النظام الدولي للوحدات

The seven SI base units and the interdependency of their definitions. Clockwise from top: kelvin (temperature), second (time), metre (length), kilogram (mass), candela (luminous intensity), mole (amount of substance) and ampere (electric current). The second of time, kelvin and kilogram are defined independently of any other base units. The metre is defined in terms of the speed of light, so depends upon the definition of the second. The definitions of the other base units are more complicated.

النظام الدولي للوحدات يرمز له بـ (SI) (الأحرف الاولى للمصطلح الفرنسي Système International d'Unités) هو نظام وحدات القياس الأوسع انتشارا في العالم، وهو يستخدم في كل بلدان العالم باستثناء الولايات الامريكية المتحدة. واشتق هذا النظام من نظام متر-كيلوغرام-ثانية للقياس باضافة بعض الوحدات، وكونه بديلا عن نظام السنتمتر-غرام-ثانية القديم يسمى هذا النظام بالنظام المتري (خاصة في الولايات الامريكية المتحدة التي لم تتبناه بشكل واسع وبربطانيا التي لا تزال في مرحلة التحول إلى النظام المتري)، وليست جميع وحدات القياس المترية مقبولة في هذا النظام.

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الوحدات الاساسية

هناك 7 وحدات اساسية في النظام الدولي للوحدات هي:

المتر ويقاس بواسطته الطول و يرمز له بال "م" و يحدد المتر الطولي بالطول الموجي لإشعاع ذرة الكريبتون
الكيلوغرام و تقاس بواسطته الكتلة و يرمز له بال "كغم"
الثانية ويقاس بها الزمن و يرمز لها بال "ث" وتحدد بمدة اشعاع ذرة السيزيوم
الامبير ويقاس به شدة التيار الكهربي و يحدد بالقوة الكهروديناميكية بين موصلين
الكلفن وتقاس به درجة الحرارة و يرمز له ب "ك"
الشمعة وتقيس شدة الضوء و ليس لها إختصار (في الإنجليزيه "cd") وهي مقدار الإشعاع الناتج من ذرة البلاتين PT المتجمده
المول وحدة لقياس كمية المادة ويستخدم عادة في الكيمياء، والمول هو عدد أفوجادرو (تقريبا 6.0221415 × 10²³) من الجزيئات الاساسية، سواء كان الحديث يدور عن ذرات او جزيئات لمركب ما.


Metre Convention

CGPM Vocabulary
French English Pages[1]
étalons [Technical] standard 5, 95
prototype prototype [kilogram/metre] 5,95
noms spéciaux [Some derived units have]
special names
16,106
mise en pratique mise en pratique
[Practical realization][Note 1]
82, 171

Towards SI

World Map showing Metrication, colour-coded by year of conversion: from ca 1800 (green) to 1980 (red). Black indicates the nations that have not yet adopted the SI-system: USA, Liberia and Burma.

At the close of the 19th century three different systems of units of measure existed for electrical measurements: a CGS-based system for electrostatic units (also known as the Gaussian or ESU system), a CGS-based system for electromechanical units


SI base units[2]:23[3][4]
Unit
name
Unit
symbol
Quantity
name
Definition (Incomplete)[n 1] Dimension
symbol
metre m length
  • Original (1793): 1/10000000 of the meridian through Paris between the North Pole and the Equator.FG
  • Interim (1960): 1650763.73 wavelengths in a vacuum of the radiation corresponding to the transition between the 2p10  and 5d5  quantum levels of the krypton-86 atom.
  • Current (1983): The distance travelled by light in vacuum in 1/299792458 second.
L
kilogram[n 2] kg mass
  • Original (1793): The grave was defined as being the weight [mass] of one cubic decimetre of pure water at its freezing point.FG
  • Current (1889): The mass of the International prototype kilogram.
M
second s time
  • Original (Medieval): 1/86400 of a day.
  • Interim (1956): 1/31556925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.
  • Current (1967): The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.
T
ampere A electric current
  • Original (1881): A tenth of the electromagnetic CGS unit of current. The [CGS] electromagnetic unit of current is that current, flowing in an arc 1 cm long of a circle 1 cm in radius creates a field of one oersted at the centre.[5] IEC
  • Current (1946): The constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2×10−7 newtons per metre of length.
I
kelvin K thermodynamic temperature Θ
mole mol amount of substance
  • Original (1900): The molecular weight of a substance in mass grams.ICAW
  • Current (1967): The amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.[n 4]
N
candela cd luminous intensity
  • Original (1946): The value of the new candle is such that the brightness of the full radiator at the temperature of solidification of platinum is 60 new candles per square centimetre.
  • Current (1979): The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
J
Note
  1. ^ Interim definitions are given only when there has been a significant difference in the definition.
  2. ^ Despite the prefix "kilo-", the kilogram is the base unit of mass. The kilogram, not the gram, is used in the definitions of derived units.
    Nonetheless, units of mass are named as if the gram were the base unit.
  3. ^ In 1954 the unit of thermodynamic temperature was known as the "degree Kelvin" (symbol °K; "Kelvin" spelt with an upper-case "K"). It was renamed the "kelvin" (symbol "K"; "kelvin" spelt with a lower case "k") in 1967.
  4. ^ When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

The original definitions of the various base units in the above table were made by the following authorities:

All other definitions result from resolutions by either CGPM or the CIPM and are catalogued in the SI Brochure.


Named units derived from SI base units[2]:3
Name Symbol Quantity Expressed in
terms of
other SI units
Expressed in
terms of
SI base units
radian rad angle 1 m/m
steradian sr solid angle 1 m2/m2
hertz Hz frequency s−1
newton N force, weight kg⋅m⋅s−2
pascal Pa pressure, stress N/m2 kg⋅m−1⋅s−2
joule J energy, work, heat N⋅m kg⋅m2⋅s−2
watt W power, radiant flux J/s kg⋅m2⋅s−3
coulomb C electric charge or quantity of electricity s⋅A
volt V voltage (electrical potential difference), electromotive force W/A kg⋅m2⋅s−3⋅A−1
farad F electric capacitance C/V kg−1⋅m−2⋅s4⋅A2
ohm Ω electric resistance, impedance, reactance V/A kg⋅m2⋅s−3⋅A−2
siemens S electrical conductance A/V kg−1⋅m−2⋅s3⋅A2
weber Wb magnetic flux V⋅s kg⋅m2⋅s−2⋅A−1
tesla T magnetic field strength Wb/m2 kg⋅s−2⋅A−1
henry H inductance Wb/A kg⋅m2⋅s−2⋅A−2
degree Celsius °C temperature relative to 273.15 K K
lumen lm luminous flux cd⋅sr cd
lux lx illuminance lm/m2 m−2⋅cd
becquerel Bq radioactivity (decays per unit time) s−1
gray Gy absorbed dose (of ionizing radiation) J/kg m2⋅s−2
sievert Sv equivalent dose (of ionizing radiation) J/kg m2⋅s−2
katal kat catalytic activity s−1⋅mol
Notes
1. The radian and steradian, once given special status, are now considered dimensionless derived units.[2]:3
2. The ordering of this table is such that any derived unit is based only on base units or derived units that precede it in the table.


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Prefixes


قالب:SI-Prefixes

Non-SI units accepted for use with SI


The litre is classed as a non-SI unit accepted for use with the SI.
Being one thousandth of a cubic metre, the litre is not a coherent unit of measure with respect to SI.


Unit names as adjectives

In English, a space is recommended between the number and the unit symbol when used as an adjective, e.g. "a 25 kg sphere".

The normal rules of English apply to unit names, where a hyphen is incorporated into the adjectival sense, e.g. "a 25-kilogram sphere".[6]

Chinese and Japanese

Chinese expressway distances road sign in eastern Beijing. Although the primary text is in Chinese, the distances use the internationally recognised numerals and symbols.


Printing SI symbols

Further rules[Note 2] are specified in respect of production of text using printing presses, word processors, typewriters and the like.


Realisation of units

Silicon sphere for the Avogadro project used for measuring the Avogadro constant to a relative uncertainty of 2×10−8 or less.[7]


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United Kingdom and the former Empire

Metrication logo of the Board of Trade

United States

Logo of the American National Metrication Council, an organization set up by American National Standards Institute (ANSI) to coordinate the country's transition to the metric system.

Even though Congress set up a framework for the use of the metric system in the nineteenth century,[8][Note 3] the United States continues to use customary units for most purposes apart from science and medicine,[9] though as a result of their Spanish heritage, metric units are used widely in Puerto Rico.[10]

A 2010 Cadillac SRX. All General Motors products are designed using metric units.

الاتحاد الاوروبي

"New SI"

Relations between proposed SI units definitions (in colour) and seven physical constants (in grey) with fixed numerical values in the proposed system. Unlike the current (2013) definition, the base units are derived from one or more constants of nature.


انظر أيضاً

الهامش

  1. ^ International Bureau of Weights and Measures (2006). Le Système international d'unités (SI) – The International System of Units (SI) (PDF) (8th ed.). ISBN 92-822-2213-6
  2. ^ أ ب ت خطأ استشهاد: وسم <ref> غير صحيح؛ لا نص تم توفيره للمراجع المسماة NIST330
  3. ^ Quantities Units and Symbols in Physical Chemistry, IUPAC
  4. ^ Page, Chester H; Vigoureux, Paul, eds. (20 May 1975). The International Bureau of Weights and Measures 1875–1975: NBS Special Publication 420. Washington, D.C.: National Bureau of Standards. pp. 238–244.
  5. ^ McKenzie, A.E.E (1961). Magnetism and Electricity. Cambridge University Press. p. 322.
  6. ^ "USMA FAQ on hyphenation for adjective forms". Retrieved 2 April 2014.
  7. ^ "Avogadro Project". National Physical Laboratory. Retrieved 2010-08-19.
  8. ^ Barbrow, Louis E.; Judson, Lewis V. (March 1976) [First published October 1963]. "Weights and Measures Standards of the United States – A brief history, Special Publication 447". National Bureau of Standards. pp. 10–20. LCCN 76-600055.
  9. ^ "Appendix G : Weights and Measures". The World Factbook. Central Intelligence Agency. Retrieved 2011-09-03.
  10. ^ Porter, Darwin; Prince, Danforth (2006). Frommer's Puerto Rico (8th ed.). Wiley Publishing. p. 58. ISBN 978-0-471-78740-2.

وصلات خارجية

رسمية
تاريخ
أبخاث
Pro-metric advocacy groups
Pro-customary measures pressure groups


خطأ استشهاد: وسوم <ref> موجودة لمجموعة اسمها "Note"، ولكن لم يتم العثور على وسم <references group="Note"/> أو هناك وسم </ref> ناقص