Physics » Notes » SI Base Units and Derived Units and Prefixes
“What are the SI Base Units ?
What are the SI Derived Units ?
What are the SI units Prefixes ?
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The SI or System International has the set of seven base units. These have been chosen to fulfill the requirements for science and technology measurements. The selection of seven base units is the responsibility of the International Committee of Weights and Measures (CIPM).
The International System of Units (SI) is the modern form of the metric system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty 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, such as lumen and watt, for other common physical quantities.
There are seven, dimensionally independent, base SI-units and two supplementary units. All other units can be derived from the base ones. Below, you can find the list of the base SI units as well as the list of the Supplementary SI units.
| SI Base Units | ||
| Unit | Symbol | Quantity |
| Meter (metre) | m | Length |
| Kilogram | kg | Mass |
| Second | s | Time |
| Ampere | A | Electric current |
| Kelvin | K | Thermodynamic temperature |
| Mole | mol | Amount of substance |
| Candela | cd | Luminous intensity |
| SI Supplementary Units | ||
| Radian | rad | Plane angle (2D angle) |
| Steradian | sr | Solid angle (3D angle) |
SI Unit Definitions :
(1) Metre: The metre is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 second.
(2) Kilogram: The kilogram is the unit of mass equal to the mass of the international prototype of kilogram.
(3) Second: The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levers (F=4, mF=0 to F=3, mF=0) of the ground state of the cesium 133 atom.
(4) Ampere: The ampere is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2×10-7 Newton per meter of length.
(5) Kelvin: The Kelvin, unit of thermodynamic temperature is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
(6) Mole: The mole is the mount of substance of a system which contains as many elementary entities as there are atoms in .012 kg of carbon 12 (about 6.022×1023 atoms). 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.
(7) Candela: The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 Hz and that has a radiant intensity in that direction of 1/683 watt per steradian.
SI derived units are units of measurement derived from the seven base units specified by the International System of Units (SI). They are either dimensionless or can be expressed as a product of one or more of the base units, possibly scaled by an appropriate power of exponentiation.
The SI has special names for 22 of these derived units (for example, hertz, the SI unit of measurement of frequency). The names of SI derived units, when written in full, are in lowercase. However, the symbols for units named after persons are written with an uppercase initial letter. For example, the symbol for hertz is “Hz”; but the symbol for metre is “m”.
| Name | Symbol | Quantity | Equivalents | SI base unit Equivalents |
| hertz | Hz | frequency | 1/s | s−1 |
| radian | rad | angle | m/m | 1 |
| steradian | sr | solid angle | m2/m2 | 1 |
| newton | N | force, weight | kg⋅m/s2 | kg⋅m⋅s−2 |
| pascal | Pa | pressure, stress | N/m2 | kg⋅m−1⋅s−2 |
| joule | J | energy, work, heat | N⋅m C⋅V W⋅s | kg⋅m2⋅s−2 |
| watt | W | power, radiant flux | J/s V⋅A | kg⋅m2⋅s−3 |
| coulomb | C | electric charge or quantity of electricity | s⋅A F⋅V | s⋅A |
| volt | V | voltage, electrical potential difference, electromotive force | W/A J/C | kg⋅m2⋅s−3⋅A−1 |
| farad | F | electrical capacitance | C/V s/Ω | kg−1⋅m−2⋅s4⋅A2 |
| ohm | Ω | electrical resistance, impedance, reactance | 1/S V/A | kg⋅m2⋅s−3⋅A−2 |
| siemens | S | electrical conductance | 1/Ω A/V | kg−1⋅m−2⋅s3⋅A2 |
| weber | Wb | magnetic flux | J/A T⋅m2 | kg⋅m2⋅s−2⋅A−1 |
| tesla | T | magnetic field strength, magnetic flux density | V⋅s/m2 Wb/m2 N/(A⋅m) | kg⋅s−2⋅A−1 |
| henry | H | electrical inductance | V⋅s/A Ω⋅s Wb/A | kg⋅m2⋅s−2⋅A−2 |
| degree Celsius | °C | temperature relative to 273.15 K | K | 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 |
There is a set of twenty prefixes to the SI unit names and unit symbols that may be used when specifying multiples and fractions of the units. Prefixes are added to unit names to produce multiples and sub-multiples of the original unit. Below, you can find the list of the SI units prefixes.
| Prefix Name | Prefix Symbol | Factor | Decimal |
| yotta | Y | 1024 | 1000000000000000000000000 |
| zetta | Z | 1021 | 1000000000000000000000 |
| exa | E | 1018 | 1000000000000000000 |
| peta | P | 1015 | 1000000000000000 |
| tera | T | 1012 | 1000000000000 |
| giga | G | 109 | 1000000000 |
| mega | M | 106 | 1000000 |
| kilo | k | 103 | 1000 |
| hecto | h | 102 | 100 |
| deca | da | 101 | 10 |
| 100 | 1 | ||
| deci | d | 10−1 | 0.1 |
| centi | c | 10−2 | 0.01 |
| milli | m | 10−3 | 0.001 |
| micro | μ | 10−6 | 0.000001 |
| nano | n | 10−9 | 0.000000001 |
| pico | p | 10−12 | 0.000000000001 |
| femto | f | 10−15 | 0.000000000000001 |
| atto | a | 10−18 | 0.000000000000000001 |
| zepto | z | 10−21 | 0.000000000000000000001 |
| yocto | y | 10−24 | 0.000000000000000000000001 |
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