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[[File:SI base unit.svg | thumb | upright=1.5 | Les 7 unitats del Sistema Internacional i les interdependències de les seves definicions]]
Una '''unitat bàsica''' o '''unitat base''' és una unitat de mesura del [[Sistema Internacional]] considerada com a primordial, ja que serveix per definir totes les altres unitats derivades. Existeixen set unitats base:
Linha 11 ⟶ 10:
* La [[candela]], per la [[intensitat lluminosa]]
 
== Les set unitats base del SI ==
{| class="wikitable"
|+ style="font-size:larger;font-weight:bold;"|[[SI]] base units
!Nom
!Símbol
!Mesura
!Definició formal actual (2005)
!Origen històric / justificació
|-
|[[metre]]
! Nom !! Símbol !! Mesura !! Definició !! Origen / Justificació
| style="text-align: center" | '''m'''
|[[longitud]]
|"El metre és la longitud del camí recorregut per la llum en el buit durant un interval de temps d'1&nbsp;{{frac}}&nbsp;{{val|299792458}} de segon."<br/>''17a CGPM (1983, Resolució 1, CR, 97)''
|1&nbsp;{{frac}}&nbsp;10,000,000 de la distància de l'equador terrestre al pol Nord mesurada sobre la circumferència que passa per París.
|-
|[[kilogram]]
| [[metre]] || '''m''' || [[longitud]] || "El metre es la longitud que recorre la llum al buit en un interval de 1/299 792 458 d'un segon." || És la {{frac|10,000,000}} de la distància en l'[[equador]] i el [[Pol Nord]] de la [[Terra]] mesurant la distància a través de [[París]].
| style="text-align: center" | '''kg'''
|[[mass]]
|"The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram."<br/>''3rd CGPM (1901, CR, 70)''
|The mass of one [[litre|lit{{#ifeq:{{{sp}}}|us|er|re}}]] of [[water]]. A lit{{#ifeq:{{{sp}}}|us|er|re}} is one thousandth of a cubic met{{#ifeq:{{{sp}}}|us|er|re}}.
|-
|[[second]]
| [[quilogram]] || '''kg''' || [[massa]] || || És la massa d'un litre d'[[aigua]].
| style="text-align: center" | '''s'''
|[[time]]
|"The second is the duration of {{val|9192631770}} periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom."<br/>''13th CGPM (1967/68, Resolution 1; CR, 103)''<br />"This definition refers to a caesium atom at rest at a temperature of 0 K."<br />''(Added by CIPM in 1997)''
|The day is divided in 24 hours, each hour divided in 60 minutes, each minute divided in 60 seconds.<br/>A second is 1&nbsp;{{frac}}&nbsp;(24&nbsp;×&nbsp;60&nbsp;×&nbsp;60) of the [[day]]
|-
|[[ampere]]
| [[segon]] || '''s''' || [[temps]] || || El [[dia]] està dividit en 24 hores, cada hora dividida en 60 [[minut]]s, cada minut dividit en 60 segons.
| style="text-align: center" | '''A'''
Un segon es 1⁄(24 × 60 × 60) d'un dia.
|[[electric current]]
|"The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 met{{#ifeq:{{{sp}}}|us|er|re}} apart in vacuum, would produce between these conductors a force equal to {{nowrap|2 × 10<sup>−7</sup>}} [[newton (unit)|newton]] per met{{#ifeq:{{{sp}}}|us|er|re}} of length."<br/>''9th CGPM (1948)''
|The original "International Ampere" was defined electrochemically as the current required to deposit 1.118 milligrams of silver per second from a solution of [[silver nitrate]]. Compared to the SI ampere, the difference is 0.015%.
|-
|[[kelvin]]
| [[ampere]] || '''A''' || [[corrent elèctric]] || ||
| style="text-align: center" | '''K'''
|[[thermodynamic temperature]]
|"The kelvin, unit of thermodynamic temperature, is the fraction 1&nbsp;{{frac}}&nbsp;{{val|273.16}} of the thermodynamic temperature of the [[triple point]] of water."<br/>''13th CGPM (1967/68, Resolution 4; CR, 104)''<br />"This definition refers to water having the isotopic composition defined exactly by the following amount of substance ratios: {{nowrap|0.000 155 76}} mole of <sup>2</sup>H per mole of <sup>1</sup>H, {{nowrap|0.000 379 9}} mole of <sup>17</sup>O per mole of <sup>16</sup>O, and {{nowrap|0.002 005 2}} mole of <sup>18</sup>O per mole of <sup>16</sup>O."<br />''(Added by CIPM in 2005)''
|The [[Celsius scale]]: the Kelvin scale uses the degree Celsius for its unit increment, but is a thermodynamic scale (0&nbsp;K is [[absolute zero]]).
|-
|[[Mole (unit)|mole]]
| [[kelvin]] || '''K''' || [[temperatura termodinàmica]] || ||
| style="text-align: center" | '''mol'''
|[[amount of substance]]
|"1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is 'mol.'<br />
2. 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."<br/>''14th CGPM (1971, Resolution 3; CR, 78)''<br />"In this definition, it is understood that unbound atoms of carbon 12, at rest and in their ground state, are referred to."<br />''(Added by CIPM in 1980)''
|[[Atomic weight]] or [[molecular weight]] divided by the [[molar mass constant]], 1&nbsp;g/mol.
|-
|[[candela]]
| [[mol]] || '''mol''' || [[quantitat de substància]] || ||
| style="text-align: center" | '''cd'''
|[[luminous intensity]]
|"The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency {{val|540|e=12}} hertz and that has a radiant intensity in that direction of 1/683 watt per [[steradian]]."<br/>''16th CGPM (1979, Resolution 3; CR, 100)''
|The [[candlepower]], which is based on the light emitted from a burning candle of standard properties.
|-
| [[candela]] || '''cd''' || [[intensitat lluminosa]] || ||
|}
 
 
{{ORDENA:Unitat Base}} <!--ORDENA generat per bot-->
 
{{Esborrany de ciència}}
 
[[Categoria:Unitats del Sistema Internacional]]
Obtingut de «https://ca.wikipedia.org/wiki/Unitat_bàsica_del_Sistema_Internacional»