International System of Units
International System of Units . Name adopted by the XI General Conference of Weights and Measures (held in Paris in 1960 ) for a universal, unified and coherent system of measurement units, based on the mks (meter-kilogram-second) system. This system is known as SI, initials of International System. At the 1960 Conference, standards were defined for six basic or fundamental units and two supplementary units ( radian and steradian ); In 1971 , a seventh fundamental unit, the mole, was added. 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 dimensionless derived units.
Summary
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- 1 Importance
- 2 Emergence
- 3 Prefixes, Symbols, and Factors in the SI
- 4 Writing Rules
- 5 Most used quantities and units
- 1 Units and Linear Equivalences
- 2 Units and superficial equivalences
- 3 Units and volume equivalents
- 4 Mass units and equivalences
- 5 Other usage equivalents
- 6 Speed units and their equivalences
- 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 achieved centuries ago by some countries such as Germany , the USA , Spain and England in science and technology; brought with it the need to use different physical magnitudes to express the technical characteristics of the different discoveries. Trade with the different countries of the world brought with it the spread of physical magnitudes and units that took root in the population.
All this exchange of technology or trade between countries with greater or lesser 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 therefore carry out conversions between units; leading to inaccuracies and errors.
For all of the above, the State Committee for Normalization , using the powers conferred on it by Decree Law No. 62 of December 30 , 1982 , by the Third Special Provision, establishes the conversion coefficients between units of measurement of legal use in the country.
Emergence
The International System of Units (SI) arises from the MKS Metric System (meter, kilogram and second) and three systems derived from it. That of MKSA Electrotechnics (meter, Kilogram, second and ampere); MKSG Thermotechnics (meter, kilogram , second and degree kelvin); of MSC Lighting Technology (meter, second and candela). These systems were used in isolation and had the meter, kilogram and second as a common element. Thus the idea of organizing on the basis of these systems arises. A single system of units, universal and coherent that covered all branches of science and technology.
As a result of the consultations made to thousands of scientists, technicians and educators from all countries, the International System of Units (SI) was established, to be adopted by all signatory countries of the meter conversion.
The General Conferences on Weights and Measures that were in charge of this arduous work, made present the need for its prompt application in all fields of science, technology and education. As a consequence of this decision, scientists and educators around the world began a campaign for the state implementation of this system as unique and universal.
This method consists of choosing some basic units of measurement as the basis of the system; considered independent of each other, from which the units of measurement of physical quantities are derived. There is another group of derived measurement units that are determined according to the physical formulas that relate physical quantities to each other. The basic SI units of measurement are: the meter (m), the kilogram (kg), the second (s), the ampere (A), the Kelvin (K), the candela (cd), and the mole (mol). .
Prefixes, Symbols, and Factors in the SI
| 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 | Q | 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 |
| beak | p | 10 -12 = 0.000 000 000 001 |
| femto | F | 10 -15 = 0.000 000 000 000 001 |
| atto | to | 10 -18 = 0.000 000 000 000 000 001 |
Other symbols
Listed below are 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 |
| Degree | ° | 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 observed that many people dedicated to this purpose make errors in writing units, physical magnitudes or their symbols. Below is a group of rules for writing these, which are intended to improve this unfortunate error that can be observed daily.
- Multiples and Sub-multiples of SI units are formed by multiplying or dividing the value of the SI unit by 10 or an integer power.
- SI prefix symbols are written with Latin characters, with no space between the prefix and the unit of measurement symbol.
- The symbols of the units of measurement and the units of relative and logarithmic measurement are set to use letters of the Latin, Greek alphabet or special signs.(see table)
- Symbols for units of measurement are printed in Roman (round) characters regardless of the characters used in the rest of the text.
- SI unit symbols are written with lowercase letters. However, when these are derived from patronymics, capital letters are used for the first letter.
- The symbols for SI units remain unchanged in plural.
- 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)
- The writing of the numbers will be done using Arabic figures. In the case of decimal numbers, the separation of the integer part of the decimal will be done by a comma. (, )
- Writing multi-figure decimal numbers, for easier reading, will be done by separating the whole part into groups of three figures from right to left, starting from the comma, leaving a blank space. The decimal part will also be written in groups of three figures, from left to right starting from 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. (65km)
- 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 complete names of the SI units; These will be written in lower case just like the number. (twenty meters). Only the full name of the unit will be written when referring to it.
- When a symbol accompanies a decimal value, it will be placed after all the figures. (368.54 dm)
- When indicating values of physical quantities with their limit deviations, when indicating an interval or when listing several numerical values, the unit symbol will be used according to the following example:
- 20 mm.25 mm or (20.25) mm
- 80;100 and 150 km
- From 18 to 25 Pa
- (20 ± 2) °c or 20 °c ± 2 °c
- from 120 to 150 kg
- 5m±3mm
- In written texts, a symbol should not begin the sentence.
- It is allowed to use symbols in column titles and in the names of table rows. The use of SI prefixes alone, without the accompanying unit of measurement, is not permitted.
- When writing numbers in text, they will be made the size of the capital letter.
- When writing several consecutive numbers, separate them by semicolons.
Most used quantities and units
The most commonly used measurement units are listed below with their respective equivalences to SI and other units. These are grouped into linear, superficial, volume and mass for better understanding.
Units and Linear Equivalences
| No | Unit | Symbol | Equivalence |
| 1 | kilometer | km | 1000m |
| 2 | hectometer | hmm | 100m |
| 3 | decameter | dam | 10m |
| 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 | vara (cuban) | 0.848m | |
| 12 | vara (Spanish) | 0.835 905 m | |
| 13 | yard | yd | 0.914 4 m = 3 foot = 36 inch |
| 14 | league | 4 240 m = 5 000 vara = 2.634 6 mile | |
| fifteen | chaín (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 superficial equivalences
| No | Units | Symbols | Equivalences |
| 1 | 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 | ha | 10,000 m² |
| 5 | area | to | 100 m² |
| 6 | centiare | AC | 10 m² |
| 7 | acre | 4,046.86 m² | |
| 8 | chivalry | cab | 134 202.06 m² = 13.420 m² = 324 square cord |
| 9 | besana or vesana | 2,588.77 m² = 3,600 square Cuban vara | |
| 10 | expensive | 13,420.2 m² | |
| eleven | square twine | 414,204 m² = 576 square vara | |
| 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 rose of 10,000 Cuban square vara | 7 191.04 m² | |
| 18 | rose or rose of 18 square cord | 7 455,670 m² |
Units and volume equivalents
| No | Units | Symbols | Equivalences |
| 1 | liter | l | 1000 mL= 1 dm 3 |
| 2 | bottle | 0, 750 L = 750 mL = 750 cm 3 | |
| 3 | american gallon | 3,785 41L = 3,785 41dm 3 | |
| 4 | english gallon | 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 equivalences
| No | Units | Symbols | Equivalences |
| 1 | at sign | @ | 11.502 3 kg = 25 lb |
| 2 | spanish pound | lb | 0.460 093 kg = 460 g = 16 ounces |
| 3 | Spanish quintal | 46.009 3 kg = 100 lb | |
| 4 | metric quintal | q | 100kg |
| 5 | short ton (Spain) | 920.19kg | |
| 6 | long ton (Spain) | 1030.61kg | |
| 7 | metric ton | 1000kg | |
| 8 | ounce (Spanish) | 28,755 8.10 3 kg |
Other usage equivalents
| No | Units | Symbols | Equivalences |
| 1 | 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) | #! | 745,700w |
| 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 | cubic foot of wood | 2,359 74.10 -3 m 3 | |
| 9 | yard | 3 foot = 36 inches | |
| 10 | foot | 12 inch = 0.304 8 m = 30.48 cm | |
| eleven | international inch | 0.025 4m = 2.54cm |
Speed units and their equivalents
| No | Unit | Equivalence |
| 1 | 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) | 3.6km/h |
Units equivalent to physical quantities
| No | Basic physical quantity | dimensional symbol | Basic unit | Unity symbol | Observations |
| 1 | Length | l | meter | m | It is defined by setting the value of the speed of light in a vacuum. |
| 2 | Time. | T | second | yes | It is defined by setting the value of the frequency of the hyperfine transition of the cesium atom |
| 3 | Mass | M | kilogram | kg | It is the mass of the “standard cylinder” kept at the International Bureau of Weights and Measures, in Sèvres, France. It is equivalent to the mass occupied by one liter of pure water at 14.5 °C or 286.75 K. |
| 4 | Electric current intensity | Yo | amp | TO | It is defined by setting the value of the magnetic constant. |
| 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 12C atom to 12 grams/mol. See also Avogadro’s number. |
| 7 | Luminous intensity | J. | candle | mole | See also related concepts: lumen, lux and physical illumination. |
- One Kelvin is equal to 273 oC
Units derived from systematic use in Physical Chemistry
| Physical magnitude | SI unit | Symbol | Definition |
| Force | Newton | N | Kg.ms -2 |
| Pressure | Pascal. | Da | 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 seconds) |
| 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 (YES) |
| Gas molar | R | 8.314 3 JK -1 .mol -1 |
| From Avogadro | Nah. | 6.022 5.10 23 mol -1 |
| By Boltzman | K | 1,380 5. 10 -23 Jk -1 |
| Faraday | F | 9.648 7. 10 4 C.mol |
| De Plank | h | 6,625 6.10 -34 J.s |
| Elemental charge | and | 1.602 1. 10 -19 C |
| Speed of light (vacuum) | c | 2.997 9.10 8 ms -1 |