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SI units

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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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