heat flow density

циркуляция морской воды
—
[ http://www.eionet.europa.eu/gemet/alphabetic?langcode=en]

EN

sea circulation
Large-scale horizontal water motion within an ocean. The way energy from the sun, stored in the sea, is transported around the world. The currents explain, for example, why the UK has ice-free ports in winter, while St. Petersburg, at the same latitude as the Shetland Islands, needs ice breakers. Evidence is growing that the world's ocean circulation was very different during the last ice age and has changed several times in the distant past, with dramatic effects on climate. The oceans are vital as storehouses, as they absorb more than half the sun's heat reaching the earth. This heat, which is primarily absorbed near the equator is carried around the world and released elsewhere, creating currents which last up to 1.000 years. As the Earth rotates and the wind acts upon the surface, currents carry warm tropical water to the cooler parts of the world. The strength and direction of the currents are affected by landmasses, bottlenecks through narrow straits, and even the shape of the sea-bed. When the warm water reaches polar regions its heat evaporates into the atmosphere, reducing its temperature and increasing its density. When sea-water freezes it leaves salt behind in the unfrozen water and this cold water sinks into the ocean and begins to flow back to the tropics. Eventually it is heated and begins the cycle all over again. (Source: MGH / WRIGHT)
[http://www.eionet.europa.eu/gemet/alphabetic?langcode=en]

Тематики

• охрана окружающей среды

EN

• sea circulation

DE

• Meereskreislauf

FR

• circulation maritime

циркуляция морской воды
—
[ http://www.eionet.europa.eu/gemet/alphabetic?langcode=en]

EN

sea circulation
Large-scale horizontal water motion within an ocean. The way energy from the sun, stored in the sea, is transported around the world. The currents explain, for example, why the UK has ice-free ports in winter, while St. Petersburg, at the same latitude as the Shetland Islands, needs ice breakers. Evidence is growing that the world's ocean circulation was very different during the last ice age and has changed several times in the distant past, with dramatic effects on climate. The oceans are vital as storehouses, as they absorb more than half the sun's heat reaching the earth. This heat, which is primarily absorbed near the equator is carried around the world and released elsewhere, creating currents which last up to 1.000 years. As the Earth rotates and the wind acts upon the surface, currents carry warm tropical water to the cooler parts of the world. The strength and direction of the currents are affected by landmasses, bottlenecks through narrow straits, and even the shape of the sea-bed. When the warm water reaches polar regions its heat evaporates into the atmosphere, reducing its temperature and increasing its density. When sea-water freezes it leaves salt behind in the unfrozen water and this cold water sinks into the ocean and begins to flow back to the tropics. Eventually it is heated and begins the cycle all over again. (Source: MGH / WRIGHT)
[http://www.eionet.europa.eu/gemet/alphabetic?langcode=en]

Тематики

• охрана окружающей среды

EN

• sea circulation

DE

• Meereskreislauf

FR

• circulation maritime

циркуляция морской воды
—
[ http://www.eionet.europa.eu/gemet/alphabetic?langcode=en]

EN

sea circulation
Large-scale horizontal water motion within an ocean. The way energy from the sun, stored in the sea, is transported around the world. The currents explain, for example, why the UK has ice-free ports in winter, while St. Petersburg, at the same latitude as the Shetland Islands, needs ice breakers. Evidence is growing that the world's ocean circulation was very different during the last ice age and has changed several times in the distant past, with dramatic effects on climate. The oceans are vital as storehouses, as they absorb more than half the sun's heat reaching the earth. This heat, which is primarily absorbed near the equator is carried around the world and released elsewhere, creating currents which last up to 1.000 years. As the Earth rotates and the wind acts upon the surface, currents carry warm tropical water to the cooler parts of the world. The strength and direction of the currents are affected by landmasses, bottlenecks through narrow straits, and even the shape of the sea-bed. When the warm water reaches polar regions its heat evaporates into the atmosphere, reducing its temperature and increasing its density. When sea-water freezes it leaves salt behind in the unfrozen water and this cold water sinks into the ocean and begins to flow back to the tropics. Eventually it is heated and begins the cycle all over again. (Source: MGH / WRIGHT)
[http://www.eionet.europa.eu/gemet/alphabetic?langcode=en]

Тематики

• охрана окружающей среды

EN

• sea circulation

DE

• Meereskreislauf

FR

• circulation maritime

In consequence of these features the axial pump has a distinct advantage for variable-speed services.

The spark was preferred to the arc on account of (or because of, or owing to) its ease of control.

As a consequence (or result) of its change in speed, a light ray passing obliquely from a vacuum to a material medium is refracted.

The body loses heat by radiation.

The hole density fluctuation is small by virtue of the condition ((₍₎)" 1).

The term thermodynamics implies a study of the flow of heat.

This sharp enhancement signalled the existence of a new kind of quark.

Such a low level does not necessarily mean (or signify) that...

These bands signify the presence of a vinyl group.

Equation (.2) implies that...

For ligands of rather similar type an increase in ligand basic strength implies an increase in its metal-chelating ability.

Bonding also refers to the fastening together of two prices by means of adhesives.

denotes mobility.

() stands for the input.

In this paper "mining" will be taken to mean "coal mining".

signifies the concentration of...

Here, represents the radiant energy density.

явление электрической дуги
-
[Интент]

Параллельные тексты EN-RU

Electric arc phenomenon

The electric arc is a phenomenon which takes place as a consequence of a discharge which occurs when the voltage between two points exceeds the insulating strength limit of the interposed gas; then, in the presence of suitable conditions, a plasma is generated which carries the electric current till the opening of the protective device on the supply side.

Gases, which are good insulating means under normal conditions, may become current conductors in consequence of a change in their chemical-physical properties due to a temperature rise or to other external factors.

To understand how an electrical arc originates, reference can be made to what happens when a circuit opens or closes.

During the opening phase of an electric circuit the contacts of the protective device start to separate thus offering to the current a gradually decreasing section; therefore the current meets growing resistance with a consequent rise in the temperature.

As soon as the contacts start to separate, the voltage applied to the circuit exceeds the dielectric strength of the air, causing its perforation through a discharge.

The high temperature causes the ionization of the surrounding air which keeps the current circulating in the form of electrical arc. Besides thermal ionization, there is also an electron emission from the cathode due to the thermionic effect; the ions formed in the gas due to the very high temperature are accelerated by the electric field, strike the cathode, release energy in the collision thus causing a localized heating which generates electron emission.

The electrical arc lasts till the voltage at its ends supplies the energy sufficient to compensate for the quantity of heat dissipated and to maintain the suitable conditions of temperature. If the arc is elongated and cooled, the conditions necessary for its maintenance lack and it extinguishes.

Analogously, an arc can originate also as a consequence of a short-circuit between phases. A short-circuit is a low impedance connection between two conductors at different voltages.

The conducting element which constitutes the low impedance connection (e.g. a metallic tool forgotten on the busbars inside the enclosure, a wrong wiring or a body of an animal entered inside the enclosure), subject to the difference of potential is passed through by a current of generally high value, depending on the characteristics of the circuit.

The flow of the high fault current causes the overheating of the cables or of the circuit busbars, up to the melting of the conductors of lower section; as soon as the conductor melts, analogous conditions to those present during the circuit opening arise. At that point an arc starts which lasts either till the protective devices intervene or till the conditions necessary for its stability subsist.

The electric arc is characterized by an intense ionization of the gaseous means, by reduced drops of the anodic and cathodic voltage (10 V and 40 V respectively), by high or very high current density in the middle of the column (of the order of 102-103 up to 107 A/cm2), by very high temperatures (thousands of °C) always in the middle of the current column and – in low voltage - by a distance between the ends variable from some microns to some centimeters.

[ABB]

Явление электрической дуги

Электрическая дуга между двумя электродами в газе представляет собой физическое явление, возникающее в тот момент, когда напряжения между двумя электродами превышает значение электрической прочности изоляции данного газа.
При наличии подходящих условий образуется плазма, по которой протекает электрический ток. Ток будет протекать до тех пор, пока на стороне электропитания не сработает защитное устройство.

Газы, являющиеся хорошим изолятором, при нормальных условиях, могут стать проводником в результате изменения их физико-химических свойств, которые могут произойти вследствие увеличения температуры или в результате воздействия каких-либо иных внешних факторов.

Для того чтобы понять механизм возникновения электрической дуги, следует рассмотреть, что происходит при размыкании или замыкании электрической цепи.

При размыкании электрической цепи контакты защитного устройства начинают расходиться, в результате чего постепенно уменьшается сечение контактной поверхности, через которую протекает ток.
Сопротивление электрической цепи возрастает, что приводит к увеличению температуры.

Как только контакты начнут отходить один от другого, приложенное напряжение превысит электрическую прочность воздуха, что вызовет электрический пробой.

Высокая температура приведет к ионизации воздуха, которая обеспечит протекание электрического тока по проводнику, представляющему собой электрическую дугу. Кроме термической ионизации молекул воздуха происходит также эмиссия электронов с катода, вызванная термоэлектронным эффектом. Образующиеся под воздействием очень высокой температуры ионы ускоряются в электрическом поле и бомбардируют катод. Высвобождающаяся, в результате столкновения энергия, вызывает локальный нагрев, который, в свою очередь, приводит к эмиссии электронов.

Электрическая дуга длится до тех пор, пока напряжение на ее концах обеспечивает поступление энергии, достаточной для компенсации выделяющегося тепла и для сохранения условий поддержания высокой температуры. Если дуга вытягивается и охлаждается, то условия, необходимые для ее поддержания, исчезают и дуга гаснет.

Аналогичным образом возникает дуга в результате короткого замыкания электрической цепи. Короткое замыкание представляет собой низкоомное соединение двух проводников, находящихся под разными потенциалами.

Проводящий элемент с малым сопротивлением, например, металлический инструмент, забытый на шинах внутри комплектного устройства, ошибка в электромонтаже или тело животного, случайно попавшего в комплектное устройство, может соединить элементы, находящиеся под разными потенциалами, в результате чего через низкоомное соединение потечет электрический ток, значение которого определяется параметрами образовавшейся короткозамкнутой цепи.

Протекание большого тока короткого замыкания вызывает перегрев кабелей или шин, который может привести к расплавлению проводников с меньшим сечением. Как только проводник расплавится, возникает ситуация, аналогичная размыканию электрической цепи. Т. е. в момент размыкания возникает дуга, которая длится либо до срабатывания защитного устройства, либо до тех пор, пока существуют условия, обеспечивающие её стабильность.

Электрическая дуга характеризуется интенсивной ионизацией газов, что приводит к падению анодного и катодного напряжений (на 10 и 40 В соответственно), высокой или очень высокой плотностью тока в середине плазменного шнура (от 102-103 до 107 А/см2), очень высокой температурой (сотни градусов Цельсия) всегда в середине плазменного шнура и низкому падению напряжения при расстоянии между концами дуги от нескольких микрон до нескольких сантиметров.

[Перевод Интент]

Тематики

• НКУ (шкафы, пульты,...)

EN

• electric arc phenomenon