International System of Units

International System of Units . Name adopted by the XI General Conference on Weights and Measures (held in Paris in 1960 ) for a universal, unified and coherent system of units of measurement, based on the mks (meter-kilogram-second) system. This system is known as SI, initials of the International System. At the 1960 Conference, standards were defined for six basic or fundamental units and two supplementary units ( radian and steradian ); in 1971A seventh fundamental unit, the mol. The two supplementary units were abolished as an independent class within the International System at the XX General Conference on Weights and Measures ( 1995 ); these two units were incorporated into the SI as derived units without dimensions.

Summary

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  • 1 Importance
  • 2 Emergence
  • 3 Prefixes, Symbols, and Factors in the SI
  • 4 Rules of writing
  • 5 Magnitudes and most used units
    • 1 Linear Units and Equivalences
    • 2 Units and surface equivalences
    • 3 Units and volume equivalences
    • 4 Units and mass equivalents
    • 5 Other equivalences of use
    • 6 Speed ​​units and their equivalents
    • 7 Units equivalent to physical quantities
    • 8 Units derived from systematic use in Physical Chemistry
    • 9 Physical constants frequently used in Chemistry and Physics
  • 6 External links
  • 7 Sources

Importance

The development reached centuries ago by some countries like Germany , the USA , Spain and England in science and technology; It brought with it the need to use different physical quantities to express the technical characteristics of the different discoveries. Trade with the different countries of the world brought with it the spread of the magnitudes and physical units that were taking root in the population.
All this exchange of technology or trade between countries with more or less development made it easier for the same characteristic to be assigned a different unit, which depended on the country that manufactured it. This diversity of magnitudes and physical units forced man to establish equivalences and consequently to carry out conversions between the units; causing inaccuracies and errors.
For all of the above, the State Committee for Standardization , in use of the powers conferred on it by Decree Law No. 62 of December 30 , 1982 , by the Third Special Provision, establishes the conversion coefficients between measurement units of legal use in the country.

Emergence

The International System of Units (SI), arises from the MKS Metric System (meter, kilogram and second) and from three systems derived from it. The one of the Electrotechnia MKSA (meter, Kilogram, second and ampere); the Termotecnia MKSG (meter, kilogram, second and kelvin); of the MSC Lighting Technology (meter, second and candle). These systems were used in isolation and had the kilogram and the second meter as a common element. Thus arises the idea of ​​organizing on the basis of these systems. A single, universal and coherent system of units covering all branches of science and technology.
As a result of the consultations made with thousands of scientists, technicians and pedagogues from all countries, the establishment of the International System of Units (SI) occurs, to be adopted by all the signatory countries of the conversion of the metro.
The General Conferences on Weights and Measures that were in charge of this arduous work, made clear the need for its prompt application in all fields of science, technology and education. As a consequence of this decision, the world’s scientists and pedagogues began a campaign for the state implementation of this system as unique and universal.
This method consists in that some basic units of measurement are chosen as the basis of the system; considered independent of each other, from which the units of measurement of the physical quantities are derived. There is another group of derived units of measurement that are determined according to the physical formulas that relate the physical quantities to each other. The basic SI units of measure are: meter (m), kilogram (kg), second (s), ampere (A), Kelvin (K), candle (cd), and mole (mol) .

SI Prefixes, Symbols, and Factors

    PREFIX    SYMBOL       FACTOR
 exa   AND  10 18 = 1,000,000,000,000,000,000
 peta   T  10 15 = 1,000,000,000,000,000
 tera   P  10 12 = 1,000,000,000,000
 jig   G  10 9 = 1,000,000,000
 mega   M  10 6 = 1,000,000
 kilo    k  10 3 = 1,000
 hecto    h  10 2 = 100
 said   gives  10 1 = 10
 i said   d  10 -1 = 0.1
 centi   c  10 -2 = 0.01
 milli   m  10 -3 = 0.001
 micro   µ  10 -6 = 0.000 001
 elder brother   n  10 -9 = 0.000 000 001
 peak   p  10 -12 = 0.000 000 000 001
 femto   F  10 -15 = 0,000,000,000,000,000
 atto   to  10 -18 = 0,000,000,000,000,000,000

 

Other symbols
The following is a group of symbols approved by the SI to designate other units of measurement. For this, letters of the Greek, Latin alphabet or special signs are used.

 UNIT  SYMBOL  UNIT  SYMBOL  UNIT  SYMBOL
 Grade  °  Percent  %  Bel  B
 Minute  ′  Per thousand  ‰  decibel  dB
 Second  ″  Part per million  ppm

 

Writing rules

When consulting texts or other documents as well as television, it can be seen that many people dedicated to this purpose make errors in the writing of the units, physical quantities or their symbols. Below are a group of rules for writing these, with which it is intended to improve this unfortunate error that can be observed daily.

  1. The Multiples and Sub-multiplesof the SI units are formed by multiplying or dividing the value of the SI unit by 10 or an integer power.
  2. SI prefix symbols are written in Latin characters, with no space between the prefix and the unit of measure symbol.
  3. The symbols of the units of measurement and the relative and logarithmic units of measurement are established using letters of the Latin, Greek alphabet or special signs. (See table)
  4. Measurement unit symbols are printed in Roman (round) characters regardless of the characters used in the rest of the text.
  5. SI unit symbols are lowercase. However, when these are derived from patronymics, the capital letter is used for the first letter.
  6. SI unit symbols remain unchanged in the plural.
  7. SI unit symbols are written without a period at the end. If the symbol appears at the end of the sentence, a space will be left between the symbol and the period. (The distance is 36 km)
  8. The writing of the numbers will be done using Arabic numerals. In the case of decimal numbers, the separation of the integer part of the decimal will be done using a comma. (,)
  9. The writing of decimal numbers of several digits, for easier reading, will be done by separating the whole part in groups of three digits from right to left, starting with the comma, leaving a blank space. The decimal part will also be written in groups of three digits, from left to right starting with the comma. (26 450 327,693 578 31) After each numerical value, symbols are written leaving a space between the number and the first letter of the symbol. (65 km)
  10. Generally in written texts the symbols of the units will be used and not their full names. In the event that it is necessary to write the names of the SI units in full; These will be written in lower case as the number. (twenty meters). Only the full name of the unit will be written when referring to it.
  11. When a symbol accompanies a decimal value, it will be placed after all the digits. (368.54 dm)
  12. When indicating values ​​of physical quantities with their limit deviations, when indicating an interval or when enumerating several numerical values, the unit symbol will be used according to the following example:
    1. 20 mm.25 mm or (20.25) mm
    2. 80; 100 and 150 km
    3. From 18 to 25 Pa
    4. (20 ± 2) ° c or 20 ° c ± 2 ° c
    5. from 120 to 150 kg
    6. 5m ± 3mm
  13. In written texts, a symbol should not start the sentence.
  14. Symbols are allowed to be used in column headings and in table row names. The use of SI prefixes alone is not supported, without the accompanying unit of measure.
  15. When writing numbers in a text they will become the size of the capital letter.
  16. When writing several consecutive numbers, it will be done separating them by semicolons.

Magnitudes and most used units

The most commonly used units of measurement are listed below with their respective equivalences to SI and other units. These are grouped into linear, superficial, volume and mass for better understanding.

Linear Units and Equivalences

 No        Unit    Symbol                  Equivalence
 one  kilometer  km  1000 m
 2  hectometer  hm  100 m
 3  decameter  dam  10 m
 4  fathom  1,671 81 m
 5  string  20,352 m
 6  foot (Cuban)  0.282 667 m
 7  foot (Spanish)  0.278 635 m
 8  inch (cuban)  0.023 556 m
 9  inch (Spanish)  0.023 219 m
 10  inch (international)  0.025 4 m (most used in Cuba)
 eleven  rod (Cuban)  0.848 m
 12  rod (Spanish)  0.835 905 m
 13  yard  yd  0.914 4m = 3ft = 36inch
 14  league  4,240 m = 5,000 rod = 2,634 6 mile
 fifteen  chain (surveyor’s chain)  20,116 8 m = 66 ft
 16  mile (statute mile)  mile  1 609,344 m
 17  international nautical mile  1,853.18 m

 

Units and surface equivalences

No        Units Symbols      Equivalences
 one  square kilometer  [[km 2]]  1,000,000 m²
 2  square hectometer hm²  10,000 m² = 1 ha (hectare)
 3  square decameter dam²  100 m²
 4  hectare he has  10,000 m²
 5  area to  100 m²
 6  centiarea AC  10 m²
 7  acre  4,046.86 m²
 8  chivalry cab  134 202.06 m² = 13,420 m² = 324 square twine
 9  besana or vesana 2,588.77 m² = 3,600 cuban square vara
 10  expensive  13 420.2 m²
 eleven  square twine  414,204 m² = 576 square rod
 12  quatrain  8 387.6 m² = 0.062 cab = 0.838
 13  square league  17,977 6.10 6 m²
 14  square foot (cuban)  0.079 9 m²
 fifteen  square inch (cuban)  554,866.10 -6 m 2
 16  square rod (cuban)  0.719 104 m²
 17  rose or slash of 10,000 cuban square rod  7 191.04 m²
 18  pink or slash 18 square cord  7 455,670 m²

 

Units and volume equivalents

No Units Symbols Equivalences
 one  liter  L  1000 mL = 1 dm 3
 2  bottle  0.750 L = 750 mL = 750 cm 3
 3  american gallon  3, 785 41 L = 3,785 41dm 3
 4  gallon english  4, 546 09 L = 4 546 09 dm 3
 5  carboy 5 gallons = 25 bottles = 18.75 L
 6  liquid pint (us) 0.473 176.10 -3 m 3
 7 tablespoon 15 dm 3 = 15 mL
 8 teaspoonful 5 dm 3 = 5 mL

 

Units and mass equivalents

No Units Symbols Equivalences
 one at  @ 11,502 3 kg = 25 lb
 2 Spanish pound  lb 0.460 093 kg = 460 g = 16 ounces
 3 Spanish quintal  what 46,009 3 kg = 100 lb
 4 metric quintal  what 100 kg
 5 short ton (Spain) 920.19 kg
 6 long ton (Spain) 1030.61 kg
 7 metric ton 1000 kg
 8 ounce (Spanish) 28,755 8.10 3 kg

 

Other equivalences of use

No  Units  Symbols     Equivalences
 one  light-year ly  9,460 53.10 15 m
 2  barrel for oil bbl  158,987 L = 158,987 dm 3 = 42 gallons
 3  horsepower (English)  hp  745,700 w
 4  steam horse  cv  735,499 w
 5  decade  10 years = 120 months
 6  century  100 years = 1200 months
 7  printing point  0.351 460.10 -3 m
 8  wooden cubic foot  2,359 74.10 -3 m 3
 9  yard  3 foot = 36 inches
 10  foot  12 inch = 0.304 8m = 30.48cm
 eleven  international inch  0.025 4m = 2.54cm

 

Speed units and their equivalents

 No       Unit       Equivalence
 one  international knot (kn)  0.514 444 m / s = 1,852 km / h
 2  knot (uk)  0.514 773 m / s = 1,853 18 km / h
 3  yard per minute (yd / min)  1,524.10 -3 m / s
 4  kilometer per hour (km / h)  0.277 778 m / s
 5  mile per hour (mile / h)  0.447 04 m / s = 1,609 344 km / h
 6  meter per second (m / s)  3.6 km / h

Units equivalent to physical quantities

 No       Basic physical magnitude       Dimensional symbol       Basic unit       Unity symbol       Observations
 one  Length  L  meter  m  It is defined by setting the value of the speed of light in a vacuum.
 2  Weather.  T  second  s  It is defined by fixing the value of the frequency of the hyperfine transition of the cesium atom
 3  Mass  M  kilogram  Kg  It is the mass of the “master cylinder” guarded at the International Office of Weights and Measures, in Sèvres, France. Equivalent to the mass that a liter of pure water occupies at 14.5 ° C or 286’75 K.
 4  Electric current intensity  I  amp  TO  It is defined by setting the magnetic constant value.
 5  Temperature  θ  Kelvin  K  It is defined by setting the value of the thermodynamic temperature of the triple point of water.
 6  Amount of substance  n  mole  mole  It is defined by setting the value of the molar mass of the atom from 12C to 12 grams / mol. See also Avogadro’s number.
 7  Luminous intensity  J  candle  mole  See also related concepts: lumen, lux, and physical lighting.
  • One Kelvin equals 273 oC

Units derived from systematic use in Physical Chemistry

 Physical magnitude       SI unit       Symbol       Definition
 Force  Newton  N  Kg.ms -2
 Pressure  Pascal.  Pa  Kg.m -1 .s -2 = Nm -2
 Energy  joule  J  Kg.m2.s-2 = Nm
 Power  Watt  W  Js -1 = kg.m 2 .s -2
 Electric charge  Coulomb  C  Ace
 Electric potential difference  volt  V  Kg.m 2 .s -3 .A -2 = VA -1
 Electric resistance  ohm  Ω  Kg.m 2 s -3 .A -2 = VA -1
 Frequency  Hertz  Hz  s -1 (cycles per second)
 Surface tension  Does not have  Does not have  Kg.s -2 = Nm -1 = Jm -2
 Dynamic viscosity  Does not have  Does not have  Kg.m -1 .s -1
 Permittivity  Does not have  Does not have  Kg -1 .m -3 .s 4 .A 2

Physical constants frequently used in Chemistry and Physics

     Constant   Symbol       Value (SI)
 Molar gases  R  8,314 3 JK -1 .mol -1
 From Avogadro  Na.  6,022 5.10 23 mol -1
 Boltzman’s  K  1,380 5. 10 -23 Jk -1
 From Faraday  F  9,648 7. 10 4 C.mol
 From Plank  h  6,625 6.10 -34 J.s
 Elemental charge  and  1,602 1. 10 -19 C
 Speed ​​of light (empty)  c  2,997 9.10 8 ms 

 

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