Posted on مايو 12, 2009 under Uncategorized |
annual plant is a plant that usually germinates, flowers, and dies in one year. True annuals will only live longer than a year if they are prevented from setting seed. Some seedless plants can also be considered annuals even though they do not grow a flower. [1]In gardening, annual often refers to a plant grown outdoors in the spring and summer and surviving just for one growing season. Many food plants are, or are grown as, annuals, including most domesticated grains. Some perennials and biennials are grown in gardens as annuals for convenience, particularly if they are not considered cold hardy for the local climate. Carrot, celery and parsley are true biennials that are usually grown as annual crops for their edible roots, petioles and leaves, respectively. Tomato, sweet potato and bell pepper are tender perennials usually grown as annuals.Ornamental annualer perennials commonly grown as annuals are impatiens, wax begonia, snapdragon, Pelargonium, coleus and petunia. Some biennials that can be grown as annuals are pansy and hollyhock.One seed-to-seed life cycle for an annual can occur in as little as a month in some species, though most last several months. Oilseed rapa can go from seed-to-seed in about five weeks under a bank of fluorescent lamps in a school classroom. Many desert annuals are termed ephemerals because their seed-to-seed life cycle is only many weeks. They spend most of the year as seeds to survive dry conditions.Examples of true annuals include corn, lettuce, pea, cauliflower, watermelon, bean, zinnia and marigoldSome pictures of annual plants:
A biennial plant is a flowering plant that takes two years to complete its biological lifecycle. In the first year the plant grows leaves, stems, and roots (vegetative structures), then it enters a period of dormancy over the colder months. Usually the stem remains very short and the leaves are low to the ground, forming a rosette. Many biennials require a cold treatment, or vernalization, before they will flower. During the next spring or summer, the stem of the biennial plant elongates greatly, or “bolting bolts”. The plant then flowers, producing fruits and seeds before it finally dies. There are far fewer biennials than either perennial plants or annual plants.Under extreme climatic conditions, a biennial plant may complete its life cycle in a very short period of time (e.g. three or four months instead of two years). This is quite common in vegetable or flower seedlings that were exposed to cold conditions, or vernalized, before they were planted in the ground. This behavior leads to many normally biennial plants being treated as annuals in some areas. Flowering can be induced in some biennials without vernalization by application of the plant hormone gibberellin, but this is rarely done commercially. The Sweet William Dwarf plant is a biennial plantFrom a gardener’s perspective, a plant’s status as annual, biennial, or perennial often varies based on location or purpose. Biennials grown for flowers, fruits, or seeds need to be grown for two years. Biennials that are grown for edible leaves or roots are grown as annuals, e.g. beets, Brussels sprouts, cabbage, carrots, celery, lettuce, parsley, and Swiss chard. If a normally biennial plant is grown in extremely harsh conditions, it is likely to be treated as an annual because it will not survive the winter cold. Conversely, an annual grown under extremely favorable conditions may have highly successful seed propagation, giving it the appearance of being biennial or perennial. Some short-lived perennials may appear to be biennial rather than perennial. True biennials flower only once, while many perennials will flower every year once mature.Examples of biennial plants are parsley, Lunaria, silverbeet, Sweet William, colic weed, and carrot. Plant breeders have produced annual cultivars of several biennials that will flower the first year from seed, e.g. foxglove and stock.Some pictures of biennial:
A perennial plant or perennial is a plant that lives for more than two years.[1] When used by gardeners or horticulturalists, this term applies specifically to perennial herbaceous plants. Scientifically, woody plants like shrubs and trees are also perennial in their habit.Perennials, especially small flowering plants, grow and bloom over the spring and summer and then die back every autumn and winter, then return in the spring from their root-stock rather than seeding themselves as an annual plant does. These are known as herbaceous perennials. However, depending on the rigors of local climate, a plant that is a perennial in its native habitat, or in a milder garden, may be treated by a gardener as an annual and planted out every year, from seed, from cuttings or from divisions.The symbol for a perennial plant, based on Species Plantarum by Linnaeus is, , which is also the astronomical symbol for the planet Jupiter.
Posted on مارس 14, 2009 under barcelona |
Futbol Club Barcelona
(Catalan IPA: [fudˈbɔɫ ˌklup bəɾsəˈlonə], Spanish IPA: [ˈfutβol ˌkluβ baɾθeˈlona]), also known simply as Barcelona and familiarly as Barça (Catalan IPA: [ˈbaɾsə], Spanish IPA: [ˈbaɾsa]), is a sports club based in Barcelona, Catalonia, Spain. It is best known for its football team, which was founded in 1899 by a group of Swiss, English and Spanish men led by Joan Gamper. The club has become a Catalan institution, hence the motto “Més que un club” (More than a club).
FC Barcelona is one of the only three clubs that have never been relegated from La Liga and the second most successful club in Spanish football having won eighteen La Liga titles, a record twenty-four Spanish Cups, seven Spanish Super Cups and two League Cups. They are also one of the most successful clubs in European football having won two European Cups, four UEFA Cup Winners’ Cups, three Inter-Cities Fairs Cups and two UEFA Super Cups.
The club’s stadium is the Camp Nou, the largest stadium in Europe. Barcelona enjoys a high rate of popularity; about 25.7% of Spanish population support the club,[1] while according to a recent survey Barcelona is the most popular football club in Europe with around 44.2 million fans.[2] With 156,366 socis in June 2007, the Catalan club is also placed among the top football clubs in the world with the most registered members, and the number of penyes, the officially-registered supporter clubs, reached the number of 1,782 worldwide in June 2006. The fans of FC Barcelona are known as culés. The club shares a great rivalry with Real Madrid and contest in one of the most famous football matches worldwide, known as El Clásico.
During the season 2007–08, Barcelona was the third richest club in the world with a revenue of €308.8 million. It was also one of the founding members of the now-defunct G-14 group of the leading European football clubs and its modern replacement, the European Club Association. The club also operates a reserve team, FC Barcelona Atlètic, while there was a youth team until 2007, FC Barcelona C.
Posted on مارس 14, 2009 under biology |
Biochemistry
is the study of the chemical processes in living organisms. It deals with the structure and function of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules.
Although there are a vast number of different biomolecules many are complex and large molecules (called polymers) that are composed of similar repeating subunits (called monomers). Each class of polymeric biomolecule has a different set of subunit types.[1] For example, a protein is a polymer whose subunits are selected from a set of 20 or more amino acids. Biochemistry studies the chemical properties of important biological molecules, like proteins, in particular the chemistry of enzyme-catalyzed reactions.
The biochemistry of cell metabolism and the endocrine system has been extensively described. Other areas of biochemistry include the genetic code (DNA, RNA), protein synthesis, cell membrane transport, and signal transduction.
Since all known life forms that are still alive today are descended from the same common ancestor, they have similar biochemistries, even for matters that seem to be essentially arbitrary, such as handedness of various biomolecules. It is unknown whether alternative biochemistries are possible or practical.Biochemistry is the study of the chemical processes in living organisms. It deals with the structure and function of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules.
Although there are a vast number of different biomolecules many are complex and large molecules (called polymers) that are composed of similar repeating subunits (called monomers). Each class of polymeric biomolecule has a different set of subunit types.[1] For example, a protein is a polymer whose subunits are selected from a set of 20 or more amino acids. Biochemistry studies the chemical properties of important biological molecules, like proteins, in particular the chemistry of enzyme-catalyzed reactions.
The biochemistry of cell metabolism and the endocrine system has been extensively described. Other areas of biochemistry include the genetic code (DNA, RNA), protein synthesis, cell membrane transport, and signal transduction.
Since all known life forms that are still alive today are descended from the same common ancestor, they have similar biochemistries, even for matters that seem to be essentially arbitrary, such as handedness of various biomolecules. It is unknown whether alternative biochemistries are possible or practical.
Biochemistry is the study of the chemical processes in living organisms. It deals with the structure and function of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules.
Although there are a vast number of different biomolecules many are complex and large molecules (called polymers) that are composed of similar repeating subunits (called monomers). Each class of polymeric biomolecule has a different set of subunit types.[1] For example, a protein is a polymer whose subunits are selected from a set of 20 or more amino acids. Biochemistry studies the chemical properties of important biological molecules, like proteins, in particular the chemistry of enzyme-catalyzed reactions.
The biochemistry of cell metabolism and the endocrine system has been extensively described. Other areas of biochemistry include the genetic code (DNA, RNA), protein synthesis, cell membrane transport, and signal transduction.
Since all known life forms that are still alive today are descended from the same common ancestor, they have similar biochemistries, even for matters that seem to be essentially arbitrary, such as handedness of various biomolecules. It is unknown whether alternative biochemistries are possible or practical.
Posted on مارس 07, 2009 under cisco |
Thermal grease
Silicone thermal compound
Metal (silver) thermal compound
Metal thermal grease applied to a chip
Surface imperfections
Thermal grease (also called thermal compound, heat paste, heat transfer compound, thermal paste, or heat sink compound) is a fluid substance, originally with properties akin to grease, which increases the thermal conductivity of a thermal interface (by compensating for the irregular surfaces of the components). In electronics, it is often used to aid a component’s thermal dissipation via a heat sink.
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[ Basic types
Thermal greases use one or more different thermally conductive substances:
- Ceramic-based thermal grease has generally good thermal conductivity and is usually composed of a ceramic powder suspended in a liquid or gelatinous silicone compound, which may be described as ’silicone paste’ or ’silicone thermal compound’. The most commonly used ceramics and their thermal conductivities (in units of W/(m ·K)) are:[1] beryllium oxide (218), aluminium nitride (170), aluminum oxide (39), zinc oxide (21), and silicon dioxide (1). Thermal grease is usually white in colour since these ceramics are all white in powder form.
- Metal-based thermal grease contain solid metal particles (usually silver). It has a better thermal conductivity (and is more expensive) than ceramic-based grease. It is also more electrically conductive which can cause problems if it contacts the electrical connections of an integrated circuit.
- Liquid metal based. Some thermal pastes are made of liquid metal alloys of gallium. Rare and expensive.
All but the last classification of compound usually use silicone grease as a medium, a heat conductor in itself, though some manufacturers prefer use of fractions of mineral oil.[citation needed]
[] Purpose
Thermal grease is primarily used in the electronics and computer industries to assist a heatsink to draw heat away from a semiconductor component such as an integrated circuit or transistor.
Thermally conductive paste improves the efficiency of a heatsink by filling air gaps that occur when the irregular surface of a heat generating component is pressed against the irregular surface of a heatsink, air being approximately 8000 times less efficient at conducting heat (see Thermal conductivity) than, for example, aluminium, a common heatsink material.[2] Surface imperfections inherently arise from limitations in manufacturing technology and range in size from visible and tactile flaws such as machining marks or casting irregularities to sub-microscopic ones not visible to the naked eye.
As such, both the thermal conductivity and the “conformability” (i.e., the ability of the material to conform to irregular surfaces) are the important characteristics of thermal grease.
Both high power handling transistors, like those in a conventional audio amplifier, and high speed integrated circuits, such as the central processing unit (CPU) of a personal computer, generate sufficient heat to require the use of thermal grease in addition to the heatsink. High temperatures cause semiconductors to change their switpoint of failure while CPU power dissipation overheating causes logic errors as heat raises electrical resistance on the multi-nanometer wide circuits of the CPU core.[]
[] Properties
The metal oxide and nitride particles suspended in silicone thermal compounds have thermal conductivities of up to 220 W/(m·K).[ (In comparison, the thermal conductivity of metals used particle additions, copper is 401 W/(m·K), silver 429 and aluminum 237.) The typical thermal conductivities of the silicone compounds are 0.7 to 3 W/(m·K). Silver thermal compounds may have a conductivity of 2 to 3 W/(m·K) or more.
In compounds containing suspended particles, the properties of the fluid may well be the most important. As seen by the thermal conductivity measures above, the conductivity is closer to that of the fluid components rather than the ceramic or metal components. Other properties of fluid components that are important for thermal grease might be:
- How well it fills the gaps and conforms to the component’s uneven surfaces and the heat sink
- How well it adheres to those surfaces
- How well it maintains its consistency over the required temperature range
- How well it resists drying out or flaking over time
- How well it insulates electrically
- Whether it degrades with oxidation or breaks down over time
The compound must also be smooth so that it is easy to apply in a very thin layer.
[] Applying and removing
Computer processor heatsinks utilize a variety of designs to promote better thermal transfer between components. Flat and smooth surfaces may use a small line method to apply material, and exposed heat-pipe surfaces will be best prepared with multiple lines.[4]
Because thermal grease’s thermal conductivity is poorer than the metals they couple, it is important to use no more than is necessary to exclude air gaps. Excess grease separating the metal surfaces further will only degrade conductivity, increasing the chances of overheating. It should also be noted that silver-based thermal grease can also be slightly electrically conductive. If excess were to flow onto the circuits, it could cause a short circuit.
The preferred way to remove typical silicone oil-based thermal grease from a component or heat sink is by using isopropyl alcohol (rubbing alcohol). If none is available, pure acetone is also a valid method of removal. There are also purpose made cleaners for removing and purifying the surfaces of the contacts.
Posted on يناير 04, 2009 under Uncategorized |
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