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'''Nhiệt độ không tuyệt đối''', '''độ không tuyệt đối''', hay đơn giản là '''không tuyệt đối''', là [[trạng thái]] [[nhiệt động học]] lý tưởng của [[vật chất]], trong đó mọi [[chuyển động nhiệt]] đều ngừng. Trạng thái này, theo các kết quả tính toán lý thuyết, đạt được đối với mọi hệ vật chất ở nhiệt độ -273.,15[[°C]]
[[Thể loại: TemperatureNhiệt độ]] ▼{{Đang dịch 2 (nguồn)
[[Thể loại:Nhiệt động lực học]]
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<!-- is the lowest possible [[temperature]], occurring when no [[heat]] energy remains in a substance. Absolute zero is the point at which molecules do not move (relative to the rest of the body) more than they are required to by a [[quantum mechanics|quantum mechanical]] effect called [[zero-point energy]]. By international agreement, absolute zero is defined as precisely 0 K on the [[Kelvin]] scale, which is a [[thermodynamic temperature|thermodynamic (absolute) temperature]] scale, and -273.15°C on the [[Celsius]] scale.<ref>[http://www1.bipm.org/en/si/si_brochure/chapter2/2-1/2-1-1/kelvin.html International Agreement (Absolute Zero)]</ref> Absolute zero is also precisely equivalent to 0 °R on the [[Rankine]] scale (also a thermodynamic temperature scale), and –459.67 °F on the [[Fahrenheit]] scale.
To establish an instrument to measure a range of temperatures, in 1593 [[Galileo Galilei]] invented a rudimentary water [[thermometer]]. One of the first to discuss the possibility of an “absolute cold” on such a scale was [[Robert Boyle]] who in his 1665 ''New Experiments and Observations touching Cold'', stated the dispute which is the ''primum frigidum'' is very well known among naturalists, some contending for the earth, others for water, others for the air, and some of the moderns for nitre, but all seeming to agree that:
<div style="font-size:115%">
{{cquote|There is some body or other that is of its own nature ''supremely cold'' and by participation of which all other bodies obtain that quality.}}</div>
The question whether there is a limit to the degree of cold possible, and, if so, where the zero must be placed, was first attacked by the French physicist [[Guillaume Amontons]], in 1702-1703, in connection with his improvements in the air thermometer. In his instrument temperatures were indicated by the height at which a column of mercury was sustained by a certain mass of air, the volume or " spring " of which of course varied with the heat to which it was exposed. Amontons therefore argued that the zero of his thermometer would be that temperature at which the spring of the air in it was reduced to nothing. On the scale he used the boiling-point of water was marked at +73 and the melting-point of ice at 511, so that the zero of his scale was equivalent to about -240 on the centigrade scale.
This remarkably close approximation to the modern value of - 273° for the zero of the air-thermometer was further improved on by [[Johann Heinrich Lambert]] (''Pyrometrie'', 1779), who gave the value -270 and observed that this temperature might be regarded as absolute cold. Values of this order for the absolute zero were not, however, universally accepted about this period. [[Laplace]] and [[Lavoisier]], for instance, in their treatise on heat (1780), arrived at values ranging from 150o to 3000 below the freezing-point of water, and thought that in any case it must be at least 60o below, while John Dalton in his Chemical Philosophy gave ten calculations of this value, and finally adopted - 3000° C. as the natural zero of temperature. After J. P. Joule had determined the mechanical equivalent of heat, Lord Kelvin approached the question from an entirely different point of view, and in 1848 devised a scale of absolute temperature which was independent of the properties of any particular substance and was based solely on the fundamental [[laws of thermodynamics]]. It followed from the principles on which this scale was constructed that its zero was placed at -273°, at almost precisely the same point as the zero of the air-thermometer.<ref>[http://www.1911encyclopedia.org/Cold Cold] – Britannica 1911</ref>
While scientists cannot fully achieve a state of “zero” heat energy in a substance, they have made great advancements in achieving temperatures ever closer to absolute zero (where matter exhibits odd [[Bose–Einstein condensate|quantum effects]]). In 1994, the [[National Institute of Standards and Technology|NIST]] achieved a record cold temperature of 700 [[Kelvin#SI prefixed forms of kelvin|nK]] (billionths of a kelvin). In 2003, researchers at [[Massachusetts Institute of Technology|MIT]] eclipsed this with a new record of 450 [[Kelvin#SI prefixed forms of kelvin|pK]] (0.45 nK).
== Record temperatures near absolute zero ==
It can be shown from the laws of [[thermodynamics]] that absolute zero can never be achieved artificially, though it is possible to reach temperatures arbitrarily close to it through the use of [[cryocoolers]]. This is the same principle that ensures no [[machine]] can be 100% efficient.
At very low temperatures in the vicinity of absolute zero, matter exhibits many unusual properties including [[superconductor|superconductivity]], [[superfluid]]ity, and [[Bose-Einstein condensate|Bose-Einstein condensation]]. In order to study such [[phenomenon|phenomena]], [[scientist]]s have worked to obtain ever lower temperatures.
*In [[September 2003]], [[Massachusetts Institute of Technology|MIT]] announced a record cold temperature of 450 [[1 E-12 K|pK]], or 4.5 × 10<sup>-10</sup>  K in a Bose-Einstein condensate of sodium atoms. This was performed by [[Wolfgang Ketterle]] and colleagues at MIT.<ref>Leanhardt, A. ''et al.'' (2003) ''Science'' '''301''' 1513. [http://physicsweb.org/article/news/7/9/8 Physicsweb news report]</ref>
*As of [[February 2003]], the [[Boomerang Nebula]], with a temperature of -272.15 Celsius; 1K, is the coldest place known outside a laboratory. The [[nebula]] is [[1 E19 m|5000 light-years]] from [[Earth]] and is in the constellation [[Centaurus]].<ref>[http://www.smh.com.au/articles/2003/02/20/1045638427695.html Press report February 21 2003]</ref>
*As of [[November 2000]], nuclear spin temperatures below 100 pK were reported for an experiment at the [[Helsinki University of Technology]]'s Low Temperature Lab. However, this was the temperature of one particular type of motion — a quantum property called nuclear spin — not the overall average thermodynamic temperature for all possible degrees of freedom.<ref>The experimental methods and results are presented in detail in T.A. Knuuttila’s Ph.D. thesis which can be accessed from [http://www.hut.fi/Yksikot/Kirjasto/Diss/2000/isbn9512252147/ this site]. Also the university’s pressure release on its achievement is [http://ltl.hut.fi/Low-Temp-Record.html here]</ref>
==Thermodynamics near absolute zero==
At temperatures near 0 K, nearly all molecular motion ceases and <math>\Delta</math>''S'' = 0 for any [[adiabatic process]]. Pure substances can (ideally) form perfect [[crystal]]s as ''T'' <math>\rightarrow</math>0. [[Max Planck|Planck's]] strong form of the [[third law of thermodynamics]] states that the [[entropy]] of a perfect crystal vanishes at absolute zero. However, if the lowest energy state is [[degenerate energy level|degenerate]] (more than one [[microstate (statistical mechanics)|microstate]]), this cannot be true. The original [[Walther Nernst|Nernst]] ''heat theorem'' makes the weaker and less controversial claim that the entropy ''change'' for any isothermal process approaches zero as ''T'' → 0
:<math> \lim_{T \to 0} \Delta S = 0 </math>
which implies that the entropy of a perfect crystal simply approaches a constant value.
''The [[Nernst postulate]] identifies the [[isotherm]] T = 0 as coincident with the [[adiabat]] S = 0, although other isotherms and adiabats are distinct. As no two adiabats intersect, no other adiabat can [[intersect]] the T = 0 isotherm. Consequently no adiabatic process initiated at nonzero temperature can lead to zero temperature.'' (≈ Callen, pp. 189-190)
An even stronger assertion is that ''It is impossible by any procedure to reduce the temperature of a system to zero in a finite number of operations.'' (≈ Guggenheim, p. 157)
A perfect crystal is one in which the internal [[lattice (group)|lattice]] structure extends uninterrupted in all directions. The perfect order can be represented by translational [[symmetry]] along three (not usually [[orthogonality|orthogonal]]) [[Cartesian coordinate system|axes]]. Every lattice element of the structure is in its proper place, whether it is a single atom or a molecular grouping. For [[chemical substance|substances]] which have two (or more) stable crystalline forms, such as [[diamond]] and [[graphite]] for [[carbon]], there is a kind of "chemical degeneracy". The question remains whether both can have zero entropy at ''T'' = 0 even though each is perfectly ordered.
Perfect crystals never occur in practice; imperfections, and even entire amorphous materials, simply get "frozen in" at low temperatures, so transitions to more stable states do not occur.
Using the [[Peter Debye|Debye]] model, the [[specific heat capacity|specific heat]] and entropy of a pure crystal are proportional to ''T''<sup> 3</sup>, while the [[enthalpy]] and [[chemical potential]] are proportional to ''T''<sup> 4</sup>. (Guggenheim, p. 111) These quantities drop toward their ''T'' = 0 limiting values and approach with ''zero'' slopes. For the specific heats at least, the limiting value itself is definitely zero, as borne out by experiments to below 10 K. Even the less detailed [[Albert Einstein|Einstein]] model shows this curious drop in specific heats. In fact, all specific heats vanish as absolute zero, not just those of crystals. Likewise for the coefficient of [[thermal expansion]]. [[Maxwell relations|Maxwell's relations]] show that various other quantities also vanish. These [[phenomenon|phenomena]] were unanticipated.
Since the relation between changes in the [[Gibbs free energy]], the enthalpy and the entropy is
:<math> \Delta G = \Delta H - T \Delta S \,</math>
it follows that as ''T'' decreases, Δ''G'' and Δ''H'' approach each other (so long as Δ''S'' is bounded). [[Experiment]]ally, it is found that most [[chemical reaction]]s are [[exothermic reaction|exothermic]] and release heat ''in the direction'' they are found to be going, toward [[thermodynamic equilibrium|equilbrium]]. That is, even at [[room temperature]] ''T'' is low enough so that the fact that (Δ''G'')<sub>''T,P''</sub> < 0 (usually) implies that Δ''H'' < 0. (In the opposite direction, each such reaction would of course absorb heat.)
More than that, the ''slopes'' of the temperature derivatives of Δ''G'' and Δ''H'' converge and ''are equal to zero'' at ''T'' = 0, which ensures that Δ''G'' and Δ''H'' are nearly the same over a considerable range of temperatures, justifying the approximate [[empiricism|empirical]] [[Principle of Thomsen and Berthelot]], which says that ''the equilibrium state to which a system proceeds is the one which evolves the greatest amount of heat'', i.e., an actual process is the ''most exothermic one''. (Callen, pp. 186-187)
==Absolute temperature scales==
As mentioned, absolute or [[thermodynamic temperature]] is conventionally measured in [[kelvin]]s ([[Celsius]]-size degrees), and increasingly rarely in the [[Rankine]] scale ([[Fahrenheit]]-size degrees). Absolute temperature is uniquely determined up to a multiplicative constant which specifies the size of the "degree", so the ''ratios'' of two absolute temperatures, ''T''<sub>2</sub>/''T''<sub>1</sub>, are the same in all scales. The most transparent definition comes from the classical [[Maxwell-Boltzmann distribution]] over energies, or from the quantum analogs: [[Fermi-Dirac statistics]] (particles of half-integer [[spin (physics)|spin]]) and [[Bose-Einstein statistics]] (particles of integer spin), all of which give the relative numbers of particles as (decreasing) [[exponential function]]s of energy over ''kT''. On a [[macroscopic]] level, a definition can be given in terms of the efficiencies of "reversible" [[heat engine]]s operating between hotter and colder thermal reservoirs.
==Negative temperatures==
{{main|Negative temperature}}
Certain semi-isolated systems (for example a system of non-interacting spins in a magnetic field) can achieve negative temperatures; however, they are not actually colder than absolute zero. They can be however thought of as "hotter than T=∞", as energy will flow from a negative temperature system to any other system with positive temperature upon contact.
== Xem thêm ==
{{col-begin}}
{{col-break}}
* [[Celsius]]
* [[Fahrenheit]]
* [[Delisle_scale| Delisle]]
* [[Heat]]
* [[International Temperature Scale of 1990|ITS-90]]
{{col-break}}
* [[Kelvin]]
* [[Rankine]]
* [[Thermodynamic temperature|Thermodynamic (absolute) temperature]]
* [[Triple point]]
* [[Nguyên lý Nernst]]
{{col-end}}
==Tham khảp==
* {{cite book | author=Herbert B. Callen | title=Thermodynamics, Chapter 10 | publisher=John Wiley & Sons, Inc. | year=1960}} Library of Congress Catalog Card No. 60-5597. The clearest presentation of the logical foundations of the subject.
* {{cite book | author=E.A. Guggenheim | title=Thermodynamics: An Advanced Treatment for Chemists and Physicists, 5th ed. | publisher=North Holland; John Wiley & Sons, Inc. | year=1967}} Library of Congress Catalog Card No. 60-20003. A remarkably astute and comprehensive treatise.
* {{cite book | author=G. S. Rushbrooke | title=Introduction to Statistical Mechanics | publisher=Oxford Univ. Press | year=1949}} The classic, compact introduction to the subject. -->
==Notes==
<references />
==Liên kết ngoài==
▲[[Thể loại:Temperature]]
[[Thể loại:Thermodynamics]]
[[af:Absolute nul]]
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