International System of Units
From the Simple English Wikipedia, the free encyclopedia that anyone can change
The International System of Units is the international standard form of the metric system today. SI is the short name for this from the French language phrase Système International d'unités.
The metric system is a system of measuring based on the metre for length, distance or displacement, kilogram for mass, and second for time.
The metre, kilogram and second can be used in combination with each other. This will make different units of measurement to mean other amounts, such as volume, energy, pressure, and velocity.
Sometimes we want to talk about larger or smaller measurements. Then we add a prefix. A prefix is something to the beginning of a word to make a word. The prefix kilo- means "1000" and the prefix milli- means "0.001". So one kilometre is 1000 metres and one milligram is a 1000th of a gram. These prefixes are shown in the table on the right side.
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[change] History and use
These were created in France after the French Revolution. They are now used almost everywhere in the world, apart from the United States, Liberia and Myanmar, where the older imperial units are still widely employed. Varios other countries, most of them historically related to the British Empire, are in different stages of the process of introduction of the metric system as replacement for or as an addition to the use of the imperial system.
Even the United Kingdom, which created the old US units of measurement, is now generally using the metric system alongside the imperial system. There is even pressure to become more metric, both as a result of UK government plans dating before Britain's entry into the European Union and because of EU rules regarding common systems of measurement.
[change] Base units of measurement
| yotta- | 1 000 000 000 000 000 000 000 000 |
|---|---|
| zetta- | 1 000 000 000 000 000 000 000 |
| exa- | 1 000 000 000 000 000 000 |
| peta- | 1 000 000 000 000 000 |
| tera- | 1 000 000 000 000 |
| giga- | 1 000 000 000 |
| mega- | 1 000 000 |
| kilo- | 1 000 |
| hecto- | 100 |
| deca- | 10 |
| 1 | |
| deci- | 1/10 |
| centi- | 1/ 100 |
| milli- | 1/ 1 000 |
| micro- | 1/ 1 000 000 |
| nano- | 1/ 1 000 000 000 |
| pico- | 1/ 1 000 000 000 000 |
| femto- | 1/ 1 000 000 000 000 000 |
| atto- | 1/ 1 000 000 000 000 000 000 |
| zepto- | 1/ 1 000 000 000 000 000 000 000 |
| yocto- | 1/ 1 000 000 000 000 000 000 000 000 |
The SI base units are measurements used by scientists and other people around the world. All the other units can be written by combining these seven base units in different ways. These other units are called "derived units".
[change] Length (l)
Unit: metre or meter (m)
One metre is defined as the distance light travels in a vacuum in 1/299,792,458 second. This standard was adopted in 1983 when the speed of light in vacuum was defined to be precisely 299,792,458 m/s.
[change] Mass (m)
Unit: kilogram (kg)
One kilogram is defined to be the mass of a specific cylinder of platinum-iridium alloy. It is kept at the International Bureau of Weights and Measures near Paris. There is an ongoing effort to introduce a definition using other basic or atomic constants.
[change] Time (t)
Unit: second (s)
One second is defined as the time required for 9,192,631,770 periods of the radiation between to two specific energy levels of the element caesium-133. This definition was adopted in 1967.
[change] Electric flow (I)
Unit: ampere (A)
The ampere is that constant electrical current (or flow) which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per metre of length.
The ampere is one of two base units (the other being the candela) that uses derived units in its definition, not just base units. One newton is 1 kg m s-2.
[change] Temperature (T)
Unit: kelvin (K)
The kelvin, unit of temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. It is named after Lord Kelvin.
[change] Amount of substance (n)
Unit: mole (mol)
- 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".
- 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.
[change] Luminous intensity (brightness of light) (I)
Unit: candela (cd)
The candela is the luminous intensity (brightness), in a given direction, of a 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 candela is one of two base units (the other being the ampere) that uses derived units in its definition, not just base units. One hertz is 1 s-1, one watt is 1 J s-1 = 1 kg m2 s-2 and the steradian is the solid angle subtended at the centre of a sphere of radius r by a portion of the surface of the sphere having an area r2.
[change] Derived units of measurement
[change] Plane angle
Unit: radian (rad)
[change] Solid angle
Unit: steradian (sr)
[change] Frequency
Unit: hertz (Hz)
[change] Force
Unit: newton (N)
[change] Pressure
Unit: pascal (Pa)
[change] Energy
Unit: joule (J)
[change] Power
Unit: watt (W)
[change] Electric charge
Unit: coulomb (C)
[change] Electric potential
Unit: volt (V)
[change] Capacitance
Unit: farad (F)
[change] Electric resistance
Unit: ohm (Ω)
[change] Derived units with special names
There are also derived units which have special names. Usually these were made to make calculating simpler.
| Name | Symbol | Quantity | Expression in terms of other units | Expression in terms of SI base units |
|---|---|---|---|---|
| hertz | Hz | frequency | 1/s | s−1 |
| newton | N | force, weight | m∙kg/s2 | m∙kg∙s−2 |
| pascal | Pa | pressure, stress | N/m2 | m−1∙kg∙s−2 |
| joule | J | energy, work, heat | N∙m | m2∙kg∙s−2 |
| watt | W | power, radiant flux | J/s | m2∙kg∙s−3 |
| coulomb | C | electric charge or electric flux | s∙A | s∙A |
| volt | V | voltage, electrical potential difference, electromotive force | W/A = J/C | m2∙kg∙s−3∙A−1 |
| farad | F | electric capacitance | C/V | m−2∙kg−1∙s4∙A2 |
| ohm | Ω | electric resistance, impedance, reactance | V/A | m2∙kg∙s−3∙A−2 |
| siemens | S | electrical conductance | 1/Ω | m−2∙kg−1∙s3∙A2 |
| weber | Wb | magnetic flux | J/A | m2∙kg∙s−2∙A−1 |
| tesla | T | magnetic field | V∙s/m2 = Wb/m2 = N/A∙m | kg∙s−2∙A−1 |
| henry | H | inductance | V∙s/A = Wb/A | m2∙kg∙s−2∙A−2 |
| Celsius | °C | Celsius Temperature | T°C = TK − 273.15 | K |
| lumen | lm | luminous flux | cd∙sr | cd |
| lux | lx | illuminance | lm/m2 | m−2∙cd |
| becquerel | Bq | radioactivity (decays per unit time) | 1/s | s−1 |
| gray | Gy | absorbed dose (of ionizing radiation) | J/kg | m2∙s−2 |
| sievert | Sv | equivalent dose (of ionizing radiation) | J/kg | m2∙s−2 |
| katal | kat | catalytic activity | mol/s | s−1∙mol |
