SI units
SI units Systéme International d'Unités: the international system of units now recommended for all scientific purposes. A coherent and rationalized system of units derived from the
m.k.s. units (a metric system based on the metre, kilogram, and second), SI units have now replaced
c.g.s. units and
Imperial units. The system has seven
base units and two
dimensionless units (formerly called
supplementary units), all other units being derived from these nine units. 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; when possible a prefix representing 10 raised to a power that is a multiple of three should be used.
TABLE 1.1 Base and dimensionless SI units
Physical quantity | Name | Symbol |
|---|
*dimensionless units |
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 |
TABLE 1.2 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 |
TABLE 1.3 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 1.4 Conversion of units of 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 |
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