The Systeme International [S I]
Le Systeme
international d'Unites officially came
into being in October 1960 and has been adopted by nearly all
countries, though
the amount of actual usage varies considerably. It is based upon 7
principal
units, 1 in each of 7 different categories. Each of these units may
take a prefix.
From these basic units many other units
are derived and named. The basic units are:-
Category
|
Name
|
Abbreviation
|
Length
|
metre
|
m
|
Mass
|
kilogram
|
kg
|
Time
|
second
|
s
|
Electric current
|
ampere
|
A
|
Temperature
|
kelvin
|
K
|
Amount of substance
|
mole
|
mol
|
Luminous intensity
|
candela
|
cd
|
Definitions
of the
Seven Basic S I Units
metre [m]
The metre is the basic unit of length. It is the distance light
travels, in a
vacuum, in 1/299792458th of a second.
kilogram [kg]
The kilogram is the basic unit of mass. It is the mass of an
international
prototype in the form of a platinum-iridium cylinder kept at Sevres in
France. It
is now the only basic unit still defined in terms of a material object,
and also
the only one with a prefix[kilo] already in place.
second [s]
The second is the basic unit of time. It is the length of time taken
for
9192631770 periods of vibration of the caesium-133 atom to occur.
ampere [A]
The ampere is the basic unit of electric current. It is that current
which
produces a specified force between two parallel wires which are 1 metre
apart in
a vacuum.It is named after the French physicist Andre Ampere
(1775-1836).
kelvin [K]
The kelvin is the basic unit of temperature. It is 1/273.16th
of the
thermodynamic temperature of the triple point of water. It is
named after the
Scottish mathematician and physicist William Thomson 1st Lord Kelvin
(1824-1907).
mole [mol]
The mole is the basic unit of substance. It is the amount of substance
that
contains as many elementary units as there are atoms in 0.012 kg of
carbon-12.
candela [cd]
The candela is the basic unit of luminous intensity. It is the
intensity of a
source of light of a specified frequency, which gives a specified
amount of
power in a given direction.
Derived Units of the S I
From the 7 basic units of the SI many other units are derived for a
variety of
purposes. Only some of them are explained here. The units printed in bold
are either basic units or else, in some cases, are themselves derived.
farad [F]
The farad is the SI unit of the capacitance of an electrical system,
that is,
its capacity to store electricity. It is a rather large unit as defined
and is
more often used as a microfarad. It is named after the
English chemist and
physicist Michael Faraday (1791-1867).
hertz [Hz]
The hertz is the SI unit of the frequency of a periodic phenomenon. One
hertz
indicates that 1 cycle of the phenomenon occurs every second.
For most
work much higher frequencies are needed such as the kilohertz [kHz] and
megahertz [MHz]. It is named after the German physicist
Heinrich Rudolph
Hertz (1857-94).
joule [J]
The joule is the SI unit of work or energy. One joule is the amount of
work done
when an applied force of 1 newton moves through a
distance of 1 metre
in the direction of the force.It is named after the English
physicist James
Prescott Joule (1818-89).
newton [N]
The newton is the SI unit of force. One newton is the force required to
give a
mass of 1 kilogram an acceleration of 1 metre
per second
per second. It is named after the English
mathematician and physicist
Sir Isaac Newton (1642-1727).
ohm
The ohm is the SI unit of resistance of an electrical conductor. Its
symbol, is
the capital Greek letter 'omega'. It is named after the
German physicist
Georg Simon Ohm (1789-1854).
pascal [Pa]
The pascal is the SI unit of pressure. One pascal is the pressure
generated by a
force of 1 newton acting on an area of 1 square metre.
It is a
rather small unit as defined and is more often used as a kilopascal
[kPa]. It
is named after the French mathematician, physicist and philosopher
Blaise Pascal
(1623-62).
volt [V]
The volt is the SI unit of electric potential. One volt is the
difference of
potential between two points of an electical conductor when a current
of 1 ampere
flowing between those points dissipates a power of 1 watt.
It is named
after the Italian physicist Count Alessandro Giuseppe Anastasio Volta
(1745-1827).
watt [W]
The watt is used to measure power or the rate of doing work. One watt
is a power
of 1 joule per second. It
is named after the Scottish engineer
James Watt (1736-1819).
The Prefixes of the S I
The S I allows the sizes of units to be made bigger or smaller by the
use of
appropriate prefixes. For example, the electrical unit of a watt is not
a big
unit even in terms of ordinary household use, so it is generally used
in terms
of 1000 watts at a time. The prefix for 1000 is kilo
so we use
kilowatts[kW] as our unit of measurement. For makers of electricity, or
bigger
users such as industry, it is common to use megawatts[MW] or even
gigawatts[GW].
The full range of prefixes with their [symbols or abbreviations] and
their
multiplying factors is
| yotta
|
Y
|
1,000,000,000,000,000,000,000,000
|
= 10^24
|
|
zetta
|
Z
|
1,000,000,000,000,000,000,000
|
= 10^21
|
|
exa
|
E
|
1,000,000,000,000,000,000
|
= 10^18
|
|
peta
|
P
|
1,000,000,000,000,000
|
= 10^15
|
|
tera
|
T
|
1,000,000,000,000,
|
= 10^12
|
|
giga
|
G
|
1,000,000,000
|
|
|
mega
|
M
|
1,000,000,
|
|
|
kilo
|
k
|
1,000,
|
|
|
hecto
|
h
|
100
|
|
|
deca
|
da
|
10
|
|
|
|
1 |
|
|
deci
|
d
|
0.1
|
|
|
centi
|
c
|
0.01
|
|
|
milli
|
m
|
0.001,
|
|
|
micro
|
�
|
0.000,001,
|
|
|
nano
|
n
|
0.000,000,001,
|
|
|
pico
|
p
|
0.000,000,000,001
|
= 10^-12
|
|
femto
|
f
|
0.000,000,000,000,001
|
= 10^-15
|
|
atto
|
a
|
0.000,000,000,000,000,001
|
= 10^-18
|
|
zepto
|
z
|
0.000,000,000,000,000,000,001
|
= 10^-21
|
|
yocto
|
y
|
0.000,000,000,000,000,000,000,001
|
= 10^-24
|
Note: � the symbol used for micro is
the Greek letter known as 'mu'
Nearly all of
the S I prefixes are multiples or sub-multiples of 1000.
However,
these are inconvenient for many purposes and so hecto,
deca, deci,
and centi are also used.
deca also appears as deka da
or dk in the USA and
Contintental Europe. So much for standards!
|