SI, Metric and Derived
Unit
Scientists use standards for the basic units of measurement. These standards allow
scientists to make accurate and precise measurements that are
repeatableand unchanging.
Measurements in the scientific world are expressed by the Sytemae International (SI). The metric system is
common system based on multiples of ten.
The SI system is just a modernized version of the metric system adopted in 1964 by the IUPAC and is
based on fundamental units. All the other units are derived from the fundamentals.
People often misuse the terms accuracy and precision and it is very important the you understand what each term
means.
Accuracy is how close the data matches the accepted value.
Precision is how well the data can be repeated.
Believe it or not it is very possible to be precise, yet still be wrong. If your data is repeatable but not in
line with the true value you are precise but not accurate. Many times a scientist will be wrong because the tools
he is using are defective or imperfect. It is this very reason that many scientists work to improve scientific
tools as much as the work on phenomena.
The SI System
The SI system (Systeme International d'Unites) seven fundamental units and 22 derived units. It also
allows for a few other units such as time but the cgs system is nit allowed.
Property 
Unit 
Symbol 
length 
meter 
m 
mass 
kilogram 
kg 
time 
second 
s 
temperature 
Kelvin 
K 
electric current 
Ampere 
A 
luminous intensity 
candela 
cd 
amount of a substance 
mole 
mol 
The Metric System
The metric system is a common measuring system based on powers of ten. Its fundamental units are the
gram, meter and liter. In 1875 the worldwide scientific community accepted the metric system as the standard for
science.
Property

Unit 
Symbol 
length 
meter 
m 
mass 
gram 
g 
volume 
liter 
L 
time 
second 
s 
temperature 
Celsius 
^{o}C 
Prefixes
Both the metric and the SI systems are based on the decimal system (powers of ten) and make use of
prefixes to indicate fractions and multiples of ten. The same prefixes are used in both systems.
Prefixes for Large 
Prefix 
symbol 
meaning 
value 
giga 
G 
billion 
10^{9} 
mega 
M 
million 
10^{6} 
kilo 
k 
thousand 
10^{3} 
Prefixes for Small 
Prefix 
symbol 
meaning 
value 
deci 
d 
tenths 
10^{1} 
centi 
c 
hundredths 
10^{2} 
milli 
m 
thousandths 
10^{3} 
micro 
µ 
millionths 
10^{6} 
nano 
n 
billionths 
10^{9} 
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