SI Units and Derived Units
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The International System of Units (SI, abbreviated from the French Système international ) is the modern form of the metric system, and is the most widely used system of measurement.
- A system of physical units ( SI units ) based on the metre, kilogram, second, ampere, kelvin, candela, and mole, together with a set of prefixes to indicate multiplication or division by a power of ten.
- It comprises a coherent system of units of measurement built on seven base units (ampere, kelvin, second, metre, kilogram, candela, mole) and a set of twenty decimal prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.
- The system also specifies names for 22 derived units for other common physical quantities like lumen, watt, etc.
- The SI base units are the building blocks of the system and all the other units are derived from them. When Maxwell first introduced the concept of a coherent system, he identified three quantities that could be used as base units: mass, length and time.
|Unit (name)||Unit (symbol)||Quantity (name)||Definition|
|metre||m||Length||The distance travelled by light in vacuum in 1/299792458 second.|
|kilogram||kg||Mass||The mass of a small squat cylinder of ~47 cubic centimetres of platinum-iridium alloy kept in a laboratory in France|
|second||s||Time||The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.|
|ampere||A||Electric current||a unit of electric current equal to a flow of one coulomb per second.|
|kelvin||K||Thermodynamic Temperature||1/273.16 of the thermodynamic temperature of the triple point of water|
|mole||mol||Amount of Substance||The amount of substance of a system which contains as many elementary entities [n 4] as there are atoms in 0.012 kilogram of carbon-12.|
|candela||cd||Luminous Intensity||The magnitude of an electromagnetic field , in a specified direction, that has a power level of 1/683 watt (1.46 x 10 -3 W) per steradian at a frequency of 540 terahertz (540 THz or 5.40 x 10 14 Hz ).|
SI Derived units
|joule||J||energy, work, heat|
|watt||W||power, radiant flux|
|coulomb||C||electric charge or quantity of electricity|
|volt||V||voltage (electrical potential), emf|
|ohm||Ω||resistance, impedance, reactance|
|tesla||T||magnetic flux density|
|degree Celsius||°C||temperature relative to 273.15 K|
|becquerel||Bq||radioactivity (decays per unit time)|
|gray||Gy||absorbed dose (of ionizing radiation)|
|sievert||Sv||equivalent dose (of ionizing radiation)|