**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**

Name | Symbol | Quantity |
---|---|---|

radian | rad | angle |

steradian | sr | solid angle |

hertz | Hz | frequency |

newton | N | force, weight |

pascal | Pa | pressure, stress |

joule | J | energy, work, heat |

watt | W | power, radiant flux |

coulomb | C | electric charge or quantity of electricity |

volt | V | voltage (electrical potential), emf |

farad | F | capacitance |

ohm | Ω | resistance, impedance, reactance |

siemens | S | electrical conductance |

weber | Wb | magnetic flux |

tesla | T | magnetic flux density |

henry | H | inductance |

degree Celsius | °C | temperature relative to 273.15 K |

lumen | lm | luminous flux |

lux | lx | illuminance |

becquerel | Bq | radioactivity (decays per unit time) |

gray | Gy | absorbed dose (of ionizing radiation) |

sievert | Sv | equivalent dose (of ionizing radiation) |

katal | kat | catalytic activity |

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