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UNITS OF THE INTERNATIONAL SYSTEM         

The International System (abbreviated SI, for Systeme International, the French name for the system) was adopted in 1960 by the 11th General Conference on Weights and Measures.  An expanded and modified version of the metric system, the International System, addresses the need of modern science for additional and more accurate units of measurement.  The key features of the International System are decimalization, system of prefixes and a standard defined in terms of an invariable physical measure.  

 
BASE UNITS

The International System has base units from which all others in the system are derived.  The standards for the base units, except for the kilogram, are defined by unchanging and reproducible physical occurrences.  For example, the meter is defined as the distance traveled by light in a vacuum in 1/299,792,458 of a second.  The standard for the kilogram is a platinum-iridium cylinder kept at the International Bureau of Weights and Standards in Svres, France.  

UNIT

QUANTITY

SYMBOL

meter

length

m

kilogram

mass

kg

second

time

s

ampere

electric current

A

kelvin

temperature

K

mole

amount of matter

mol

candela

luminous intensity

cd


SUPPLEMENTARY UNITS

The International System uses two supplementary units that are based on abstract geometrical concepts rather than physical standards.
 

UNIT

QUANTITY

SYMBOL

radian

plane angles

rad

steradian

solid angles

sr

 

 

DERIVED UNITS

Most of the units in the International System are a derived unit, that is units defined in terms of base units and supplementary units.  Derived units can be divided into two groups those that have a special name and symbol, and those that do not.
 

WITH NAMES AND SYMBOLS

MEASURE OF

DERIVATION

acceleration

m/s2

angular acceleration

rad/s2

angular velocity

rad/s

density

kg/m3

electric field strength

V/m

luminance

cd/m2

magnetic field strength

A/m

velocity

m/s

 
PREFIXES

A multiple of unit in the International System is formed by adding a prefix to the name of that unit.  The prefixes change the magnitude of the unit by orders of 10 from_1018_to_10-18. For example, a kilometer is 1000 meters, a hectoliter is 100 liters, a milligram is 1/1000 of a gram, a megawatt is 1,000,000 watt, etc.
 

PREFIX

SYMBOL

MULTIPLYING FACTOR

exa-

E

1018 = 1,000,000,000,000,000,000

peta-

P

1015 = 1,000,000,000,000,000

tera-

T

1012 = 1,000,000,000,000

giga-

G

109  = 1,000,000,000

mega-

M

106  = 1,000,000

kilo-

K

103  = 1,000

hecto-

h

102  = 100

deca-

da

10   = 10

deci

d

10-1   = 0.1

centi-

c

10-2   = 0.01

milli-

m

10-3   = 0.001

micro-

μ

10-6   = 0.000,001

nano-

n

10-9   = 0.000,000,001

pico-

p

10-12  = 0.000,000,000,001

femto-

f

10-15  = 0.000,000,000,000,001

atto-

a

10-18  = 0.000,000,000,000,000,001

 

 

 

 

 

 

 

 

 

 

ADDITIONAL UNITS

Listed below are a few of the non-SI units that are commonly used with the International System:
 

UNIT

QUANITY

SYMBOL

angstrom (=10.10m)

length

electron-volt (=0.160 aJ)

energy

eV

hectare (=10,000 m2)

land area

ha

liter (= 1.0 dm3)

volume or capacity

l

standard atmosphere (=101.3 kPa)

pressure

atm

 

WITH NAMES AND SYMBOLS
 

UNIT

MEASURE OF

SYMBOL

DERIVATION

coulomb

electric charge

C

A ·s

farad

electric capacitance

F

A · s/V

henry

inductance

H

V ·s/A

hertz

frequency

Hz

cycles/s

joule

quantity of energy

J

N · m

lumen

flux of light

lm

cd · sr

lux

illumination

lx

lm/m2

newton

fource

N

kg · m/s2

ohm

electric resistance

Ω

V/A

pascal

pressure

Pa

N/m2

tesla

magnetic flux density

T

Wb/ m2

volt

voltage

V

W/A

watt

power

W

J/s

weber

magnetic flux

Wb

V ·s

   

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