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S.I. Units: Difference between revisions

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The modern metric system is the [https://en.wikipedia.org/wiki/International_System_of_Units '''International System of Units'''] ''(French: Système International d'Unités, SI)''.  The system was published in 1960 and was based on the metre-kilogram-second system of units (MKS). There are now internationally agreed units covering all aspects of technology together with a set of [https://en.wikipedia.org/wiki/Metric_prefix#List_of_SI_prefixes '''prefixes'''] which serve as multipliers or fractions of them. The preferred prefixes multiply or divide the unit names by 1000 ''(or 10<sup>-3</sup>)'' thus, 10mm is preferred to a centimetre, especially in construction and engineering.
[[File:SIbaseunits.png|300px|right]]
__TOC__
 
 
The modern metric system is the [https://en.wikipedia.org/wiki/International_System_of_Units '''International System of Units'''] ''(French: Système International d'Unités, SI)''.  The system was published in 1960 and was based on the metre-kilogram-second system of units (MKS). There are now seven internationally agreed units plus units derived from them to cover all aspects of technology together with a set of [https://en.wikipedia.org/wiki/Metric_prefix#List_of_SI_prefixes '''prefixes'''] which serve as multipliers or fractions of them.
 
 
The preferred prefixes multiply or divide the unit names by 1000 ''(or 10<sup>-3</sup>)'' thus, 10mm is preferred to a centimetre, especially in construction and engineering.


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__TOC__


=====Measurement of Length=====
=====Measurement of Length=====
{|
{|
|-
|-
| colspan="5"|[https://en.wikipedia.org/wiki/Metre '''Metre''']
! colspan="5"|[https://en.wikipedia.org/wiki/Metre '''Metre''']
|-
|-
| '''Prefix Name'''
! '''Prefix Name'''
| '''Symbol'''
! '''Symbol'''
| '''10<sup>n</sup>'''
! '''10<sup>n</sup>'''
| '''Decimal'''
! '''Decimal'''
| '''Description'''
! '''Description'''
|-
|-
| kilometre
| kilometre
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| A millionth of a metre
| A millionth of a metre
|}
|}
=====Units used in Mechanics=====
{|
|-
! '''Prefix Name'''
! '''Symbol'''
! '''10<sup>n</sup>'''
! '''Decimal'''
! '''Description'''
|-
| gram
| g
| 10<sup>-3</sup>
| 1/1000
| Originally the weight of a cubic centimetre of pure water but now defined as one-thousandth of a [https://en.wikipedia.org/wiki/Kilogram '''kilogram''']
|-
| kilogram
| kg
| 10<sup>0</sup>
| 1
| '''S.I. Base Unit of Mass''' - equal to the mass of the ''International Prototype of the Kilogram''
|-
| tonne
| t
| 10<sup>3</sup>
| 1000
| Known as '''Metric Ton''' in USA ''(i.e. equivalent to approximately 2,204.6lbs whereas Imperial Ton = 2240lbs)''
|-
| megatonne
| Mt
| 10<sup>9</sup>
| 1,000,000,000
| Used for the energy of nuclear explosions and other similar events.
|-
| Newton
| N
| 10<sup>0</sup>
| 1
| '''S.I. Base Unit of Force''' named after [https://en.wikipedia.org/wiki/Isaac_Newton '''Sir Isaac Newton'''] - the amount needed to accelerate a 1kg mass at 1 metre per second each second ''(1N = 1kg X ms<sup>2</sup>)''
|-
| Newton Metre
| Nm
| 10<sup>0</sup>
| 1
| '''S.I. Base Unit of Torque or Moment''' - resulting from applying a force of one newton perpendicularly to a moment arm which is one metre long ''(1Nm)''. It is calculated as for a joule but is preferred for Torque to avoid confusion with Work and Energy.
|-
| Joule
| J
| 10<sup>0</sup>
| 1
| '''S.I. Base Unit of Work or Energy''' named after [https://en.wikipedia.org/wiki/James_Prescott_Joule '''James Prescott Joule'''] - the amount needed to apply a newton over the distance of a metre ''(1J = 1Nm)''
|-
| Pascal
| Pa
| 10<sup>0</sup>
| 1
| '''S.I. Base Unit of Pressure or Stress''' named after [https://en.wikipedia.org/wiki/Blaise_Pascal '''Blaise Pascal'''] - the pressure exerted by applying a newton over a square metre ''(1Pa = 1Nm<sup>2</sup>)''
|}
=====Units used in Electronics=====
=====Units used in Electronics=====
{|
{|
|-
|-
| colspan="5"|[https://en.wikipedia.org/wiki/Ohm '''Ohms (resistance)'''] - named after [https://en.wikipedia.org/wiki/Georg_Ohm '''Georg Simon Ohm''']
! colspan="5"|[https://en.wikipedia.org/wiki/Ohm '''Ohms (resistance)'''] - named after [https://en.wikipedia.org/wiki/Georg_Ohm '''Georg Simon Ohm''']
|-
|-
| '''Prefix Name'''
! '''Prefix Name'''
| '''Symbol'''
! '''Symbol'''
| '''10<sup>n</sup>'''
! '''10<sup>n</sup>'''
| '''Decimal'''
! '''Decimal'''
| '''Description'''
! '''Description'''
|-
|-
| mega-ohm
| mega-ohm
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</span>
</span>
|-
|-
| colspan="5"|[https://en.wikipedia.org/wiki/Ampere '''Ampere (electric current)'''] - named after [https://en.wikipedia.org/wiki/Andr%C3%A9-Marie_Amp%C3%A8re ''André-Marie Ampère''']
! colspan="5"|[https://en.wikipedia.org/wiki/Ampere '''Ampere (electric current)'''] - named after [https://en.wikipedia.org/wiki/Andr%C3%A9-Marie_Amp%C3%A8re ''André-Marie Ampère''']
|-
|-
| '''Prefix Name'''
! '''Prefix Name'''
| '''Symbol'''
! '''Symbol'''
| '''10<sup>n</sup>'''
! '''10<sup>n</sup>'''
| '''Decimal'''
! '''Decimal'''
| '''Description'''
! '''Description'''
|-
|-
| ampere
| ampere
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| A thousandth
| A thousandth
|-
|-
| colspan="5"|[https://en.wikipedia.org/wiki/Farad '''Farads (electric capacitance)'''] - named after [https://en.wikipedia.org/wiki/Michael_Faraday '''Michael Faraday''']
! colspan="5"|[https://en.wikipedia.org/wiki/Farad '''Farads (electric capacitance)'''] - named after [https://en.wikipedia.org/wiki/Michael_Faraday '''Michael Faraday''']
|-
|-
| '''Prefix Name'''
! '''Prefix Name'''
| '''Symbol'''
! '''Symbol'''
| '''10<sup>n</sup>'''
! '''10<sup>n</sup>'''
| '''Decimal'''
! '''Decimal'''
| '''Description'''
! '''Description'''
|-
|-
| farad
| farad

Revision as of 14:33, 13 November 2016

SIbaseunits.png


The modern metric system is the International System of Units (French: Système International d'Unités, SI). The system was published in 1960 and was based on the metre-kilogram-second system of units (MKS). There are now seven internationally agreed units plus units derived from them to cover all aspects of technology together with a set of prefixes which serve as multipliers or fractions of them.


The preferred prefixes multiply or divide the unit names by 1000 (or 10-3) thus, 10mm is preferred to a centimetre, especially in construction and engineering.

Note: Units named after people use capital letters for the letter symbol - but not for the unit name itself.

Measurement of Length
Metre
Prefix Name Symbol 10n Decimal Description
kilometre km 103 1000 A thousand metres
metre m 100 1 A metre
centimetre cm 10-2 0.01 A hundredth of a metre
millimetre mm 10-3 0.001 A thousandth of a metre
micrometre μm 10-6 0.000001 A millionth of a metre


Units used in Mechanics
Prefix Name Symbol 10n Decimal Description
gram g 10-3 1/1000 Originally the weight of a cubic centimetre of pure water but now defined as one-thousandth of a kilogram
kilogram kg 100 1 S.I. Base Unit of Mass - equal to the mass of the International Prototype of the Kilogram
tonne t 103 1000 Known as Metric Ton in USA (i.e. equivalent to approximately 2,204.6lbs whereas Imperial Ton = 2240lbs)
megatonne Mt 109 1,000,000,000 Used for the energy of nuclear explosions and other similar events.
Newton N 100 1 S.I. Base Unit of Force named after Sir Isaac Newton - the amount needed to accelerate a 1kg mass at 1 metre per second each second (1N = 1kg X ms2)
Newton Metre Nm 100 1 S.I. Base Unit of Torque or Moment - resulting from applying a force of one newton perpendicularly to a moment arm which is one metre long (1Nm). It is calculated as for a joule but is preferred for Torque to avoid confusion with Work and Energy.
Joule J 100 1 S.I. Base Unit of Work or Energy named after James Prescott Joule - the amount needed to apply a newton over the distance of a metre (1J = 1Nm)
Pascal Pa 100 1 S.I. Base Unit of Pressure or Stress named after Blaise Pascal - the pressure exerted by applying a newton over a square metre (1Pa = 1Nm2)


Units used in Electronics
Ohms (resistance) - named after Georg Simon Ohm
Prefix Name Symbol 10n Decimal Description
mega-ohm MΩ 106 1000000 A million ohms
kilo-ohm kΩ (or K) 103 1000 A thousand ohms
ohm Ω (or R) 100 1 An ohm
Note:

The unit or its prefix may be used to replace a decimal point - e.g. 2.2mega-ohm = 2M2; 2.2kilo-ohm = 2K2; and 2.2ohm = 2R2.

Ampere (electric current)' - named after André-Marie Ampère
Prefix Name Symbol 10n Decimal Description
ampere A 100 1 Amp
milli-amp mA 10-3 0.001 A thousandth
Farads (electric capacitance) - named after Michael Faraday
Prefix Name Symbol 10n Decimal Description
farad F 100 1 A Farad
micro-Farad μF 10-6 0.000001 A millionth
nano-Farad nF 10-9 0.000000001 A thousand millionth
pico-Farad pF 10-12 0.000000000001 A trillionth
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