| 释义 |
SI units
Système International d'Unités: the international system of units now recommended for most scientific purposes. A coherent and rationalized system of units derived from the m.k.s. units, SI units have now replaced c.g.s. units and Imperial units for many purposes. The system has seven base units and two dimensionless units (formerly called supplementary units ) from which all other units are derived. There are 18 derived units with special names. Each unit has an agreed symbol (a capital letter or an initial capital letter if it is named after a scientist, otherwise the symbol consists of one or two lower-case letters). Decimal multiples of the units are indicated by a set of prefixes; whenever possible a prefix representing 10 raised to a power that is a multiple of three should be used. TABLE2.1 Base and dimensionless SI units | Physical quantity | Name | Symbol | | length | metre | m | | mass | kilogram | kg | | time | second | s | | electric current | ampere | A | | thermodynamic temperature | kelvin | K | | luminous intensity | candela | cd | | amount of substance | mole | mol | | plane angle | radian | rad | | solid angle | steradian | sr | | dimensionless units | TABLE2.2 Derived SI units with special names | Physical quantity | Name of SI unit | Symbol of SI unit | | frequency | hertz | Hz | | energy | joule | J | | force | newton | N | | power | watt | W | | pressure | pascal | Pa | | electric charge | coulomb | C | | electric potential difference | volt | V | | electric resistance | ohm | ω | | electric conductance | siemens | S | | electric capacitance | farad | F | | magnetic flux | weber | Wb | | inductance | henry | H | | magnetic flux density (magnetic induction) | tesla | T | | luminous flux | lumen | lm | | illuminance | lux | lx | | absorbed dose | gray | Gy | | activity | becquerel | Bq | | dose equivalent | sievert | Sv | TABLE2.3 Decimal multiples and submultiples to be used with SI units | Submultiple | Prefix | Symbol | Multiple | Prefix | Symbol | | 10-1 | deci | d | 10 | deca | da | | 10-2 | centi | c | 102 | hecto | h | | 10-3 | milli | m | 103 | kilo | k | | 10-6 | micro | µ | 106 | mega | M | | 10-9 | nano | n | 109 | giga | G | | 10-12 | pico | p | 1012 | tera | T | | 10-15 | femto | f | 1015 | peta | P | | 10-18 | atto | a | 1018 | exa | E | | 10-21 | zepto | z | 1021 | zetta | Z | | 10-24 | yocto | y | 1024 | yotta | Y | TABLE 2.4 Conversion of units to SI units | From | To | Multiply by | | in | m | 2.54 × 10-2 | | ft | m | 0.3048 | | sq.in | m2 | 6.4516 × 10-4 | | sq. ft | m2 | 9.2903 × 10-2 | | cu. in | m3 | 1.63871 × 10-5 | | cu. ft | m3 | 2.83168 × 10-2 | | l(itre) | m3 | 10-3 | | gal(lon) | l(itre) | 4.546 09 | | miles/hr | m s-1 | 0.477 04 | | km/hr | m s-1 | 0.277 78 | | lb | kg | 0.453 592 | | g cm-3 | kg m-3 | 103 | | lb/in3 | kg m-3 | 2.767 99 × 104 | | dyne | N | 10-5 | | poundal | N | 0.138 255 | | lbf | N | 4.448 22 | | mmHg | Pa | 133.322 | | atmosphere | Pa | 1.013 25 × 105 | | hp | W | 745.7 | | erg | J | 10-7 | | eV | J | 1.602 10 × 10-19 | | kW h | J | 3.6 × 106 | | cal | J | 4.1868 | |