgl\sin \theta .} According to the Lagrange's equations, the equations of motion are: d d t ∂ T ∂ x ˙ − ∂ T ∂ x = F , {\displaystyle {\frac {\mathrm {d} }{\mathrm {d} t}}{\partial {T} \over \partial {\dot {x}}}-{\partial {T} \over \partial x}=F,} d d t ∂ T ∂ θ ˙ − ∂ T ∂ θ = m g l sin θ , {\displaystyle {\frac {\mathrm {d} }{\mathrm {d} t}}{\partial {T} \over \partial {\dot {\theta }}}-{\partial {T} \over \partial \theta }=mgl\sin \theta ,} substituting T {\displaystyle T} in these equations and simplifying leads to the equations that describe the motion of the inverted pendulum: ( M + m ) x ¨ − m ℓ θ ¨ cos θ + m ℓ θ ˙ 2 sin θ = F , {\displaystyle \left(M+m\right){\ddot {x}}-m\ell {\ddot {\theta }}\cos \theta +m\ell
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"page_id": 265421,
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{\dot {\theta }}^{2}\sin \theta =F,} ℓ θ ¨ − g sin θ = x ¨ cos θ . {\displaystyle \ell {\ddot {\theta }}-g\sin \theta ={\ddot {x}}\cos \theta .} These equations are nonlinear, but since the goal of a control system would be to keep the pendulum upright, the equations can be linearized around θ ≈ 0 {\displaystyle \theta \approx 0} . === From Euler-Lagrange equations === The generalized forces can be both written as potential energy V x {\displaystyle V_{x}} and V θ {\displaystyle V_{\theta }} , According to the D'Alembert's principle, generalized forces and potential energy are connected: Q j = d d t ∂ V ∂ q ˙ j − ∂ V ∂ q j , {\displaystyle Q_{j}={\frac {\mathrm {d} }{\mathrm {d} t}}{\frac {\partial V}{\partial {\dot {q}}_{j}}}-{\frac {\partial V}{\partial q_{j}}}\,,} However, under certain circumstances, the potential energy is not accessible, only generalized forces are available. After getting the Lagrangian L = T − V {\displaystyle L=T-V} , we can also use Euler–Lagrange equation to solve for equations of motion: ∂ L ∂ x − d d t ( ∂ L ∂ x ˙ ) = 0 {\displaystyle {\frac {\partial L}{\partial x}}-{\frac {\mathrm {d} }{\mathrm {d} t}}\left({\frac {\partial L}{\partial {\dot {x}}}}\right)=0} , ∂ L ∂ θ − d d t ( ∂ L ∂ θ ˙ ) = 0 {\displaystyle {\frac {\partial L}{\partial \theta }}-{\frac {\mathrm {d} }{\mathrm {d} t}}\left({\frac {\partial L}{\partial {\dot {\theta }}}}\right)=0} . The only difference is whether to incorporate the generalized forces into the potential energy V j {\displaystyle V_{j}} or write them explicitly as Q j {\displaystyle Q_{j}} on the right side, they all lead to the same equations in the final. === From Newton's second law === Oftentimes it is beneficial to use Newton's second law instead of Lagrange's equations because
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"page_id": 265421,
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Newton's equations give the reaction forces at the joint between the pendulum and the cart. These equations give rise to two equations for each body; one in the x-direction and the other in the y-direction. The equations of motion of the cart are shown below where the LHS is the sum of the forces on the body and the RHS is the acceleration. F − R x = M x ¨ {\displaystyle F-R_{x}=M{\ddot {x}}} F N − R y − M g = 0 {\displaystyle F_{N}-R_{y}-Mg=0} In the equations above R x {\displaystyle R_{x}} and R y {\displaystyle R_{y}} are reaction forces at the joint. F N {\displaystyle F_{N}} is the normal force applied to the cart. This second equation depends only on the vertical reaction force, thus the equation can be used to solve for the normal force. The first equation can be used to solve for the horizontal reaction force. In order to complete the equations of motion, the acceleration of the point mass attached to the pendulum must be computed. The position of the point mass can be given in inertial coordinates as r → P = ( x − ℓ sin θ ) x ^ I + ℓ cos θ y ^ I {\displaystyle {\vec {r}}_{P}=(x-\ell \sin \theta ){\hat {x}}_{I}+\ell \cos \theta {\hat {y}}_{I}} Taking two derivatives yields the acceleration vector in the inertial reference frame. a → P / I = ( x ¨ + ℓ θ ˙ 2 sin θ − ℓ θ ¨ cos θ ) x ^ I + ( − ℓ θ ˙ 2 cos θ − ℓ θ ¨ sin θ ) y ^ I {\displaystyle {\vec {a}}_{P/I}=({\ddot {x}}+\ell {\dot {\theta }}^{2}\sin \theta -\ell {\ddot {\theta }}\cos \theta ){\hat {x}}_{I}+(-\ell {\dot {\theta }}^{2}\cos \theta
|
{
"page_id": 265421,
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-\ell {\ddot {\theta }}\sin \theta ){\hat {y}}_{I}} Then, using Newton's second law, two equations can be written in the x-direction and the y-direction. Note that the reaction forces are positive as applied to the pendulum and negative when applied to the cart. This is due to Newton's third law. R x = m ( x ¨ + ℓ θ ˙ 2 sin θ − ℓ θ ¨ cos θ ) {\displaystyle R_{x}=m({\ddot {x}}+\ell {\dot {\theta }}^{2}\sin \theta -\ell {\ddot {\theta }}\cos \theta )} R y − m g = m ( − ℓ θ ˙ 2 cos θ − ℓ θ ¨ sin θ ) {\displaystyle R_{y}-mg=m(-\ell {\dot {\theta }}^{2}\cos \theta -\ell {\ddot {\theta }}\sin \theta )} The first equation allows yet another way to compute the horizontal reaction force in the event the applied force F {\displaystyle F} is not known. The second equation can be used to solve for the vertical reaction force. The first equation of motion is derived by substituting F − R x = M x ¨ {\displaystyle F-R_{x}=M{\ddot {x}}} into R x = m ( x ¨ + ℓ θ ˙ 2 sin θ − ℓ θ ¨ cos θ ) {\displaystyle R_{x}=m({\ddot {x}}+\ell {\dot {\theta }}^{2}\sin \theta -\ell {\ddot {\theta }}\cos \theta )} , which yields ( M + m ) x ¨ − m ℓ θ ¨ cos θ + m ℓ θ ˙ 2 sin θ = F {\displaystyle \left(M+m\right){\ddot {x}}-m\ell {\ddot {\theta }}\cos \theta +m\ell {\dot {\theta }}^{2}\sin \theta =F} By inspection this equation is identical to the result from Lagrange's Method. In order to obtain the second equation, the pendulum equation of motion must be dotted with a unit vector that runs perpendicular to the pendulum at all times and is
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"page_id": 265421,
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typically noted as the x-coordinate of the body frame. In inertial coordinates this vector can be written using a simple 2-D coordinate transformation x ^ B = cos θ x ^ I + sin θ y ^ I {\displaystyle {\hat {x}}_{B}=\cos \theta {\hat {x}}_{I}+\sin \theta {\hat {y}}_{I}} The pendulum equation of motion written in vector form is ∑ F → = m a → P / I {\displaystyle \sum {\vec {F}}=m{\vec {a}}_{P/I}} . Dotting x ^ B {\displaystyle {\hat {x}}_{B}} with both sides yields the following on the LHS (note that a transpose is the same as a dot product) ( x ^ B ) T ∑ F → = ( x ^ B ) T ( R x x ^ I + R y y ^ I − m g y ^ I ) = ( x ^ B ) T ( R p y ^ B − m g y ^ I ) = − m g sin θ {\displaystyle ({\hat {x}}_{B})^{T}\sum {\vec {F}}=({\hat {x}}_{B})^{T}(R_{x}{\hat {x}}_{I}+R_{y}{\hat {y}}_{I}-mg{\hat {y}}_{I})=({\hat {x}}_{B})^{T}(R_{p}{\hat {y}}_{B}-mg{\hat {y}}_{I})=-mg\sin \theta } In the above equation the relationship between body frame components of the reaction forces and inertial frame components of reaction forces is used. The assumption that the bar connecting the point mass to the cart is massless implies that this bar cannot transfer any load perpendicular to the bar. Thus, the inertial frame components of the reaction forces can be written simply as R p y ^ B {\displaystyle R_{p}{\hat {y}}_{B}} , which signifies that the bar can transfer loads only along the axis of the bar itself. This gives rise to another equation that can be used to solve for the tension in the rod itself: R p = R x 2 + R y 2 {\displaystyle R_{p}={\sqrt {R_{x}^{2}+R_{y}^{2}}}} The RHS
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"page_id": 265421,
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of the equation is computed similarly by dotting x ^ B {\displaystyle {\hat {x}}_{B}} with the acceleration of the pendulum. The result (after some simplification) is shown below. m ( x ^ B ) T ( a → P / I ) = m ( x ¨ cos θ − ℓ θ ¨ ) {\displaystyle m({\hat {x}}_{B})^{T}({\vec {a}}_{P/I})=m({\ddot {x}}\cos \theta -\ell {\ddot {\theta }})} Combining the LHS with the RHS and dividing through by m yields ℓ θ ¨ − g sin θ = x ¨ cos θ {\displaystyle \ell {\ddot {\theta }}-g\sin \theta ={\ddot {x}}\cos \theta } which again is identical to the result of Lagrange's method. The benefit of using Newton's method is that all reaction forces are revealed to ensure that nothing is damaged. For a derivation of the equations of motions from Newton's second law, as above, using the Symbolic Math Toolbox and references therein. == Variants == Achieving stability of an inverted pendulum has become a common engineering challenge for researchers. There are different variations of the inverted pendulum on a cart ranging from a rod on a cart to a multiple segmented inverted pendulum on a cart. Another variation places the inverted pendulum's rod or segmented rod on the end of a rotating assembly. In both, (the cart and rotating system) the inverted pendulum can fall only in a plane. The inverted pendulums in these projects can either be required to maintain balance only after an equilibrium position is achieved, or can achieve equilibrium by itself. Another platform is a two-wheeled balancing inverted pendulum. The two wheeled platform has the ability to spin on the spot offering a great deal of maneuverability. Yet another variation balances on a single point. A spinning top, a unicycle, or an inverted pendulum atop a
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"page_id": 265421,
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spherical ball all balance on a single point. === Kapitza's pendulum === An inverted pendulum in which the pivot is oscillated rapidly up and down can be stable in the inverted position. This is called Kapitza's pendulum, after Russian physicist Pyotr Kapitza who first analysed it. The equation of motion for a pendulum connected to a massless, oscillating base is derived the same way as with the pendulum on the cart. The position of the point mass is now given by: ( − ℓ sin θ , y + ℓ cos θ ) {\displaystyle \left(-\ell \sin \theta ,y+\ell \cos \theta \right)} and the velocity is found by taking the first derivative of the position: v 2 = y ˙ 2 − 2 ℓ y ˙ θ ˙ sin θ + ℓ 2 θ ˙ 2 . {\displaystyle v^{2}={\dot {y}}^{2}-2\ell {\dot {y}}{\dot {\theta }}\sin \theta +\ell ^{2}{\dot {\theta }}^{2}.} The Lagrangian for this system can be written as: L = 1 2 m ( y ˙ 2 − 2 ℓ y ˙ θ ˙ sin θ + ℓ 2 θ ˙ 2 ) − m g ( y + ℓ cos θ ) {\displaystyle L={\frac {1}{2}}m\left({\dot {y}}^{2}-2\ell {\dot {y}}{\dot {\theta }}\sin \theta +\ell ^{2}{\dot {\theta }}^{2}\right)-mg\left(y+\ell \cos \theta \right)} and the equation of motion follows from: d d t ∂ L ∂ θ ˙ − ∂ L ∂ θ = 0 {\displaystyle {\mathrm {d} \over \mathrm {d} t}{\partial {L} \over \partial {\dot {\theta }}}-{\partial {L} \over \partial \theta }=0} resulting in: ℓ θ ¨ − y ¨ sin θ = g sin θ . {\displaystyle \ell {\ddot {\theta }}-{\ddot {y}}\sin \theta =g\sin \theta .} If y represents a simple harmonic motion, y = A sin ω t {\displaystyle y=A\sin \omega t} , the
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"page_id": 265421,
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following differential equation is: θ ¨ − g ℓ sin θ = − A ℓ ω 2 sin ω t sin θ . {\displaystyle {\ddot {\theta }}-{g \over \ell }\sin \theta =-{A \over \ell }\omega ^{2}\sin \omega t\sin \theta .} This equation does not have elementary closed-form solutions, but can be explored in a variety of ways. It is closely approximated by the Mathieu equation, for instance, when the amplitude of oscillations are small. Analyses show that the pendulum stays upright for fast oscillations. The first plot shows that when y {\displaystyle y} is a slow oscillation, the pendulum quickly falls over when disturbed from the upright position. The angle θ {\displaystyle \theta } exceeds 90° after a short time, which means the pendulum has fallen on the ground. If y {\displaystyle y} is a fast oscillation the pendulum can be kept stable around the vertical position. The second plot shows that when disturbed from the vertical position, the pendulum now starts an oscillation around the vertical position ( θ = 0 {\displaystyle \theta =0} ). The deviation from the vertical position stays small, and the pendulum doesn't fall over. == Examples == Arguably the most prevalent example of a stabilized inverted pendulum is a human being. A person standing upright acts as an inverted pendulum with their feet as the pivot, and without constant small muscular adjustments would fall over. The human nervous system contains an unconscious feedback control system, the sense of balance or righting reflex, that uses proprioceptive input from the eyes, muscles and joints, and orientation input from the vestibular system consisting of the three semicircular canals in the inner ear, and two otolith organs, to make continual small adjustments to the skeletal muscles to keep us standing upright. Walking, running, or balancing
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on one leg puts additional demands on this system. Certain diseases and alcohol or drug intoxication can interfere with this reflex, causing dizziness and disequilibration, an inability to stand upright. A field sobriety test used by police to test drivers for the influence of alcohol or drugs, tests this reflex for impairment. Some simple examples include balancing brooms or meter sticks by hand. The inverted pendulum has been employed in various devices and trying to balance an inverted pendulum presents a unique engineering problem for researchers. The inverted pendulum was a central component in the design of several early seismometers due to its inherent instability resulting in a measurable response to any disturbance. The inverted pendulum model has been used in some recent personal transporters, such as the two-wheeled self-balancing scooters and single-wheeled electric unicycles. These devices are kinematically unstable and use an electronic feedback servo system to keep them upright. Swinging a pendulum on a cart into its inverted pendulum state is considered a traditional optimal control toy problem/benchmark. == See also == Double inverted pendulum Inertia wheel pendulum Furuta pendulum iBOT Humanoid robot Ballbot == References == D. Liberzon Switching in Systems and Control (2003 Springer) pp. 89ff == Further reading == Franklin; et al. (2005). Feedback control of dynamic systems, 5, Prentice Hall. ISBN 0-13-149930-0 == External links == YouTube - Inverted Pendulum - Demo #3 YouTube - inverted pendulum YouTube - Double Pendulum on a Cart YouTube - Triple Pendulum on a Cart A dynamical simulation of an inverse pendulum on an oscillatory base Archived 2019-09-13 at the Wayback Machine Inverted Pendulum: Analysis, Design, and Implementation Non-Linear Swing-Up and Stabilizing Control of an Inverted Pendulum System Stabilization fuzzy control of inverted pendulum systems Blog post on inverted pendulum, with Python code Equations of Motion for the
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Cart and Pole Control Task
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Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. Autoxidation leads to degradation of organic compounds, including living matter. Antioxidants are frequently added to industrial products, such as polymers, fuels, and lubricants, to extend their usable lifetimes. Foods are also treated with antioxidants to prevent spoilage, in particular the rancidification of oils and fats. In cells, antioxidants such as glutathione, mycothiol, or bacillithiol, and enzyme systems like superoxide dismutase inhibit damage from oxidative stress. Known dietary antioxidants are vitamins A, C, and E, but the term has also been applied to various compounds that exhibit antioxidant properties in vitro, having little evidence for antioxidant properties in vivo. Dietary supplements marketed as antioxidants have not been shown to maintain health or prevent disease in humans. == History == As part of their adaptation from marine life, terrestrial plants began producing non-marine antioxidants such as ascorbic acid (vitamin C), polyphenols, and tocopherols. The evolution of angiosperm plants between 50 and 200 million years ago resulted in the development of many antioxidant pigments – particularly during the Jurassic period – as chemical defences against reactive oxygen species that are byproducts of photosynthesis. Originally, the term antioxidant specifically referred to a chemical that prevented the consumption of oxygen. In the late 19th and early 20th centuries, extensive study concentrated on the use of antioxidants in important industrial processes, such as the prevention of metal corrosion, the vulcanization of rubber, and the polymerization of fuels in the fouling of internal combustion engines. Early research on the role of antioxidants in biology focused on their use in preventing the oxidation of unsaturated fats, which is the cause of rancidity. Antioxidant activity could be measured simply by placing the fat in a closed container with oxygen and measuring the rate of oxygen
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{
"page_id": 3277,
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"title": "Antioxidant"
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consumption. However, it was the identification of vitamins C and E as antioxidants that revolutionized the field and led to the realization of the importance of antioxidants in the biochemistry of living organisms. The possible mechanisms of action of antioxidants were first explored when it was recognized that a substance with anti-oxidative activity is likely to be one that is itself readily oxidized. Research into how vitamin E prevents the process of lipid peroxidation led to the identification of antioxidants as reducing agents that prevent oxidative reactions, often by scavenging reactive oxygen species before they can damage cells. == Uses == === Food preservatives === Antioxidants are added to food to prevent deterioration. Exposure to oxygen and sunlight are the two main factors in the oxidation of food, so food is preserved by keeping in the dark and sealing it in containers or even coating it in wax, as with cucumbers. However, as oxygen is also important for plant respiration, storing plant materials in anaerobic conditions produces unpleasant flavors and unappealing colors. Consequently, packaging of fresh fruits and vegetables contains an ≈8% oxygen atmosphere. Antioxidants are an especially important class of preservatives as, unlike bacterial or fungal spoilage, oxidation reactions still occur relatively rapidly in frozen or refrigerated food. These preservatives include natural antioxidants such as ascorbic acid (AA, E300) and tocopherols (E306), as well as synthetic antioxidants such as propyl gallate (PG, E310), tertiary butylhydroquinone (TBHQ), butylated hydroxyanisole (BHA, E320) and butylated hydroxytoluene (BHT, E321). Unsaturated fats can be highly susceptible to oxidation, causing rancidification. Oxidized lipids are often discolored and can impart unpleasant tastes and flavors. Thus, these foods are rarely preserved by drying; instead, they are preserved by smoking, salting, or fermenting. Even less fatty foods such as fruits are sprayed with sulfurous antioxidants prior to air
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{
"page_id": 3277,
"source": null,
"title": "Antioxidant"
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drying. Metals catalyse oxidation. Some fatty foods such as olive oil are partially protected from oxidation by their natural content of antioxidants. Fatty foods are sensitive to photooxidation, which forms hydroperoxides by oxidizing unsaturated fatty acids and ester. Exposure to ultraviolet (UV) radiation can cause direct photooxidation and decompose peroxides and carbonyl molecules. These molecules undergo free radical chain reactions, but antioxidants inhibit them by preventing the oxidation processes. === Pharmaceutical excipients === Some pharmaceutical products require protection from oxidation. A number of antioxidants can be used as excipients. Sequestrants such as disodium EDTA can also be used to prevent metal-catalyzed oxidation. === Cosmetics preservatives === Antioxidant stabilizers are also added to fat-based cosmetics such as lipstick and moisturizers to prevent rancidity. Antioxidants in cosmetic products prevent oxidation of active ingredients and lipid content. For example, phenolic antioxidants such as stilbenes, flavonoids, and hydroxycinnamic acid strongly absorb UV radiation due to the presence of chromophores. They reduce oxidative stress from sun exposure by absorbing UV light. === Industrial uses === Antioxidants may be added to industrial products, such as stabilizers in fuels and additives in lubricants, to prevent oxidation and polymerization that leads to the formation of engine-fouling residues. Antioxidant polymer stabilizers are widely used to prevent the degradation of polymers, such as rubbers, plastics and adhesives, that causes a loss of strength and flexibility in these materials. Polymers containing double bonds in their main chains, such as natural rubber and polybutadiene, are especially susceptible to oxidation and ozonolysis. They can be protected by antiozonants. Oxidation can be accelerated by UV radiation in natural sunlight to cause photo-oxidation. Various specialised light stabilisers, such as HALS may be added to plastics to prevent this. Antioxidants for polymer materials are: Primary antioxidants scavenge free radicals formed during the initial (thermal) oxidation process
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"page_id": 3277,
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(ROO•), thus preventing chain reactions that lead to polymer degradation. Phenolics: They are more specifically "hindered phenols", which means a bulky group (typically a tert-butyl) is put near the phenol OH. Examples: butylated hydroxytoluene, 2,4-dimethyl-6-tert-butylphenol, para tertiary butyl phenol, 2,6-di-tert-butylphenol, 1,3,5-Tris(4-(tert-butyl)-3-hydroxy-2,6-dimethylbenzyl)-1,3,5-triazinane-2,4,6-trione Secondary aromatic amines: Not as hindered, which make them more active. Very few FDA approvals. Hindered amine light stabilizers (HALS): Unlike other primary antioxidants, HALS scavenges free radicals generated during photo-oxidation, thus preventing the polymer material from UV radiation. Secondary antioxidants act to decompose peroxides (ROOH) into non-radical products, thus preventing further generation of free radicals, and contributing to the overall oxidate stability of the polymer. Often used in combination with phenolic antioxidants for syngeristic effects. Phosphites: Example: tris(2,4-di-tert-butylphenyl)phosphite. Thiosynergists: Most of this class are "thio-esters" (not to be confused with thioesters): an ester of 3,3-thiodipropionic acid. Other organic sulfide (R1-S-R2) compounds also have a similar effect. Multifunctional antioxidants: an antioxidant can have both primary and secondary functional groups to act as both. Having multiple functional groups is what "multifunctional" means in chemistry. The hydroxylamine functional group on its own can act as both. Radical scavengers: scavenges free radicals to halt the chain reaction. This can be any radical in the oxidation cycle (R•, ROO•, RO•, •OH), though in practice RO• and •OH are too reactive to "trap". Common types include lactones (esp. substituted benzofuranone) and acrylated bis-phenols. === Use as pharmaceutical === Probucol was originally designed as an antioxidant polymer stabilizer for rubber tires. It was later found to reduce LDL-C levels independently of the LDL receptor and became a prescription drug. Its approval predated statins by a decade. === Environmental and health hazards === Synthetic phenolic antioxidants (SPAs) and aminic antioxidants have potential human and environmental health hazards. SPAs are common in indoor dust, small air particles,
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"page_id": 3277,
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sediment, sewage, river water and wastewater. They are synthesized from phenolic compounds and include 2,6-di-tert-butyl-4-methylphenol (BHT), 2,6-di-tert-butyl-p-benzoquinone (BHT-Q), 2,4-di-tert-butyl-phenol (DBP) and 3-tert-butyl-4-hydroxyanisole (BHA). BHT can cause hepatotoxicity and damage to the endocrine system and may increase the carcinogenicity of 1,1-dimethylhydrazine exposure. BHT-Q can cause DNA damage and mismatches through the cleavage process, generating superoxide radicals. DBP is toxic to marine life if exposed long-term. Phenolic antioxidants have low biodegradability, but they do not have severe toxicity toward aquatic organisms at low concentrations. Another type of antioxidant, diphenylamine (DPA), is commonly used in the production of commercial, industrial lubricants and rubber products and it also acts as a supplement for automotive engine oils. == Oxidative challenge in biology == The vast majority of complex life on Earth requires oxygen for its metabolism, but this same oxygen is a highly reactive element that can damage living organisms. Organisms contain chemicals and enzymes that minimize this oxidative damage without interfering with the beneficial effect of oxygen. In general, antioxidant systems either prevent these reactive species from being formed, or remove them, thus minimizing their damage. Reactive oxygen species can have useful cellular functions, such as redox signaling. Thus, ideally, antioxidant systems do not remove oxidants entirely, but maintain them at some optimum concentration. Reactive oxygen species produced in cells include hydrogen peroxide (H2O2), hypochlorous acid (HClO), and free radicals such as the hydroxyl radical (·OH), and the superoxide anion (O2−). The hydroxyl radical is particularly unstable and will react rapidly and non-specifically with most biological molecules. This species is produced from hydrogen peroxide in metal-catalyzed redox reactions such as the Fenton reaction. These oxidants can damage cells by starting chemical chain reactions such as lipid peroxidation, or by oxidizing DNA or proteins. Damage to DNA can cause mutations and possibly cancer, if not
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"page_id": 3277,
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"title": "Antioxidant"
}
|
reversed by DNA repair mechanisms, while damage to proteins causes enzyme inhibition, denaturation, and protein degradation. The use of oxygen as part of the process for generating metabolic energy produces reactive oxygen species. In this process, the superoxide anion is produced as a by-product of several steps in the electron transport chain. Particularly important is the reduction of coenzyme Q in complex III, since a highly reactive free radical is formed as an intermediate (Q·−). This unstable intermediate can lead to electron "leakage", when electrons jump directly to oxygen and form the superoxide anion, instead of moving through the normal series of well-controlled reactions of the electron transport chain. Peroxide is also produced from the oxidation of reduced flavoproteins, such as complex I. However, although these enzymes can produce oxidants, the relative importance of the electron transfer chain to other processes that generate peroxide is unclear. In plants, algae, and cyanobacteria, reactive oxygen species are also produced during photosynthesis, particularly under conditions of high light intensity. This effect is partly offset by the involvement of carotenoids in photoinhibition, and in algae and cyanobacteria, by large amount of iodide and selenium, which involves these antioxidants reacting with over-reduced forms of the photosynthetic reaction centres to prevent the production of reactive oxygen species. === Examples of bioactive antioxidant compounds === Physiological antioxidants are classified into two broad divisions, depending on whether they are soluble in water (hydrophilic) or in lipids (lipophilic). In general, water-soluble antioxidants react with oxidants in the cell cytosol and the blood plasma, while lipid-soluble antioxidants protect cell membranes from lipid peroxidation. These compounds may be synthesized in the body or obtained from the diet. The different antioxidants are present at a wide range of concentrations in body fluids and tissues, with some such as glutathione or ubiquinone mostly
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{
"page_id": 3277,
"source": null,
"title": "Antioxidant"
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present within cells, while others such as uric acid are more systemically distributed (see table below). Some antioxidants are only found in a few organisms, and can be pathogens or virulence factors. The interactions between these different antioxidants may be synergistic and interdependent. The action of one antioxidant may therefore depend on the proper function of other members of the antioxidant system. The amount of protection provided by any one antioxidant will also depend on its concentration, its reactivity towards the particular reactive oxygen species being considered, and the status of the antioxidants with which it interacts. Some compounds contribute to antioxidant defense by chelating transition metals and preventing them from catalyzing the production of free radicals in the cell. The ability to sequester iron for iron-binding proteins, such as transferrin and ferritin, is one such function. Selenium and zinc are commonly referred to as antioxidant minerals, but these chemical elements have no antioxidant action themselves, but rather are required for the activity of antioxidant enzymes, such as glutathione reductase and superoxide dismutase. (See also selenium in biology and zinc in biology.) ==== Uric acid ==== Uric acid has the highest concentration of any blood antioxidant and provides over half of the total antioxidant capacity of human serum. Uric acid's antioxidant activities are also complex, given that it does not react with some oxidants, such as superoxide, but does act against peroxynitrite, peroxides, and hypochlorous acid. Concerns over elevated UA's contribution to gout must be considered one of many risk factors. By itself, UA-related risk of gout at high levels (415–530 μmol/L) is only 0.5% per year with an increase to 4.5% per year at UA supersaturation levels (535+ μmol/L). Many of these aforementioned studies determined UA's antioxidant actions within normal physiological levels, and some found antioxidant activity at levels
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{
"page_id": 3277,
"source": null,
"title": "Antioxidant"
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as high as 285 μmol/L. ==== Vitamin C ==== Ascorbic acid or vitamin C, an oxidation-reduction (redox) catalyst found in both animals and plants, can reduce, and thereby neutralize, reactive oxygen species such as hydrogen peroxide. In addition to its direct antioxidant effects, ascorbic acid is also a substrate for the redox enzyme ascorbate peroxidase, a function that is used in stress resistance in plants. Ascorbic acid is present at high levels in all parts of plants and can reach concentrations of 20 millimolar in chloroplasts. ==== Glutathione ==== Glutathione has antioxidant properties since the thiol group in its cysteine moiety is a reducing agent and can be reversibly oxidized and reduced. In cells, glutathione is maintained in the reduced form by the enzyme glutathione reductase and in turn reduces other metabolites and enzyme systems, such as ascorbate in the glutathione-ascorbate cycle, glutathione peroxidases and glutaredoxins, as well as reacting directly with oxidants. Due to its high concentration and its central role in maintaining the cell's redox state, glutathione is one of the most important cellular antioxidants. In some organisms glutathione is replaced by other thiols, such as by mycothiol in the Actinomycetes, bacillithiol in some gram-positive bacteria, or by trypanothione in the Kinetoplastids. ==== Vitamin E ==== Vitamin E is the collective name for a set of eight related tocopherols and tocotrienols, which are fat-soluble vitamins with antioxidant properties. Of these, α-tocopherol has been most studied as it has the highest bioavailability, with the body preferentially absorbing and metabolising this form. It has been claimed that the α-tocopherol form is the most important lipid-soluble antioxidant, and that it protects membranes from oxidation by reacting with lipid radicals produced in the lipid peroxidation chain reaction. This removes the free radical intermediates and prevents the propagation reaction from continuing. This reaction
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{
"page_id": 3277,
"source": null,
"title": "Antioxidant"
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produces oxidised α-tocopheroxyl radicals that can be recycled back to the active reduced form through reduction by other antioxidants, such as ascorbate, retinol or ubiquinol. This is in line with findings showing that α-tocopherol, but not water-soluble antioxidants, efficiently protects glutathione peroxidase 4 (GPX4)-deficient cells from cell death. GPx4 is the only known enzyme that efficiently reduces lipid-hydroperoxides within biological membranes. However, the roles and importance of the various forms of vitamin E are presently unclear, and it has even been suggested that the most important function of α-tocopherol is as a signaling molecule, with this molecule having no significant role in antioxidant metabolism. The functions of the other forms of vitamin E are even less well understood, although γ-tocopherol is a nucleophile that may react with electrophilic mutagens, and tocotrienols may be important in protecting neurons from damage. === Pro-oxidant activities === Antioxidants that are reducing agents can also act as pro-oxidants. For example, vitamin C has antioxidant activity when it reduces oxidizing substances such as hydrogen peroxide; however, it will also reduce metal ions such as iron and copper that generate free radicals through the Fenton reaction. While ascorbic acid is effective antioxidant, it can also oxidatively change the flavor and color of food. With the presence of transition metals, there are low concentrations of ascorbic acid that can act as a radical scavenger in the Fenton reaction. 2 Fe3+ + Ascorbate → 2 Fe2+ + Dehydroascorbate 2 Fe2+ + 2 H2O2 → 2 Fe3+ + 2 OH· + 2 OH− The relative importance of the antioxidant and pro-oxidant activities of antioxidants is an area of current research, but vitamin C, which exerts its effects as a vitamin by oxidizing polypeptides, appears to have a mostly antioxidant action in the human body. === Enzyme systems === As with
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{
"page_id": 3277,
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"title": "Antioxidant"
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the chemical antioxidants, cells are protected against oxidative stress by an interacting network of antioxidant enzymes. Here, the superoxide released by processes such as oxidative phosphorylation is first converted to hydrogen peroxide and then further reduced to give water. This detoxification pathway is the result of multiple enzymes, with superoxide dismutases catalysing the first step and then catalases and various peroxidases removing hydrogen peroxide. As with antioxidant metabolites, the contributions of these enzymes to antioxidant defenses can be hard to separate from one another, but the generation of transgenic mice lacking just one antioxidant enzyme can be informative. ==== Superoxide dismutase, catalase, and peroxiredoxins ==== Superoxide dismutases (SODs) are a class of closely related enzymes that catalyze the breakdown of the superoxide anion into oxygen and hydrogen peroxide. SOD enzymes are present in almost all aerobic cells and in extracellular fluids. Superoxide dismutase enzymes contain metal ion cofactors that, depending on the isozyme, can be copper, zinc, manganese or iron. In humans, the copper/zinc SOD is present in the cytosol, while manganese SOD is present in the mitochondrion. There also exists a third form of SOD in extracellular fluids, which contains copper and zinc in its active sites. The mitochondrial isozyme seems to be the most biologically important of these three, since mice lacking this enzyme die soon after birth. In contrast, the mice lacking copper/zinc SOD (Sod1) are viable but have numerous pathologies and a reduced lifespan (see article on superoxide), while mice without the extracellular SOD have minimal defects (sensitive to hyperoxia). In plants, SOD isozymes are present in the cytosol and mitochondria, with an iron SOD found in chloroplasts that is absent from vertebrates and yeast. Catalases are enzymes that catalyse the conversion of hydrogen peroxide to water and oxygen, using either an iron or manganese cofactor.
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{
"page_id": 3277,
"source": null,
"title": "Antioxidant"
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This protein is localized to peroxisomes in most eukaryotic cells. Catalase is an unusual enzyme since, although hydrogen peroxide is its only substrate, it follows a ping-pong mechanism. Here, its cofactor is oxidised by one molecule of hydrogen peroxide and then regenerated by transferring the bound oxygen to a second molecule of substrate. Despite its apparent importance in hydrogen peroxide removal, humans with genetic deficiency of catalase — "acatalasemia" — or mice genetically engineered to lack catalase completely, experience few ill effects. Peroxiredoxins are peroxidases that catalyze the reduction of hydrogen peroxide, organic hydroperoxides, as well as peroxynitrite. They are divided into three classes: typical 2-cysteine peroxiredoxins; atypical 2-cysteine peroxiredoxins; and 1-cysteine peroxiredoxins. These enzymes share the same basic catalytic mechanism, in which a redox-active cysteine (the peroxidatic cysteine) in the active site is oxidized to a sulfenic acid by the peroxide substrate. Over-oxidation of this cysteine residue in peroxiredoxins inactivates these enzymes, but this can be reversed by the action of sulfiredoxin. Peroxiredoxins seem to be important in antioxidant metabolism, as mice lacking peroxiredoxin 1 or 2 have shortened lifespans and develop hemolytic anaemia, while plants use peroxiredoxins to remove hydrogen peroxide generated in chloroplasts. ==== Thioredoxin and glutathione systems ==== The thioredoxin system contains the 12-kDa protein thioredoxin and its companion thioredoxin reductase. Proteins related to thioredoxin are present in all sequenced organisms. Plants, such as Arabidopsis thaliana, have a particularly great diversity of isoforms. The active site of thioredoxin consists of two neighboring cysteines, as part of a highly conserved CXXC motif, that can cycle between an active dithiol form (reduced) and an oxidized disulfide form. In its active state, thioredoxin acts as an efficient reducing agent, scavenging reactive oxygen species and maintaining other proteins in their reduced state. After being oxidized, the active thioredoxin is regenerated
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{
"page_id": 3277,
"source": null,
"title": "Antioxidant"
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by the action of thioredoxin reductase, using NADPH as an electron donor. The glutathione system includes glutathione, glutathione reductase, glutathione peroxidases, and glutathione S-transferases. This system is found in animals, plants and microorganisms. Glutathione peroxidase is an enzyme containing four selenium-cofactors that catalyzes the breakdown of hydrogen peroxide and organic hydroperoxides. There are at least four different glutathione peroxidase isozymes in animals. Glutathione peroxidase 1 is the most abundant and is a very efficient scavenger of hydrogen peroxide, while glutathione peroxidase 4 is most active with lipid hydroperoxides. Surprisingly, glutathione peroxidase 1 is dispensable, as mice lacking this enzyme have normal lifespans, but they are hypersensitive to induced oxidative stress. In addition, the glutathione S-transferases show high activity with lipid peroxides. These enzymes are at particularly high levels in the liver and also serve in detoxification metabolism. == Health research == === Relation to diet === The dietary antioxidant vitamins A, C, and E are essential and required in specific daily amounts to prevent diseases. Polyphenols, which have antioxidant properties in vitro due to their free hydroxy groups, are extensively metabolized by catechol-O-methyltransferase which methylates free hydroxyl groups, and thereby prevents them from acting as antioxidants in vivo. === Interactions === Common pharmaceuticals (and supplements) with antioxidant properties may interfere with the efficacy of certain anticancer medication and radiation therapy. Pharmaceuticals and supplements that have antioxidant properties suppress the formation of free radicals by inhibiting oxidation processes. Radiation therapy induce oxidative stress that damages essential components of cancer cells, such as proteins, nucleic acids, and lipids that comprise cell membranes. === Adverse effects === Relatively strong reducing acids can have antinutrient effects by binding to dietary minerals such as iron and zinc in the gastrointestinal tract and preventing them from being absorbed. Examples are oxalic acid, tannins and phytic acid,
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{
"page_id": 3277,
"source": null,
"title": "Antioxidant"
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which are high in plant-based diets. Calcium and iron deficiencies are not uncommon in diets in developing countries where less meat is eaten and there is high consumption of phytic acid from beans and unleavened whole grain bread. However, germination, soaking, or microbial fermentation are all household strategies that reduce the phytate and polyphenol content of unrefined cereal. Increases in Fe, Zn and Ca absorption have been reported in adults fed dephytinized cereals compared with cereals containing their native phytate. High doses of some antioxidants may have harmful long-term effects. The Beta-Carotene and Retinol Efficacy Trial (CARET) study of lung cancer patients found that smokers given supplements containing beta-carotene and vitamin A had increased rates of lung cancer. Subsequent studies confirmed these adverse effects. These harmful effects may also be seen in non-smokers, as one meta-analysis including data from approximately 230,000 patients showed that β-carotene, vitamin A or vitamin E supplementation is associated with increased mortality, but saw no significant effect from vitamin C. No health risk was seen when all the randomized controlled studies were examined together, but an increase in mortality was detected when only high-quality and low-bias risk trials were examined separately. As the majority of these low-bias trials dealt with either elderly people, or people with disease, these results may not apply to the general population. This meta-analysis was later repeated and extended by the same authors, confirming the previous results. These two publications are consistent with some previous meta-analyses that also suggested that vitamin E supplementation increased mortality, and that antioxidant supplements increased the risk of colon cancer. Beta-carotene may also increase lung cancer. Overall, the large number of clinical trials carried out on antioxidant supplements suggest that either these products have no effect on health, or that they cause a small increase in mortality
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{
"page_id": 3277,
"source": null,
"title": "Antioxidant"
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in elderly or vulnerable populations. === Exercise and muscle soreness === A 2017 review showed that taking antioxidant dietary supplements before or after exercise is unlikely to produce a noticeable reduction in muscle soreness after a person exercises. == Levels in food == Antioxidant vitamins are found in vegetables, fruits, eggs, legumes and nuts. Vitamins A, C, and E can be destroyed by long-term storage or prolonged cooking. The effects of cooking and food processing are complex, as these processes can also increase the bioavailability of antioxidants, such as some carotenoids in vegetables. Processed food contains fewer antioxidant vitamins than fresh and uncooked foods, as preparation exposes food to heat and oxygen. Other antioxidants are not obtained from the diet, but instead are made in the body. For example, ubiquinol (coenzyme Q) is poorly absorbed from the gut and is made through the mevalonate pathway. Another example is glutathione, which is made from amino acids. As any glutathione in the gut is broken down to free cysteine, glycine and glutamic acid before being absorbed, even large oral intake has little effect on the concentration of glutathione in the body. Although large amounts of sulfur-containing amino acids such as acetylcysteine can increase glutathione, no evidence exists that eating high levels of these glutathione precursors is beneficial for healthy adults. === Measurement and invalidation of ORAC === Measurement of polyphenol and carotenoid content in food is not a straightforward process, as antioxidants collectively are a diverse group of compounds with different reactivities to various reactive oxygen species. In food science analyses in vitro, the oxygen radical absorbance capacity (ORAC) was once an industry standard for estimating antioxidant strength of whole foods, juices and food additives, mainly from the presence of polyphenols. Earlier measurements and ratings by the United States Department of Agriculture
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{
"page_id": 3277,
"source": null,
"title": "Antioxidant"
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|
were withdrawn in 2012 as biologically irrelevant to human health, referring to an absence of physiological evidence for polyphenols having antioxidant properties in vivo. Consequently, the ORAC method, derived only from in vitro experiments, is no longer considered relevant to human diets or biology, as of 2010. Alternative in vitro measurements of antioxidant content in foods – also based on the presence of polyphenols – include the Folin-Ciocalteu reagent, and the Trolox equivalent antioxidant capacity assay. == References == == Further reading == == External links == Media related to Antioxidants at Wikimedia Commons
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{
"page_id": 3277,
"source": null,
"title": "Antioxidant"
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Activated alumina is manufactured from aluminium hydroxide by dehydroxylating it in a way that produces a highly porous material; this material can have a surface area significantly over 200 m2/g. The compound is used as a desiccant (to keep things dry by adsorbing water from the air) and as a filter of fluoride, arsenic and selenium in drinking water. It is made of aluminium oxide (alumina; Al2O3). It has a very high surface-area-to-weight ratio, due to the many "tunnel like" pores that it has. Activated alumina in its phase composition can be represented only by metastable forms (gamma-Al2O3 etc.). Corundum (alpha-Al2O3), the only stable form of aluminum oxide, does not have such a chemically active surface and is not used as a sorbent. == Uses == === Catalyst applications === Activated alumina is used for a wide range of adsorbent and catalyst applications including the adsorption of catalysts in polyethylene production, in hydrogen peroxide production, as a selective adsorbent for many chemicals including arsenic, fluoride, in sulfur removal from fluid streams (Claus Catalyst process). === Desiccant === Used as a desiccant, it works by a process called adsorption. The water in the air actually sticks to the alumina itself in between the tiny passages as the air passes through them. The water molecules become trapped so that the air is dried out as it passes through the filter. This process is reversible. If the alumina desiccant is heated to ~200 °C, it will release the trapped water. This process is called regenerating the desiccant. === Fluoride adsorbent === Activated alumina is also widely used to remove fluoride from drinking water. In the US, there are widespread programs to fluoridate drinking water. However, in certain regions, such as the Rajasthan region of India, there is enough fluoride in the water to
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{
"page_id": 1969364,
"source": null,
"title": "Activated alumina"
}
|
cause fluorosis. A study from the Harvard school of Public Health found exposure to high levels of fluoride as a child correlated with lower IQ. Activated alumina filters can easily reduce fluoride levels from 10 ppm to less than 1 ppm. The amount of fluoride leached from the water being filtered depends on how long the water is actually touching the alumina filter media. Basically, the more alumina in the filter, the less fluoride will be in the final, filtered water. Lower temperature water, and lower pH water (acidic water) are filtered more effectively too. Ideal pH for treatment is 5.5, which allows for up to a 95% removal rate. As per researches conducted by V.K.Chhabra, Chief Chemist (retd.) P.H.E.D. Rajasthan, activated alumina, when used as a fluoride filter, under field conditions can best be regenerated by a solution of lye (sodium hydroxide; NaOH), sulfuric acid (H2SO4). The fluoride uptake capacity (FUC) of commercial activated alumina can be up to 700 mg/kg. The FUC using V.K. Chhabra's method can be determined as follows: Fluoride solution: Dissolve 22.1 g anhydrous NaF in distilled water and dilute to 1,000 mL. 1 mL = 10 mg fluoride. 10 mL/L = 100 mg/L fluoride. Procedure: To one litre of simulated distilled water containing 100 mg/L of fluoride, agitate at 100 rpm using the jar test machine. Add 10 g of the AA under test. After one hour, switch off the machine and take out the solution. After 5 minutes, carefully decant the supernatant solution and determine the fluoride. Calculate the difference between the original and treated water fluoride concentration. Multiply the difference by 100 to give the fluoride uptake capacity of AA in mg/kg. === Vacuum systems === In high vacuum applications, activated alumina is used as a charge material in fore-line traps to
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{
"page_id": 1969364,
"source": null,
"title": "Activated alumina"
}
|
prevent oil generated by rotary vane pumps from back streaming into the system. A baffle of activated alumina can also replace the refrigerated trap often required for diffusion pumps, though this is rarely used. === Biomaterial === Its mechanical properties and non-reactivity in the biological environment allow it to be a suitable material used to cover surfaces in friction in body prostheses (e.g. hip or shoulder prostheses). Defluoridation Defluoridation is the downward adjustment of the level of fluoride in drinking water. Activated Alumina process is one of the widely used adsorption methods for the defluoridation of drinking water. == See also == Activated carbon Silica gel Synthetic Magnesium Silicate == References ==
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{
"page_id": 1969364,
"source": null,
"title": "Activated alumina"
}
|
In condensed matter physics, second sound is a quantum mechanical phenomenon in which heat transfer occurs by wave-like motion, rather than by the more usual mechanism of diffusion. Its presence leads to a very high thermal conductivity. It is known as "second sound" because the wave motion of entropy and temperature is similar to the propagation of pressure waves in air (sound). The phenomenon of second sound was first described by Lev Landau in 1941. == Description == Normal sound waves are fluctuations in the displacement and density of molecules in a substance; second sound waves are fluctuations in the density of quasiparticle thermal excitations (rotons and phonons). Second sound can be observed in any system in which most phonon-phonon collisions conserve momentum, like superfluids and in some dielectric crystals when Umklapp scattering is small. Contrary to molecules in a gas, quasiparticles are not necessarily conserved. Also gas molecules in a box conserve momentum (except at the boundaries of box), while quasiparticles can sometimes not conserve momentum in the presence of impurities or Umklapp scattering. Umklapp phonon-phonon scattering exchanges momentum with the crystal lattice, so phonon momentum is not conserved, but Umklapp processes can be reduced at low temperatures. Normal sound in gases is a consequence of the collision rate τ between molecules being large compared to the frequency of the sound wave ω ≪ 1/τ. For second sound, the Umklapp rate τu has to be small compared to the oscillation frequency ω ≫ 1/τu for energy and momentum conservation. However analogous to gasses, the relaxation time τN describing the collisions has to be large with respect to the frequency ω ≪ 1/τN, leaving a window: 1 τ u ≪ ω ≪ 1 τ N {\displaystyle {\frac {1}{\tau _{\rm {u}}}}\ll \omega \ll {\frac {1}{\tau _{N}}}} for sound-like behaviour or second
|
{
"page_id": 10685654,
"source": null,
"title": "Second sound"
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sound. The second sound thus behaves as oscillations of the local number of quasiparticles (or of the local energy carried by these particles). Contrary to the normal sound where energy is related to pressure and temperature, in a crystal the local energy density is purely a function of the temperature. In this sense, the second sound can also be considered as oscillations of the local temperature. Second sound is a wave-like phenomenon which makes it very different from usual heat diffusion. == In helium II == Second sound is observed in liquid helium at temperatures below the lambda point, 2.1768 K, where 4He becomes a superfluid known as helium II. Helium II has the highest thermal conductivity of any known material (several hundred times higher than copper). Second sound can be observed either as pulses or in a resonant cavity. The speed of second sound is close to zero near the lambda point, increasing to approximately 20 m/s around 1.8 K, about ten times slower than normal sound waves. At temperatures below 1 K, the speed of second sound in helium II increases as the temperature decreases. Second sound is also observed in superfluid helium-3 below its lambda point 2.5 mK. As per the two-fluid, the speed of second sound is given by c 2 = ( T S 2 C ρ s ρ n ) 1 / 2 {\displaystyle c_{2}=\left({\frac {TS^{2}}{C}}\,{\frac {\rho _{s}}{\rho _{n}}}\right)^{1/2}} where T {\displaystyle T} is the temperature, S {\displaystyle S} is the entropy, C {\displaystyle C} is the specific heat, ρ s {\displaystyle \rho _{s}} is the superfluid density and ρ n {\displaystyle \rho _{n}} is the normal fluid density. As T → 0 {\displaystyle T\rightarrow 0} , c 2 = c / 3 {\displaystyle c_{2}=c/{\sqrt {3}}} , where c = ( ∂ p /
|
{
"page_id": 10685654,
"source": null,
"title": "Second sound"
}
|
∂ ρ ) S ≈ ( ∂ p / ∂ ρ ) T {\displaystyle c=(\partial p/\partial \rho )_{S}\approx (\partial p/\partial \rho )_{T}} is the ordinary (or first) sound speed. == In other media == Second sound has been observed in solid 4He and 3He, and in some dielectric solids such as Bi in the temperature range of 1.2 to 4.0 K with a velocity of 780 ± 50 m/s, or solid sodium fluoride (NaF) around 10 to 20 K. In 2021 this effect was observed in a BKT superfluid as well as in a germanium semiconductor === In graphite === In 2019 it was reported that ordinary graphite exhibits second sound at 120 K. This feature was both predicted theoretically and observed experimentally, and was by far the highest temperature at which second sound has been observed. However, this second sound is observed only at the microscale, because the wave dies out exponentially with characteristic length 1-10 microns. Therefore, presumably graphite in the right temperature regime has extraordinarily high thermal conductivity but only for the purpose of transferring heat pulses distances of order 10 microns, and for pulses of duration on the order of 10 nanoseconds. For more "normal" heat-transfer, graphite's observed thermal conductivity is less than that of, e.g., copper. The theoretical models, however, predict longer absorption lengths would be seen in isotopically pure graphite, and perhaps over a wider temperature range, e.g. even at room temperature. (As of March 2019, that experiment has not yet been tried.) == Applications == Measuring the speed of second sound in 3He-4He mixtures can be used as a thermometer in the range 0.01-0.7 K. Oscillating superleak transducers (OST) use second sound to locate defects in superconducting accelerator cavities. == See also == Zero sound Third sound == References == == Bibliography ==
|
{
"page_id": 10685654,
"source": null,
"title": "Second sound"
}
|
Sinyan Shen, Surface Second Sound in Superfluid Helium. PhD Dissertation (1973). http://adsabs.harvard.edu/abs/1973PhDT.......142S V. Peshkov, "'Second Sound' in Helium II," J. Phys. (Moscow) 8, 381 (1944) U. Piram, "Numerical investigation of second sound in liquid helium," Dipl.-Ing. Dissertation (1991). Retrieved on April 15, 2007.
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{
"page_id": 10685654,
"source": null,
"title": "Second sound"
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|
Methylfentanyl may refer to: 3-Methylfentanyl α-Methylfentanyl β-Methylfentanyl
|
{
"page_id": 36375767,
"source": null,
"title": "Methylfentanyl"
}
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In mathematics and physics, Herglotz's variational principle, named after German mathematician and physicist Gustav Herglotz, is an extension of the Hamilton's principle, where the Lagrangian L explicitly involves the action S {\displaystyle S} as an independent variable, and S {\displaystyle S} itself is represented as the solution of an ordinary differential equation (ODE) whose right hand side is the Lagrangian L {\displaystyle L} , instead of an integration of L {\displaystyle L} . Herglotz's variational principle is known as the variational principle for nonconservative Lagrange equations and Hamilton equations. == Mathematical formulation == Suppose there is a Lagrangian L = L ( t , q , u , S ) {\displaystyle L=L(t,{\boldsymbol {q}},{\boldsymbol {u}},S)} of 2 n + 2 {\displaystyle 2n+2} variables, where q = ( q 1 , q 2 , … , q n ) {\displaystyle {\boldsymbol {q}}=(q_{1},q_{2},\dots ,q_{n})} and u = ( u 1 , u 2 , … , u n ) {\displaystyle {\boldsymbol {u}}=(u_{1},u_{2},\dots ,u_{n})} are n {\displaystyle n} dimensional vectors, and t , S {\displaystyle t,S} are scalar values. A time interval [ t 0 , t 1 ] {\displaystyle [t_{0},t_{1}]} is fixed. Given a time-parameterized curve q = q ( t ) {\displaystyle {\boldsymbol {q}}={\boldsymbol {q}}(t)} , consider the ODE { S ˙ ( t ) = L ( t , q ( t ) , q ˙ ( t ) , S ( t ) ) , t ∈ [ t 0 , t 1 ] S ( t 0 ) = S 0 {\displaystyle {\begin{cases}{\dot {S}}(t)=L(t,{\boldsymbol {q}}(t),{\boldsymbol {\dot {q}}}(t),S(t)),&t\in [t_{0},t_{1}]\\S(t_{0})=S_{0}\end{cases}}} When L ( t , q , u , S ) , q ( t ) , u ( t ) {\displaystyle L(t,{\boldsymbol {q}},{\boldsymbol {u}},S),{\boldsymbol {q}}(t),{\boldsymbol {u}}(t)} are all well-behaved functions, this equation allows a unique solution, and thus S 1
|
{
"page_id": 79891671,
"source": null,
"title": "Herglotz's variational principle"
}
|
:= S ( t 1 ) {\displaystyle S_{1}:=S(t_{1})} is a well defined number which is determined by the curve q ( t ) {\displaystyle {\boldsymbol {q}}(t)} . Herglotz's variation problem aims to minimize S 1 {\displaystyle S_{1}} over the family of curves q ( t ) {\displaystyle {\boldsymbol {q}}(t)} with fixed value q 0 {\displaystyle {\boldsymbol {q}}_{0}} at t = t 0 {\displaystyle t=t_{0}} and fixed value q 1 {\displaystyle {\boldsymbol {q}}_{1}} at t = t 1 {\displaystyle t=t_{1}} , i.e. the problem arg min q : q ( t 0 ) = q 0 , q ( t 1 ) = q 1 S 1 [ q ] {\displaystyle {\underset {{\boldsymbol {q}}:{\boldsymbol {q}}(t_{0})={\boldsymbol {q}}_{0},{\boldsymbol {q}}(t_{1})={\boldsymbol {q}}_{1}}{\arg \min }}S_{1}[{\boldsymbol {q}}]} Note that, when L {\displaystyle L} does not explicitly depend on S {\displaystyle S} , i.e. L = L ( t , q , u ) {\displaystyle L=L(t,{\boldsymbol {q}},{\boldsymbol {u}})} , the above ODE system gives exactly S ( t ) = ∫ t 0 t L ( t , q ( τ ) , q ( τ ) ) d τ {\textstyle S(t)=\int _{t_{0}}^{t}L(t,{\boldsymbol {q}}(\tau ),{\boldsymbol {q}}(\tau )){\rm {d}}\tau } , and thus S 1 = S ( t 1 ) = ∫ t 0 t 1 L ( t , q ( t ) , q ( t ) ) d t {\textstyle S_{1}=S(t_{1})=\int _{t_{0}}^{t_{1}}L(t,{\boldsymbol {q}}(t),{\boldsymbol {q}}(t)){\rm {d}}t} , which degenerates to the classical Hamiltonian action. The resulting Euler-Lagrange-Herglotz equation is d d t ( ∂ L ∂ q ˙ ) − ∂ L ∂ q = ∂ L ∂ S ∂ L ∂ q ˙ {\displaystyle {\frac {\mathrm {d} }{\mathrm {d} t}}\left({\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}\right)-{\frac {\partial L}{\partial {\boldsymbol {q}}}}={\frac {\partial L}{\partial S}}{\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}} which involves an extra term ∂ L
|
{
"page_id": 79891671,
"source": null,
"title": "Herglotz's variational principle"
}
|
∂ S ∂ L ∂ q ˙ {\textstyle {\frac {\partial L}{\partial S}}{\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}} that can describe the dissipation of the system. == Derivation == In order to solve this minimization problem, we impose a variation δ q {\displaystyle \delta {\boldsymbol {q}}} on q {\displaystyle {\boldsymbol {q}}} , and suppose S ( t ) {\displaystyle S(t)} undergoes a variation δ S ( t ) {\displaystyle \delta S(t)} correspondingly, then δ S ˙ ( t ) = L ( t , q ( t ) + δ q ( t ) , q ˙ ( t ) + δ q ˙ ( t ) , S ( t ) + δ S ( t ) ) − L ( t , q ( t ) , q ˙ ( t ) , S ( t ) ) = ∂ L ∂ q δ q ( t ) + ∂ L ∂ q ˙ δ q ˙ ( t ) + ∂ L ∂ S δ S ( t ) {\displaystyle {\begin{aligned}\delta {\dot {S}}(t)&=L(t,{\boldsymbol {q}}(t)+\delta {\boldsymbol {q}}(t),{\dot {\boldsymbol {q}}}(t)+\delta {\dot {\boldsymbol {q}}}(t),S(t)+\delta S(t))-L(t,{\boldsymbol {q}}(t),{\dot {\boldsymbol {q}}}(t),S(t))\\&={\frac {\partial L}{\partial {\boldsymbol {q}}}}\delta {\boldsymbol {q}}(t)+{\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}\delta {\dot {\boldsymbol {q}}}(t)+{\frac {\partial L}{\partial S}}\delta S(t)\end{aligned}}} and since the initial condition is not changed, δ S 0 = 0 {\displaystyle \delta S_{0}=0} . The above equation a linear ODE for the function δ S ( t ) {\displaystyle \delta S(t)} , and it can be solved by introducing an integrating factor μ ( t ) = e ∫ t 0 t ∂ L ∂ S d t {\displaystyle \mu (t)=\mathrm {e} ^{\int _{t_{0}}^{t}{\frac {\partial L}{\partial S}}\mathrm {d} t}} , which is uniquely determined by the ODE μ ˙ ( t ) = − μ ( t ) ∂ L ∂ S
|
{
"page_id": 79891671,
"source": null,
"title": "Herglotz's variational principle"
}
|
, u ( t 0 ) = 1. {\displaystyle {\dot {\mu }}(t)=-\mu (t){\frac {\partial L}{\partial S}},\quad u(t_{0})=1.} By multiplying μ ( t ) {\displaystyle \mu (t)} on both sides of the equation of δ S ˙ {\displaystyle \delta {\dot {S}}} and moving the term μ ( t ) ∂ L ∂ S δ S ( t ) {\textstyle \mu (t){\frac {\partial L}{\partial S}}\delta S(t)} to the left hand side, we get μ ( t ) δ S ˙ ( t ) − μ ( t ) ∂ L ∂ S δ S ( t ) = μ ( t ) ( ∂ L ∂ q δ q ( t ) + ∂ L ∂ q ˙ δ q ˙ ( t ) ) . {\displaystyle \mu (t)\delta {\dot {S}}(t)-\mu (t){\frac {\partial L}{\partial S}}\delta S(t)=\mu (t)\left({\frac {\partial L}{\partial {\boldsymbol {q}}}}\delta {\boldsymbol {q}}(t)+{\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}\delta {\dot {\boldsymbol {q}}}(t)\right).} Note that, since μ ˙ ( t ) = − μ ( t ) ∂ L ∂ S {\textstyle {\dot {\mu }}(t)=-\mu (t){\frac {\partial L}{\partial S}}} , the left hand side equals to μ ( t ) δ S ˙ ( t ) + μ ˙ ( t ) δ S ( t ) = d ( μ ( t ) δ S ( t ) ) d t {\displaystyle \mu (t)\delta {\dot {S}}(t)+{\dot {\mu }}(t)\delta S(t)={\frac {\mathrm {d} (\mu (t)\delta S(t))}{\mathrm {d} t}}} and therefore we can do an integration of the equation above from t = t 0 {\displaystyle t=t_{0}} to t = t 1 {\displaystyle t=t_{1}} , yielding μ ( t 1 ) δ S 1 − μ ( t 0 ) δ S 0 = ∫ t 0 t 1 μ ( t ) ( ∂ L ∂ q δ q ( t ) + ∂ L
|
{
"page_id": 79891671,
"source": null,
"title": "Herglotz's variational principle"
}
|
∂ q ˙ δ q ˙ ( t ) ) d t {\displaystyle \mu (t_{1})\delta S_{1}-\mu (t_{0})\delta S_{0}=\int _{t_{0}}^{t_{1}}\mu (t)\left({\frac {\partial L}{\partial {\boldsymbol {q}}}}\delta {\boldsymbol {q}}(t)+{\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}\delta {\dot {\boldsymbol {q}}}(t)\right)\mathrm {d} t} where the δ S 0 = 0 {\displaystyle \delta S_{0}=0} so the left hand side actually only contains one term μ ( t 1 ) δ S 1 {\displaystyle \mu (t_{1})\delta S_{1}} , and for the right hand side, we can perform the integration-by-part on the ∂ L ∂ q ˙ δ q ˙ ( t ) {\textstyle {\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}\delta {\dot {\boldsymbol {q}}}(t)} term to remove the time derivative on δ q {\textstyle \delta {\boldsymbol {q}}} : ∫ t 0 t 1 μ ( t ) ( ∂ L ∂ q δ q ( t ) + ∂ L ∂ q ˙ δ q ˙ ( t ) ) d t = ∫ t 0 t 1 μ ( t ) ∂ L ∂ q δ q ( t ) d t + ∫ t 0 t 1 μ ( t ) ∂ L ∂ q ˙ δ q ˙ ( t ) d t = ∫ t 0 t 1 μ ( t ) ∂ L ∂ q δ q ( t ) d t + μ ( t 1 ) ∂ L ∂ q ˙ δ q ( t 1 ) ⏟ = 0 − μ ( t 0 ) ∂ L ∂ q ˙ δ q ( t 0 ) ⏟ = 0 − ∫ t 0 t 1 d d t ( μ ( t ) ∂ L ∂ q ˙ ) δ q ( t ) d t = ∫ t 0 t 1 μ ( t ) ∂ L ∂ q δ q (
|
{
"page_id": 79891671,
"source": null,
"title": "Herglotz's variational principle"
}
|
t ) d t − ∫ t 0 t 1 d d t ( μ ( t ) ∂ L ∂ q ˙ ) δ q ( t ) d t = ∫ t 0 t 1 μ ( t ) ∂ L ∂ q δ q ( t ) d t − ∫ t 0 t 1 ( μ ˙ ( t ) ∂ L ∂ q ˙ + μ ( t ) d d t ∂ L ∂ q ˙ ) δ q ( t ) d t = ∫ t 0 t 1 μ ( t ) ∂ L ∂ q δ q ( t ) d t − ∫ t 0 t 1 ( − μ ( t ) ∂ L ∂ S ∂ L ∂ q ˙ + μ ( t ) d d t ∂ L ∂ q ˙ ) δ q ( t ) d t = ∫ t 0 t 1 μ ( t ) ( ∂ L ∂ q + ∂ L ∂ S ∂ L ∂ q ˙ − d d t ∂ L ∂ q ˙ ) _ δ q ( t ) d t , {\displaystyle {\begin{aligned}&\int _{t_{0}}^{t_{1}}\mu (t)\left({\frac {\partial L}{\partial {\boldsymbol {q}}}}\delta {\boldsymbol {q}}(t)+{\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}\delta {\dot {\boldsymbol {q}}}(t)\right)\mathrm {d} t\\=&\int _{t_{0}}^{t_{1}}\mu (t){\frac {\partial L}{\partial {\boldsymbol {q}}}}\delta {\boldsymbol {q}}(t)\mathrm {d} t+\int _{t_{0}}^{t_{1}}\mu (t){\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}\delta {\dot {\boldsymbol {q}}}(t)\mathrm {d} t\\=&\int _{t_{0}}^{t_{1}}\mu (t){\frac {\partial L}{\partial {\boldsymbol {q}}}}\delta {\boldsymbol {q}}(t)\mathrm {d} t+\mu (t_{1}){\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}\underbrace {\delta {\boldsymbol {q}}(t_{1})} _{=0}-\mu (t_{0}){\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}\underbrace {\delta {\boldsymbol {q}}(t_{0})} _{=0}-\int _{t_{0}}^{t_{1}}{\frac {\mathrm {d} }{\mathrm {d} t}}\left(\mu (t){\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}\right)\delta {\boldsymbol {q}}(t)\mathrm {d} t\\=&\int _{t_{0}}^{t_{1}}\mu (t){\frac {\partial L}{\partial {\boldsymbol {q}}}}\delta {\boldsymbol {q}}(t)\mathrm {d} t-\int _{t_{0}}^{t_{1}}{\frac {\mathrm {d} }{\mathrm {d}
|
{
"page_id": 79891671,
"source": null,
"title": "Herglotz's variational principle"
}
|
t}}\left(\mu (t){\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}\right)\delta {\boldsymbol {q}}(t)\mathrm {d} t\\=&\int _{t_{0}}^{t_{1}}\mu (t){\frac {\partial L}{\partial {\boldsymbol {q}}}}\delta {\boldsymbol {q}}(t)\mathrm {d} t-\int _{t_{0}}^{t_{1}}\left({\dot {\mu }}(t){\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}+\mu (t){\frac {\mathrm {d} }{\mathrm {d} t}}{\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}\right)\delta {\boldsymbol {q}}(t)\mathrm {d} t\\=&\int _{t_{0}}^{t_{1}}\mu (t){\frac {\partial L}{\partial {\boldsymbol {q}}}}\delta {\boldsymbol {q}}(t)\mathrm {d} t-\int _{t_{0}}^{t_{1}}\left(-\mu (t){\frac {\partial L}{\partial S}}{\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}+\mu (t){\frac {\mathrm {d} }{\mathrm {d} t}}{\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}\right)\delta {\boldsymbol {q}}(t)\mathrm {d} t\\=&\int _{t_{0}}^{t_{1}}\mu (t){\underline {\left({\frac {\partial L}{\partial {\boldsymbol {q}}}}+{\frac {\partial L}{\partial S}}{\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}-{\frac {\mathrm {d} }{\mathrm {d} t}}{\frac {\partial L}{\partial {\dot {\boldsymbol {q}}}}}\right)}}\delta {\boldsymbol {q}}(t)\mathrm {d} t,\end{aligned}}} and when S 1 {\displaystyle S_{1}} is minimized, δ S 1 = 0 {\displaystyle \delta S_{1}=0} for all δ q {\displaystyle \delta {\boldsymbol {q}}} , which indicates that the underlined term in the last line of the equation above has to be zero on the entire interval [ t 0 , t 1 ] {\displaystyle [t_{0},t_{1}]} , this gives rise to the Euler-Lagrange-Herglotz equation. == Examples == One simple one-dimensional ( n = 1 {\displaystyle n=1} ) example is given by the Lagrangian L ( t , x , x ˙ , S ) = 1 2 m x ˙ 2 − V ( x ) − γ S {\displaystyle L(t,x,{\dot {x}},S)={\frac {1}{2}}m{\dot {x}}^{2}-V(x)-\gamma S} The corresponding Euler-Lagrange-Herglotz equation is given as d d t ( m x ˙ ) + V ′ ( x ) = − γ x ˙ , {\displaystyle {\frac {\mathrm {d} }{\mathrm {d} t}}(m{\dot {x}})+V'(x)=-\gamma {\dot {x}},} which simplifies into m x ¨ = − V ′ ( x ) − γ x ˙ . {\displaystyle m{\ddot {x}}=-V'(x)-\gamma {\dot {x}}.} This equation describes the damping motion of a particle in a potential field V {\displaystyle V} ,
|
{
"page_id": 79891671,
"source": null,
"title": "Herglotz's variational principle"
}
|
where γ {\displaystyle \gamma } is the damping coefficient. == References ==
|
{
"page_id": 79891671,
"source": null,
"title": "Herglotz's variational principle"
}
|
The hydrogen hypothesis is a model proposed by William F. Martin and Miklós Müller in 1998 that describes a possible way in which the mitochondrion arose as an endosymbiont within a prokaryotic host in the archaea, giving rise to a symbiotic association of two cells from which the first eukaryotic cell could have arisen (symbiogenesis). According to the hydrogen hypothesis: The hosts that acquired the mitochondria were hydrogen-dependent archaea, possibly similar in physiology to modern methanogenic archaea, which use hydrogen and carbon dioxide to produce methane; The future mitochondrion was a facultatively anaerobic eubacterium which produced hydrogen and carbon dioxide as byproducts of anaerobic respiration; A symbiotic relationship between the two started, based on the host's hydrogen dependence (anaerobic syntrophy). == Mechanism == The hypothesis differs from many alternative views within the endosymbiotic theory framework, which suggest that the first eukaryotic cells evolved a nucleus but lacked mitochondria, the latter arising as a eukaryote engulfed a primitive bacterium that eventually became the mitochondrion. The hypothesis attaches evolutionary significance to hydrogenosomes and provides a rationale for their common ancestry with mitochondria. Hydrogenosomes are anaerobic mitochondria that produce ATP by, as a rule, converting pyruvate into hydrogen, carbon dioxide and acetate. Examples from modern biology are known where methanogens cluster around hydrogenosomes within eukaryotic cells. Most theories within the endosymbiotic theory framework do not address the common ancestry of mitochondria and hydrogenosomes. The hypothesis provides a straightforward explanation for the observation that eukaryotes are genetic chimeras with genes of archaeal and eubacterial ancestry. Furthermore, it would imply that archaea and eukarya split after the modern groups of archaea appeared. Most theories within the endosymbiotic theory framework predict that some eukaryotes never possessed mitochondria. The hydrogen hypothesis predicts that no primitively mitochondrion-lacking eukaryotes ever existed. In the 15 years following the publication of
|
{
"page_id": 593115,
"source": null,
"title": "Hydrogen hypothesis"
}
|
the hydrogen hypothesis, this specific prediction has been tested many times and found to be in agreement with observation. In 2015, the discovery and placement of the Lokiarchaeota (an archaeal lineage possessing an expanded genetic repertoire including genes involved in membrane remodeling and actin cytoskeletal structure) as the sister group to eukaryotes called into question particular tenets of the hydrogen hypothesis, as Lokiarchaeota appear to lack methanogenesis. == See also == Archezoa Eocyte hypothesis == References ==
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{
"page_id": 593115,
"source": null,
"title": "Hydrogen hypothesis"
}
|
A glacial relict is a population of a species that was common in the Northern Hemisphere prior to the onset of glaciation in the late Tertiary that was forced by climate change to retreat into refugia when continental ice sheets advanced. They are typically cold-adapted species with a distribution restricted to regions and microhabitats that allow them to survive despite climatic changes. == Examples == There are a wide variety of plant species which fit the category of glacial relict. The ones given here are a small selection of the much larger group. A tall deciduous tree genus, Liriodendron, was widespread across temperate regions of the Northern Hemisphere until the onset of continental glaciations. The genus took refuge in southeast Asia and southeast North America, expanding to occupy today's temperate habitats. The east-west orientation of mountains in Europe is thought to be the geographic barrier that prevented the genus from migrating far enough southward to avoid extinction. The biogeography of various aquatic species deemed glacial relicts that are found in Lake Sommen is likely related to a different geography during the early history of the lake. One theory claims that aquatic species were transferred from the Baltic Ice Lake through a natural lock system in connection with a temporary advance of the ice-front during the Younger Dryas. On land, the unusual occurrence of dwarf birch near Sund is also judged to be a leftover from a cold geological past. The Franklin tree (Franklinia alatamaha) was a glacial relict in the American Southeast and endemic to the Altamaha River valley in Georgia before going extinct in the wild in the early part of the 19th century. Like various other plants in this region of the United States, it grew in a lowland glacial refuge. Due to changing temperatures in the Holocene,
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{
"page_id": 60951772,
"source": null,
"title": "Glacial relict"
}
|
it was unable to survive - it likely originally dispersed as seeds floating down the Altamaha River, but due to the nature of rivers, it was unable to make the reverse journey to cooler upland climes and survive rising temperatures. This species was the subject of a number of enthusiastic searches to locate potential wild populations in the 20th century, but it was never found in the wild after its original extinction and reintroduction efforts in the early 21st century failed. Examples of other endemic plants in the Southeastern USA which were limited by the same environmental factors include the Florida torreya, the Florida yew, and the now-extinct Critchfield spruce. == See also == Biodiversity hotspot Ecological island Last Glacial Maximum refugia Nunatak hypothesis Rapoport's rule Relict (biology) Sky island Wrangel Island (home to last population of mammoths) == References ==
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{
"page_id": 60951772,
"source": null,
"title": "Glacial relict"
}
|
The toroidal ring model, known originally as the Parson magneton or magnetic electron, is a physical model of subatomic particles. It is also known as the plasmoid ring, vortex ring, or helicon ring. This physical model treated electrons and protons as elementary particles, and was first proposed by Alfred Lauck Parson in 1915. == Theory == Instead of a single orbiting charge, the toroidal ring was conceived as a collection of infinitesimal charge elements, which orbited or circulated along a common continuous path or "loop". In general, this path of charge could assume any shape, but tended toward a circular form due to internal repulsive electromagnetic forces. In this configuration the charge elements circulated, but the ring as a whole did not radiate due to changes in electric or magnetic fields since it remained stationary. The ring produced an overall magnetic field ("spin") due to the current of the moving charge elements. These elements circulated around the ring at the speed of light c, but at frequency ν = c/2πR, which depended inversely on the radius R. The ring's inertial energy increased when compressed, like a spring, and was also inversely proportional to its radius, and therefore proportional to its frequency ν. The theory claimed that the proportionality constant was the Planck constant h, the conserved angular momentum of the ring. According to the model, electrons or protons could be viewed as bundles of "fibers" or "plasmoids" with total charge ±e. The electrostatic repulsion force between charge elements of the same sign was balanced by the magnetic attraction force between the parallel currents in the fibers of a bundle, per Ampère's law. These fibers twisted around the torus of the ring as they progressed around its radius, forming a Slinky-like helix. Circuit completion demanded that each helical plasmoid fiber twisted
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{
"page_id": 13372642,
"source": null,
"title": "Toroidal ring model"
}
|
around the ring an integer number of times as it proceeded around the ring. This requirement was thought to account for "quantum" values of angular momentum and radiation. Chirality demanded the number of fibers to be odd, probably three, like a rope. The helicity of the twist, was thought to distinguish the electron from the proton. The toroidal or "helicon" model did not demand a constant radius or inertial energy for a particle. In general its shape, size, and motion adjusted according to the external electromagnetic fields from its environment. These adjustments or reactions to external field changes constituted the emission or absorption of radiation for the particle. The model, then, claimed to explain how particles linked together to form atoms. == History == === Beginnings === The development of the helicon or toroidal ring began with André-Marie Ampère, who in 1823 proposed tiny magnetic "loops of charge" to explain the attractive force between current elements. In that same era Carl Friedrich Gauss and Michael Faraday also uncovered foundational laws of classical electrodynamics, later collected by James Maxwell as Maxwell's equations. When Maxwell expressed the laws of Gauss, Faraday, and Ampère in differential form, he assumed point particles, an assumption that remains foundational to relativity theory and quantum mechanics today. In 1867 Lord Kelvin suggested that the vortex rings of a perfect fluid discovered by Hermann von Helmholtz represented "the only true atoms". Then shortly before 1900, as scientists still debated over the very existence of atoms, J. J. Thomson and Ernest Rutherford sparked a revolution with experiments confirming the existence and properties of electrons, protons, and nuclei. Max Planck added to the fire when he solved the blackbody radiation problem by assuming not only discrete particles, but discrete frequencies of radiation emanating from these "particles" or "resonators". Planck's famous
|
{
"page_id": 13372642,
"source": null,
"title": "Toroidal ring model"
}
|
paper, which incidentally calculated both the Planck constant h and the Boltzmann constant kB, suggested that something in the "resonators" themselves provided these discrete frequencies. Numerous theories about the structure of the atom developed in the wake of all the new information, of which the 1913 model of Niels Bohr came to predominate. The Bohr model proposed electrons in circular orbit around the nucleus with quantized values of angular momentum. Instead of radiating energy continuously, as classical electrodynamics demanded from an accelerating charge, Bohr's electron radiated discretely when it "leaped" from one state of angular momentum to another. === Parson magneton === In 1915, Alfred Lauck Parson proposed his "magneton" as an improvement over the Bohr model, depicting finite-sized particles with the ability to maintain stability and emit and absorb radiation from electromagnetic waves. At about the same time Leigh Page developed a classical theory of blackbody radiation assuming rotating "oscillators", able to store energy without radiating. Gilbert N. Lewis was inspired in part by Parson's model in developing his theory of chemical bonding. Then David L. Webster wrote three papers connecting Parson's magneton with Page's oscillator and explaining mass and alpha scattering in terms of the magneton. In 1917 Lars O. Grondahl confirmed the model with his experiments on free electrons in iron wires. Parson's theory next attracted the attention of Arthur Compton, who wrote a series of papers on the properties of the electron, and H. Stanley Allen, whose papers also argued for a "ring electron". == Current status == The aspect of the Parson magneton with the most experimental relevance (and the aspect investigated by Grondahl and Webster) was the existence of an electron magnetic dipole moment; this dipole moment is indeed present. However, later work by Paul Dirac and Alfred Landé showed that a pointlike particle
|
{
"page_id": 13372642,
"source": null,
"title": "Toroidal ring model"
}
|
could have an intrinsic quantum spin, and also a magnetic moment. The highly successful modern theory, Standard Model of particle physics describes a pointlike electron with an intrinsic spin and magnetic moment. On the other hand, the usual assertion that an electron is pointlike may be conventionally associated only with a "bare" electron. The pointlike electron would have a diverging electromagnetic field, which should create a strong vacuum polarization. In accordance with QED, deviations from the Coulomb law are predicted at Compton scale distances from the centre of electron, 10−11 cm. Virtual processes in the Compton region determine the spin of electron and renormalization of its charge and mass. It shows that the Compton region of the electron should be considered as a coherent whole with its pointlike core, forming a physical ("dressed") electron. Notice that the Dirac theory of electron also exhibits the peculiar behaviour of the Compton region. In particular, electrons display zitterbewegung at the Compton scale. From this point of view, the ring model does not contradict QED or the Dirac theory and some versions could possibly be used to incorporate gravity in quantum theory. The question of whether the electron has a substructure of any sort must be decided by experiment. All experiments to date agree with the Standard Model of the electron, with no substructure, ring-like or otherwise. The two major approaches are high-energy electron–positron scattering and high-precision atomic tests of quantum electrodynamics, both of which agree that the electron is point-like at resolutions down to 10−20 m. At present, the Compton region of virtual processes, 10−11 cm across, is not exhibited in the high-energy experiments on electron–positron scattering. Nikodem Popławski use the Papapetrou method of multipole expansion to show that torsion modifies Burinskii’s model of the Dirac electron by replacing the Kerr–Newman singular ring
|
{
"page_id": 13372642,
"source": null,
"title": "Toroidal ring model"
}
|
of the Compton size with a toroidal structure with the outer radius of the Compton size and the inner radius of the Cartan size (10−27 m) in the Einstein–Cartan theory of gravity. == References == == Further reading == David L. Bergman, J. Paul Wesley ; Spinning Charged Ring Model of Electron Yielding Anomalous Magnetic Moment, Galilean Electrodynamics. Vol. 1, 63-67 (Sept./Oct. 1990).
|
{
"page_id": 13372642,
"source": null,
"title": "Toroidal ring model"
}
|
100% English is a Channel 4 television programme shown in November 2006 in the United Kingdom. It looked at the genetic makeup of English people who considered themselves to be ethnically English and found that while all had an ethnic makeup similar to people of European descent, a minority discovered genetic markers from North Africa and the Middle East from several generations before they were born. The presenter was Andrew Graham-Dixon. The test results were interpreted by DNAPrint Genomics, based in Sarasota, Florida, United States. The concept of the show was to: Take eight people – all of whom are convinced they are 100% English. Then submit a sample of their DNA to a series of state-of-the-art tests ... Lord Tebbit, Garry Bushell and Carol Thatcher are among the participants who have agreed to place their genetic make-up under the microscope ... Garry Bushell, who appeared on the show, later criticised the slant of the programme and the portrayal of English people. On his website he stated: "Only Nazis, and it appears C4, think of national identity in terms of racial purity ... Besides, you could apply the same tests to the French, Greeks, or Italians and get similar results, but no-one questions their right to nationhood." == See also == Genetic genealogy == References ==
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{
"page_id": 7998691,
"source": null,
"title": "100% English"
}
|
PRAC (Probabilistic Action Cores) is an interpreter for natural-language instructions for robotic applications developed at the Institute for Artificial Intelligence at the University of Bremen, Germany, and is supported in parts by the European Commission and the German Research Foundation (DFG). == Goals == The ultimate goal of the PRAC system is to make knowledge about everyday activities from websites like wikiHow available for service robots, such that they can autonomously acquire new high-level skills by browsing the Web. PRAC addresses the problem that natural language is inherently vague and unspecific. To this end, PRAC maintains probabilistic first-order knowledge bases over semantic networks represented in Markov logic networks. As opposed to other semantic learning initiatives like NELL or IBM's Watson, PRAC does not aim at answering questions in natural language, but to disambiguate and infer information pieces that are missing in natural-language instructions, such that they can be executed by a robot. "This problem formulation is substantially different to the problem of text understanding for question answering or machine translation. In those reasoning tasks, the vagueness and ambiguity of natural-language expressions can often be kept and translated into other languages. In contrast, robotic agents have to infer missing information pieces and disambiguate the meaning of the instruction in order to perform the instruction successfully." In addition to probabilistic relational models, PRAC uses the principles of analogical reasoning and instance-based learning to infer completions of roles in semantic networks. PRAC has been successfully applied to teach robots to conduct chemical experiments and to make pancakes and pizza from wikiHow articles. == References == == External links == Project homepage Institute for Artificial Intelligence
|
{
"page_id": 53480676,
"source": null,
"title": "Probabilistic Action Cores"
}
|
Tissue growth is the process by which a tissue increases its size. In animals, tissue growth occurs during embryonic development, post-natal growth, and tissue regeneration. The fundamental cellular basis for tissue growth is the process of cell proliferation, which involves both cell growth and cell division occurring in parallel. How cell proliferation is controlled during tissue growth to determine final tissue size is an open question in biology. Uncontrolled tissue growth is a cause of cancer. Differential rates of cell proliferation within an organ can influence proportions, as can the orientation of cell divisions, and thus tissue growth contributes to shaping tissues along with other mechanisms of tissue morphogenesis. == Mechanisms of tissue growth control in animals == === Mechanical control of tissue growth in animal skin === For some animal tissues, such as mammalian skin, it is clear that the growth of the skin is ultimately determined by the size of the body whose surface area the skin covers. This suggests that cell proliferation in skin stem cells within the basal layer is likely to be mechanically controlled to ensure that the skin covers the surface of the entire body. Growth of the body causes mechanical stretching of the skin, which is sensed by skin stem cells within the basal layer and consequently leads to both an increased rate of cell proliferation as well as promoting the planar orientation of stem cell divisions to produce new skin stem cells, rather than only producing differentiating supra-basal daughter cells. Cell proliferation in skin stem cells within the basal layer can be driven by the mechanically-regulated YAP/TAZ family of transcriptional co-activators, which bind to TEAD-family DNA binding transcription factors in the nucleus to activate target gene expression and thereby drive cell proliferation. For other animal tissues, such as the bones of the
|
{
"page_id": 51514596,
"source": null,
"title": "Tissue growth"
}
|
skeleton or the internal mammalian organs intestine, pancreas, kidney or brain, it remains unclear how developmental gene regulatory networks encoded in the genome lead to organs of such different sizes and proportions. === Hormonal control of tissue growth in the entire animal body === Although different animal tissues grow at different rates and produce organs of very different proportions, the overall growth rate of the entire animal body can be modulated by circulating hormones of the Insulin/IGF-1 family, which activate the PI3K/AKT/mTOR pathway in many cells of the body to increase the average rate of both cell growth and cell division, leading to increased cell proliferation rates in many tissues. In mammals, production of IGF-1 is induced by another circulating hormone called Growth Hormone. Excessive production of Growth Hormone or IGF-1 is responsible for giantism while insufficient production of these hormones is responsible for dwarfism. === Developmental control of tissue growth during adult tissue homeostasis === Adult animal tissues such as skin or intestine maintain their size but undergo constant turnover of cells by proliferation of stem cells and progenitor cells while undergoing an equivalent loss of differentiated daughter cells via sloughing off. Gradients of Wnt signaling pathway activity appear to have a fundamental role in maintaining proliferation of stem and progenitor cells, at least in the intestine, and possibly also in skin. === Regenerative tissue growth after wounding or other types of damage === Upon tissue damage, there is an upregulation in the activity of many pathways that control tissue growth, including the YAP/TAZ pathway, Wnt signaling pathway, and growth factors that activate the PI3K/AKT/mTOR pathway. == References ==
|
{
"page_id": 51514596,
"source": null,
"title": "Tissue growth"
}
|
The minimum bactericidal concentration (MBC) is the lowest concentration of an antibacterial agent required to kill a particular bacterium. It can be determined from broth dilution minimum inhibitory concentration (MIC) tests by subculturing to agar plates that do not contain the test agent. The MBC is identified by determining the lowest concentration of antibacterial agent that reduces the viability of the initial bacterial inoculum by ≥99.9%. The MBC is complementary to the MIC; whereas the MIC test demonstrates the lowest level of antimicrobial agent that inhibits growth, the MBC demonstrates the lowest level of antimicrobial agent that results in microbial death. This means that even if a particular MIC shows inhibition, plating the bacteria onto agar might still result in organism proliferation because the antimicrobial did not cause death. Antibacterial agents are usually regarded as bactericidal if the MBC is no more than four times the MIC. Because the MBC test uses colony-forming units as a proxy measure of bacterial viability, it can be confounded by antibacterial agents which cause aggregation of bacterial cells. Examples of antibacterial agents which do this include flavonoids and peptides. == References ==
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{
"page_id": 10816739,
"source": null,
"title": "Minimum bactericidal concentration"
}
|
A lamella (pl.: lamellae) is a small plate or flake, from the Latin, and may also refer to collections of fine sheets of material held adjacent to one another in a gill-shaped structure, often with fluid in between though sometimes simply a set of "welded" plates. The term is used in biological contexts for thin membranes of plates of tissue. In the context of materials science, the microscopic structures in bone and nacre are called lamellae. Moreover, the term lamella is often used to describe crystal structure of some materials. == Uses of the term == In surface chemistry (especially mineralogy and materials science), lamellar structures are fine layers, alternating between different materials. They can be produced by chemical effects (as in eutectic solidification), biological means, or a deliberate process of lamination, such as pattern welding. Lamellae can also describe the layers of atoms in the crystal lattices of materials such as metals. In surface anatomy, a lamella is a thin plate-like structure, often one amongst many lamellae very close to one another, with open space between. In chemical engineering, the term is used for devices such as filters and heat exchangers. In mycology, a lamella (or gill) is a papery hymenophore rib under the cap of some mushroom species, most often agarics. The term has been used to describe the construction of lamellar armour, as well as the layered structures that can be described by a lamellar vector field. In medical professions, especially orthopedic surgery, the term is used to refer to 3D printed titanium technology which is used to create implantable medical devices (in this case, orthopedic implants). In context of water-treatment, lamellar filters may be referred to as plate filters or tube filters. This term is used to describe a certain type of ichthyosis, a congenital skin
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{
"page_id": 1641702,
"source": null,
"title": "Lamella (materials)"
}
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condition. Lamellar Ichthyosis often presents with a "colloidal" membrane at birth. It is characterized by generalized dark scaling. The term lamella(e) is used in the flooring industry to describe the finished top-layer of an engineered wooden floor. For example, an engineered walnut floor will have several layers of wood and a top walnut lamella. In archaeology, the term is used for a variety of small flat and thin objects, such as Amulet MS 5236, a very thin gold plate with a stamped text from Ancient Greece in the 6th century BC. In crystallography, the term was first used by Christopher Chantler and refers to a very thin layer of a perfect crystal, from which curved crystal physics may be derived. In textile industry, a lamella is a thin metallic strip used alone or wound around a core thread for goldwork embroidery and tapestry weaving. In September 2010, the U.S. Food and Drug Administration (FDA) announced a recall of two medications which contained "extremely thin glass flakes (lamellae) that are barely visible in most cases. The lamellae result from the interaction of the formulation with glass vials over the shelf life of the product." == See also == Lamella (cell biology) Middle lamella Annulate lamella Lamella (structure) == References ==
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{
"page_id": 1641702,
"source": null,
"title": "Lamella (materials)"
}
|
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