Sistema Internacional de Unidades
O Sistema Internacional de Unidades (Francês Système International ou SI abreviadamente) é o sistema de medição acordado tal como aprovado pelas reuniões periódicas dos quarenta e seis membros da Conférence Générale des Poids et Mesures (Conferência Geral de Pesos e Medidas) ou, abreviadamente, como CGPM. Ela foi desenvolvida em 1960 a partir do sistema metro-quilograma-segundo (mks) e substitui o sistema centímetro-grama-segundo (cgs).
As definições foram sujeitas a alterações ao longo dos anos e, geralmente, dependem de pressupostos que o CGPM faz sobre o que é permanente e o que não é.
As unidades de base
O SI depende de sete unidades de base de medição a partir das quais todas as outras unidades derivam. Elas são:
Segundo
O segundo (abreviado s ou seg) é a unidade de tempo, definida como
a duração de 9,192,631,770 períodos da radiação correspondente à transição entre dois níveis hiperfinos do estado fundamental do átomo de césio-133 (13ª CGPM, 1967).
Historicamente, o segundo ficou sem definição, até 1820, quando foi definido como 1/86,400 do dia solar médio. Em 1956, a CGPM adotou o segundo efemérides definição de 1/31 556 925.9747do ano tropical de 1900. A definição atual data de 1963.
Metro
O metro (abreviado m) é a unidade de comprimento. Originalmente (ver sistema métrico) ele tinha uma definição, dependendo das dimensões da terra. Mais tarde, teve uma definição dependente de um determinado tipo de radiação. Hoje sua definição depende do segundo e da velocidade da luz no vácuo--que a CGPM assume agora ser constante. Assim, um metro é 1/299,792,458º de um segundo-luz. (Portanto, a velocidade da luz é exatamente 299.792.458 metros por segundo.)
Quilograma
O quilograma (abreviado kg) (e não a grama métrica) é a unidade de massa. Ela é definida como uma quantidade de massa igual à do quilograma protótipo padrão mantido na International Bureau of Weights and Measures emt Sèvres, França. Ela é a única unidade de medida baseada num artefato físico. Este é o mesmo artefato que tem proporcionado a definição de gramas e quilogramas por mais de um século. Também é a única unidade SI de base a reter a sua forma decimal prefixa, kilo-, que provém da palavra grega para "mil".
Ampére
O ampére (abreviado A) é a unidade de corrente elétrica. (Estranhamente a carga elétrica não é a quantidade de base). A definição é a seguinte:
Um ampere é a corrente constante que, mantida em dois condutores paralelos, retilíneos, de comprimento infinito, de seção circular desprezível, e colocados a 1 metro no vácuo, produziria entre estes condutores uma força igual a 2 × 10−7 newtons por metro de comprimento.
Para a definição de newton, veja abaixo em "unidades derivadas".
Kelvin
The kelvin (abbreviated K, named for Lord Kelvin) is the unit of temperature, and specifically of absolute temperature. It is 1/273.16 of the absolute temperature of the triple point of water. The size of the kelvin is the same as the size of the Celsius degree, and thus to convert from degrees Celsius to kelvins, one adds 273.15. (The zero of the Celsius scale, which is the melting point of water ice under standard atmospheric pressure, happens to be 273.15 kelvins.)
Mole
The mole (abbreviated mol) is the unit of amount of substance. Its definition depends on that of the kilogram. It is that mass of any substance having as many elementary entities of that substance as are to be counted in 0.012 kilogram of carbon-12. An elementary entity is either an atom or a molecule--but for an ionic compound, the number of elementary entities is the total number of cations and anions required to make the compound electrically neutral.
Effectively, one mole of any substance is its formula mass in grams.
Candela
The candela (abbreviated cd) is the unit of luminous intensity. Its definition is
the luminous intensity, in a given direction, of a [light] 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.
The term hertz means "vibrations, transitions, or other events per second." For the definitions of the terms watt and steradian, see below. Light having the stated frequency is generally perceived as red.
Supplementary Units
SI originally had the two units of plane angle and solid angle. CGPM decided not to retain them in the SI definition. However, their definitions, deriving as they do from geometry, are still valid.
Radian
The radian (abbreviated rad) is the plane angle unit. It is the measure of a central angle (one having its origin as the center of a circle) that subtends an arc of the circle having a length equal to that of the radius. By convention, the circumference of the unit circle (a circle having a radius of one) is 2π, and therefore the maximum "major angle" of any circle is 2π radians.
Steradian
The steradian (abbreviated sr) is the solid angle unit. A solid angle is actually one nappe of a right circular conical surface. One steradian is the measure of a solid angle that subtends a portion of the surface of a sphere having an area equal to that of a square having sides equal in length to the radius. By convention, the surface area of a sphere is 4πr2, where r is the length of the radius, and thus the maximum "major solid angle" of any sphere is 4π steradians.
Derived Units
SI has multiple derived units that measure quantities in several areas of measurement and inquiry. The following is a comprehensive list, grouped by category.
Dynamics
Newton
The newton (abbreviated N, named for Sir Isaac Newton) is the SI unit of force. One newton is that amount of force required to accelerate a one-kilogram mass one meter per second per second. To calculate, multiply mass by acceleration.
Pascal
The pascal (abbreviated Pa) is that amount of pressure that exerts a force of one newton in every square meter of area.
Work and Power
Joule
The joule (abbreviated J, named for James Joule) is that amount of energy required to do the work of exerting a force of one newton over a distance of one meter. To calculate, multiply force by distance.
Watt
The watt (abbreviated W, named for James Watt) is the power required to do one Joule of work in one second. To calculate, divide work by time.
Electricity and Magnetism
Coulomb
The coulomb (abbreviated coul, for C. A. Coulomb) is that amount of electric charge that passes through a conductor in one second at a current of one ampere. To calculate, multiply current by time.
Volt
The volt (abbreviated V) is the electric potential difference between two conductors having a current flow of one ampere from one to another, and dissipating one watt of power. Most people find the phrase "electric potential" hard to understand and the alternative "electromotive force" to be hard to pronounce. They therefore use the word "voltage" for this quantity. To calculate, divide power by current--or divide work (or energy) by electric charge.
Ohm
The ohm (abbreviated Ω, for Georg Simon Ohm) is that amount of electric resistance that requires an electric potential difference of one volt to push a current of one ampere through it. To calculate, divide electrical potential by current.
Siemens
The siemens (abbreviated S, for Werner von Siemens) is the unit of electric conductance, which is merely the reciprocal of resistance. In essence, the siemens is that level of conductance required to allow a current of one ampere to flow through a load after an electric potential of one volt is applied. To calculate it, divide current by electric potential or "voltage," rather than dividing voltage by current.
Farad
The farad (abbreviated F, for Michael Faraday) is that amount of capacitance that exists in a capacitor (essentially a pair or collection of metal plates that store charge) if a charge of one coulomb increases the electrical potential by one volt. To calculate, divide charge by electrical potential.
Weber
The weber (abbreviated Wb, for W. E. Weber) is
the magnetic flux which, linking a circuit of one turn, would produce in it an electromotive force of 1 volt if it were reduced to zero at a uniform rate in 1 second.
To calculate, multiply electric potential (or electromotive force) by time.
Henry
The henry (abbreviated H) is that inductance in a closed loop or coil that produces a magnetic flux of one weber given a current of one ampere. To calculate, divide magnetic flux by current.
Tesla
The tesla (abbreviated T, for Nicola Tesla) is the magnetic flux density required to produce one weber of magnetic flux in one square meter of surface area. To calculate, divide flux by area.
Light
Lumen
The lumen (abbreviated lm) is that amount of luminous flux emitted in a solid-angle region of one steradian that produces one candela of intensity. To calculate, multiply luminous intensity by solid angle.
Lux
The lux (abbreviated lx) is that illuminance that represents one lumen in one square meter. To calculate, divide luminous flux by area.
Radioactivity, Radiation, Health Physics, and Radiation Safety
Becquerel
The becquerel (abbreviated Bq, for Henri Becquerel) is that amount of activity of a radioactive substance represented by one decaying atom per second.
Curie
The curie (abbreviated Ci, for Marie Curie) is obsolete today. Its original definition was "the amount of radioactivity in one gram of radium-226." Madame Curie, of course, is most famous as the discoverer of radium. One curie is equal to 3.7 * 1010 becquerel.
Gray (Unit)
The gray (abbreviated Gy) is that absorbed dose of radiation that imparts one joule of energy to one kilogram of target mass. To calculate, divide imparted energy by mass.
Sievert
The sievert (abbreviated Sv) is the unit of dose equivalent. It is that dose of any given form of ionizing radiation that would cause a human being or an animal as much injury as would a one-gray dose of X rays.
Ligações externas
- SI Sizes.com
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