SI UNITS ( le Système Internationale d’unités.) ? THINGS YOU SHOULD KNOW ABOUT THE MEASURMENT SYSTEMS.
The abbreviation comes from the French
title, le Système Internationale d’unités. This system is used worldwide now;
in fact, only three countries have not adopted SI for commerce as well as for
science: Burma (also known as Myanmar), Liberia, and the United States.
Base SI Units SI is an integrated metric
system designed so that it can readily be used throughout the sciences as well
as engineering and commerce. SI has seven base units, chosen such that all
quantities can be expressed in some combination of the base units.
SI also has
a set of derived units, defined in terms of the base units, to provide
convenient measures of particular quantities. below Table lists SI base units and
the quantities they
Unit name
|
Unit symbol
|
Unit measures
|
Metre
|
m
|
Length
|
Seconds
|
s
|
Time
|
Kilogramme
|
kg
|
Mass
|
Kelvin
|
K
|
Temperature
|
Mole
|
mol
|
Amount of substance
|
Ampere
|
A
|
Electric current
|
Candela
|
cd
|
Luminous intensity
|
The metre, kilogram, and second are units that
you likely know well from everyday use. For historical reasons, the kilogram—not
the gram—is the SI base unit of mass. The kilogram is the only base unit that
has a prefix.
A
mole is the amount of a substance that contains the same number
of elementary entities (e.g., atoms or
molecules) as exactly 12 g of the
isotope carbon-12, about 6.022 3 1023. In other words, 1 mol contains
Avogadro’s number of atoms or molecules.
A
kelvin is equal to a Celsius
degree, but the zero point of the Kelvin
temperature scale, denoted 0 K, is absolute zero, the lowest temperature
possible (approximately 2273 °C).
The ampere is a measure of electric
current, the f low of electric charge past a given point. When you study
electromagnetism later in this book, you will see why the designated base unit
for electrical quantities is based on current rather than charge.
The candela is the unit of luminous
intensity, a measure of the light output of a source. As you might guess from
the name, the candela is based on the light output of a standard candle with a
specified composition and rate of combustion. In fact, a typical candle has
brightness in the order of 1 cd. The candela is a precisely defined unit that
replaces older units.
OTHER UNITS
As well as the base units, there are a
number of fundamental derived SI units. Some of these are simply combinations
of the units listed.
For example, speed and velocity have units of m/s, and acceleration (the rate of change of velocity) has units of m/s2.
There are also commonly used non-SI units, including the minute (min); the solar day (d); the year (a or y or yr);
The astronomical unit (AU), which is the mean distance between the centres of Earth and the Sun; and the light year (ly), which is the distance that light travels in one year.
For example, speed and velocity have units of m/s, and acceleration (the rate of change of velocity) has units of m/s2.
There are also commonly used non-SI units, including the minute (min); the solar day (d); the year (a or y or yr);
The astronomical unit (AU), which is the mean distance between the centres of Earth and the Sun; and the light year (ly), which is the distance that light travels in one year.
Unit
Name
|
Unit
Symbol
|
Quantity
|
Base Unit
Equivalent
|
newton
|
N
|
force
|
m - kg - s-2
|
joule
|
J
|
energy, work
|
m2- kg - s-2
|
watt
|
W
|
power
|
m2- kg - s-3
|
hertz
|
Hz
|
frequency
|
s-1
|
radian
|
rad
|
angle
|
m- m-1
(dimensionless)
|
steradian
|
sr
|
solid angle
|
m2- m-2
|
pascal
|
Pa
|
pressure
|
m-1- kg - s-2
|
coulomb
|
C
|
electric charge
|
s - A
|
volt
|
V
|
electric potential
difference
|
m2- kg - s-3-
A-1
|
ohm
|
Ω
|
electric resistance
|
m2- kg - s-3- A-2
|
farad
|
F
|
electric capacitance
|
m2- kg-1 – s4-
A2
|
henry
|
H
|
electromagnetic
inductance
|
m2- kg - s-2- A-2
|
tesla
|
T
|
magnetic field
strength
|
kg - s-2- A-1
|
weber
|
Wb
|
magnetic flux
|
m2- kg - s-2- A-1
|
becquerel
|
Bq
|
radioactivity
|
s-1
|
lumen
|
lm
|
luminous flux
|
cd
|
lux
|
lx
|
luminous flux per
unit area
|
m-2 - cd
|
SI Prefixes
SI has a set of prefixes that indicate
different powers of 10;
For example,
A kilometre is 1000 m, and a millimetre is
0.001 m. below table lists the names and abbreviations
For the most commonly used SI prefixes.
Note that the prefix symbols greater than
“kilo” are capitalized
Name
|
Symbol
|
Multiplication
|
zetta
|
Z
|
1021
|
exa
|
E
|
1018
|
peta
|
P
|
1015
|
tera
|
T
|
1012
|
giga
|
G
|
109
|
mega
|
M
|
106
|
kilo
|
k
|
103
|
hecto
|
h
|
102
|
deca
|
da
|
101
|
deci
|
d
|
10-1
|
centi
|
c
|
10-2
|
milli
|
m
|
10-3
|
micro
|
μ
|
10-6
|
nano
|
n
|
10-9
|
pico
|
p
|
10-12
|
femto
|
f
|
10-15
|
atto
|
a
|
10-18
|
zepto
|
z
|
10-21
|
Writing SI
SI specifies rules for writing
measurements. These rules combine aspects of traditional styles used in various
countries into a single, unambiguous convention that is recognized worldwide.
1. We always use a space between the number and the unit symbol (e.g., 10 m, not 10m).
2. Abbreviations for units are capitalized
when they are named after a person but not otherwise (e.g., N for newton but kg
for kilogram).
3. Units are not capitalized, even when named
after a person, when written out in full (e.g., newton not Newton). This rule
prevents confusion over whether one is referring to the person or the unit.
4. SI unit symbols do not end with a period
(e.g., W and cd, not W. and cd.).
5. Unit symbols stand for both single units
and multiples of the unit, so we never add an “s” to make a symbol plural
(e.g., 10 m means ten metres, but 10 ms means ten milliseconds).
6. The unit symbols are written in normal
(roman or upright) type to distinguish them from variables, which are written
in italic (e.g., “m” always represents the unit metre, while m usually
represents the variable mass).
7. When two (or more) units are multiplied
together, we use a small dot to separate them (e.g., N.m.).
8. Normally we write simple units in the denominator
using the slash (e.g., kg.m/s2). When the situation is complex, it
is sometimes more clear to use negative exponents (e.g., C2 N -1
m-2). Either format may be used.
9. When writing numbers with many digits, sets
of 3 digits on either side of the decimal point are separated by spaces instead
of commas. In a number with 4 digits, the space is optional (e.g., 52 469 450, 7.630
941 23, and 4123 or 4 123).
10. Normally we use the symbol for the unit
when it is along with a numerical value (e.g., 12.5 N), but we write the unit
in full otherwise (e.g., measure the force in newtons). The full form is also
used when we write the number out (e.g., a force of five newtons).
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