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
reproducible and
unchanging.
Measurements in the scientific world are expressed by the
metric system or SI system. 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 true value.
Precision is how well the data can
be reproduced, that is 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} 
Conversion
Rule:
