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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