Kelvin

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Trong hệ thống đo lường quốc tế, kelvin là một đơn vị đo lường cơ bản cho nhiệt độ. Nó được kí hiệu bằng chữ K. Mỗi độ K trong nhiệt giai Kenvin (1K) bằng một độ trong nhiệt giai Celsius (1°C) và 0°C ứng với 273,15K. Thang nhiệt độ này được lấy theo tên của nhà vật lý, kỹ sư người Ireland William Thomson, 1st Baron Kelvin

Lord Kelvin
Sinh	26 June 1824 Belfast, Co. Antrim, Ireland
Mất	17 December 1907 Largs, Ayrshire, Scotland

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Kelvin temperature conversion formulas

To find	From	Formula
Celsius	kelvin	°C = K − 273.15
kelvin	Celsius	K = °C + 273.15
Rankine	kelvin	°R = K × 1.8
kelvin	Rankine	K = °R ÷ 1.8
Fahrenheit	kelvin	°F = (K × 1.8) − 459.67
kelvin	Fahrenheit	K = (°F + 459.67) ÷ 1.8
electronvolts	kelvin	eV ≈ K ÷ 11,604.5
kelvin	electronvolts	K ≈ eV × 11,604.5
For temperature intervals rather than specific temperatures, 1 kelvin = 1°C and 1 kelvin = 1.8°R Comparisons among various temperature scales Conversion calculator for units of temperature

The kelvin (symbol: K) is the SI unit of temperature and is one of the seven SI base units. It is used to specify temperature on a thermodynamic (absolute) temperature scale where absolute zero—the lowest possible temperature where nothing could be colder and the least possible heat energy remains in a substance—is defined as being equivalent to zero kelvins (0 K).

The unit is named after the Irish physicist and engineer William Thomson, 1st Baron Kelvin (1824 – 1907), who wrote of the need for an "absolute thermometric scale."

Mục lục 1 Definition of the kelvin 2 SI prefixed forms of kelvin 3 Typographical and usage conventions 3.1 Uppercase/lowercase and plural form usage 3.2 Temperatures and intervals 3.3 Formatting and typestyle for the K symbol 3.4 The special Unicode kelvin sign 3.5 Why technical articles use a mix of kelvin units and the Celsius scale 4 Color temperature 5 History of the Kelvin scale 6 See also 7 References

[sửa] Definition of the kelvin

The kelvin is defined, by international agreement ^[1], in terms of the triple point of specially prepared (VSMOW) water. This definition also relates the kelvin and absolute zero to the Celsius scale. Absolute zero—the temperature at which nothing could be colder and the least possible heat energy remains in a substance—is defined as being precisely 0 K and −273.15°C. The triple point of water is defined as being precisely 273.16 K and 0.01°C. This definition does three things: 1) it fixes the magnitude of the kelvin as being precisely 1 part in 273.16 parts of the difference between absolute zero and the triple point of water; 2) it establishes that one kelvin has precisely the same magnitude as a one-degree increment on the Celsius scale; and 3) it establishes the difference between the two scales' null points as being precisely 273.15 kelvins (0 K = −273.15°C and 273.16 K = 0.01°C). Temperatures in kelvins can be converted to other units according to the table at top right.

Some key temperatures relating temperatures in kelvins to the Celsius scale are shown in the table below.

	kelvin	Celsius	Fahrenheit
Absolute zero (precisely, by definition)	0 K	−273.15 °C	−459.67 °F
Melting point of ice (approximate) [2]	273.15 K	0°C	32°F
Water’s triple point (precisely, by definition)	273.16 K	0.01°C	32.018°F
Water's boiling point (approximate) [3]	373.1339 K	99.9839°C	211.9710°F

[sửa] SI prefixed forms of kelvin

SI prefixes are often employed to denote decimal multiples and submultiples of the kelvin. The most commonly used factors of kelvin are listed below.^[4]

Submultiples				Multiples
Factor	Name	Symbol		Factor	Name	Symbol
10−1	decikelvin	dK		101	decakelvin	daK
10−2	centikelvin	cK		102	hectokelvin	hK
10−3	millikelvin	mK		103	kilokelvin	kK
10−6	microkelvin	µK		106	megakelvin	MK
10−9	nanokelvin	nK		109	gigakelvin	GK
10−12	picokelvin	pK		1012	terakelvin	TK
10−15	femtokelvin	fK		1015	petakelvin	PK
10−18	attokelvin	aK		1018	exakelvin	EK
10−21	zeptokelvin	zK		1021	zettakelvin	ZK
10−24	yoctokelvin	yK		1024	yottakelvin	YK

[sửa] Typographical and usage conventions

[sửa] Uppercase/lowercase and plural form usage

The kelvin is always spelled with a lowercase k unless it is the first word in a sentence or title.

Until the 13th General Conference on Weights and Measures (CGPM) in 1967–1968, the kelvin was called a "degree", the same as with the other temperature scales at the time. It was distinguished from the others with either the adjectival suffix "Kelvin" ("degree Kelvin") or informally with "absolute" ("degree absolute"). The latter was ambiguous since it could also refer to the Rankine scale. At this time, the plural form of "degree Kelvin" was "degrees Kelvin". After the name change to simply "kelvin", the plural became "kelvins". ^[5]

[sửa] Temperatures and intervals

Because the kelvin is an individual unit of measure, it is particularly well-suited for expressing temperature intervals: differences between temperatures or their uncertainties (e.g. "Agar exhibited a melting point hysteresis of 25 kelvins," and "The uncertainty was 10 millikelvins"). Of course, the kelvin is also used to express specific temperatures along the thermodynamic scale (e.g. "Gallium melts at 302.9146 kelvins").

[sửa] Formatting and typestyle for the K symbol

The kelvin symbol is always a roman (non-italic) capital K since the lowercase version is the SI prefix for 1 × 10³. The admonition against italicizing the symbol K applies to all SI unit symbols; only symbols for variables and constants (e.g. P = pressure, and c = 299,792,458 m/s) are italicized in scientific and engineering papers. As with most other SI unit symbols (angle symbols, e.g. 45° 3′ 4″, are the exception,) there is a space between the numeric value and the kelvin symbol (e.g. "99.987 K").^[6]

[sửa] The special Unicode kelvin sign

Unicode, which is an industry standard designed to allow text and symbols from all of the writing systems of the world to be consistently represented and manipulated by computers, includes a special "kelvin sign" at U+212A. One types K when encoding this special kelvin character in a Web page. Its appearance is similar to an ordinary uppercase K. To better see the difference between the two, below in maroon text is the kelvin character followed immediately by a simple uppercase K:

KK

When viewed on computers that properly support Unicode, the above line appears as follows (size may vary):

Hình:Kelvin symbol plus K.jpg

Depending on the operating system, Web browser, and the default font, the "K" in the Unicode character may be narrower and slightly taller than a plain uppercase K; precisely the opposite may be true on other platforms. However, there will usually be a discernible difference between the two. If the computer being used to view a particular Web page doesn't support the Unicode kelvin sign character (K), it may be canonically decomposed by the browser into U+004B (uppercase K) and the two would appear identical. In still other computers, the kelvin symbol is mapped incorrectly and produces an odd character.

Accordingly, for Web use, it is better to use the simple uppercase K to represent the kelvin symbol so it can be properly viewed by the widest possible audience.

[sửa] Why technical articles use a mix of kelvin units and the Celsius scale

In science (especially) and in engineering, the Celsius scale and the kelvin are often used simultaneously in the same article (e.g. "...its measured value was 0.01023 °C with an uncertainty of 70 µK"). This practice is permissible because the degree Celsius is a special name for the kelvin for use in expressing Celsius temperatures^[7] and the magnitude of the degree Celsius is precisely equal to that of the kelvin. Notwithstanding the official endorsement provided by decision #3 of Resolution 3 of the 13th CGPM, which stated "a temperature interval may also be expressed in degrees Celsius," the practice of simultaneously using both "°C" and "K" remains widespread throughout the scientific world as the use of SI prefixed forms of the degree Celsius (such as "µ°C" or "millidegrees Celsius") to express a temperature interval has not been well-adopted.

This practice should be avoided for literature directed to lower-level technical fields and in non-technical articles intended for the general public where both the kelvin and its symbol, K, are not well recognized and could be confusing.

[sửa] Color temperature

The kelvin is often used in the measure of the color temperature of light sources. Color temperature is based upon the principle that a black body radiator emits light whose color depends on the temperature of the radiator. Black bodies with temperatures below about 4000 K appear reddish whereas those above about 7500 K appear bluish. Color temperature is important in the fields of image projection and photography where a color temperature of approximately 5500 K is required to match "daylight" film emulsions. In astronomy, the stellar classification of stars and their place on the Hertzsprung-Russell diagram are based, in part, upon their surface temperature. The Sun for instance, has an effective photosphere temperature of 5778 K.

[sửa] History of the Kelvin scale

Below are some historic milestones in the development of the Kelvin scale and its unit increment, the kelvin. For more on the history of thermodynamic temperature, see Thermodynamic temperature: History of thermodynamic temperature.

• 1848: William Thomson, (1824 – 1907) also known as Lord Kelvin, wrote in his paper, On an Absolute Thermometric Scale, of the need for a scale whereby "infinite cold" (absolute zero) was the scale's null point, and which used the degree Celsius for its unit increment. Thomson calculated that absolute zero was equivalent to −273 °C on the air thermometers of the time. This absolute scale is known today as the Kelvin thermodynamic temperature scale. Thomson's value of "−273" was actually derived from 0.00366, which was the accepted expansion coefficient of gas per degree Celsius relative to the ice point. The inverse of −0.00366 expressed to four significant digits is −273.2 °C which is remarkably close to the true value of −273.15 °C.

• 1954: Resolution 3 of the 10th CGPM (Conférence Générale des Poids et Mesures, also known as the General Conference on Weights and Measures) gave the Kelvin scale its modern definition by choosing the triple point of water as its second defining point and assigned it a temperature of precisely 273.16 kelvin (what was actually written 273.16 "degrees Kelvin" at the time).

• 1967/1968: Resolution 3 of the 13th CGPM created the unit increment of thermodynamic temperature (as distinct from the scale) and gave it the name "kelvin", symbol K, instead of "degree Kelvin", symbol °K. In so doing, feeling it useful to explicitly define the magnitude of this new unit increment, 13th CGPM also decided in Resolution 4 that "The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water."

• 2005: The CIPM (Comité International des Poids et Mesures, also known as the International Committee for Weights and Measures) affirmed that for the purposes of delineating the temperature of the triple point of water, the definition of the Kelvin thermodynamic temperature scale would refer to water having an isotopic composition defined as being precisely equal to the nominal specification of VSMOW water.

[sửa] See also

Absolute zero Celsius Fahrenheit Heat

ITS-90 Rankine Thermodynamic temperature Triple point

[sửa] References

1. ▲ [1]
2. ▲ The ice point of purified water has been measured to be 0.000089 +/- 0.00001 degrees Celsius - see Magnum, B.W. (June năm 1995). “Reproducibility of the Temperature of the Ice Point in Routine Measurements” (PDF). Nist Technical Note 1411. Địa chỉ URL được truy nhập 2007-02-11.
3. ▲ For Vienna Standard Mean Ocean Water at one standard atmosphere (101.325 kPa) when calibrated solely per the two-point definition of thermodynamic temperature. Older definitions of the Celsius scale once defined the boiling point of water under one standard atmosphere as being precisely 100°C. However, the current definition results in a boiling point that is actually 0.0161 K less. For more about the actual boiling point of water, see VSMOW water in temperature measurement.
4. ▲ The term "most commonly used" is based on those with more than 500 Google hits on the name.
5. ▲ Webster's 11th Collegiate; NIST SP 811
6. ▲ For more information on conventions used in technical writing, see the informative SI Unit rules and style conventions by the NIST as well as the BIPM’s SI brochure: Subsection 5.3.3, Formatting the value of a quantity.
7. ▲ Note (e) of SI Brochure, Section, 2.2.2, Table 3

Nhiệt độ trong nhiệt giai Kelvin đôi khi còn được gọi là nhiệt độ tuyệt đối, do 0K ứng với nhiệt độ nhỏ nhất mà vật chất có thể đạt được. Tại 0K, trên lý thuyết, mọi chuyển động nhiệt hỗn loạn đều ngừng. Thực tế chưa quan sát được vật chất nào đạt tới chính xác 0K; chúng luôn có nhiệt độ cao hơn 0K một chút, tức là vẫn có chuyển động nhiệt hỗn loạn ở mức độ nhỏ. Ngay cả những trạng thái vật chất rất lạnh như ngưng tụ Bose-Einstein cũng có nhiệt độ lớn hơn 0K. Quan sát này phù hợp với nguyên lý bất định Heisenberg; nếu vật chất ở chính xác 0K, luôn tìm được hệ quy chiếu trong đó vận tốc chuyển động của chúng là 0 và vị trí không thay đổi, nghĩa là đo được chính xác cùng lúc vị trí và động lượng của hệ, vi phạm nguyên lý bất định.