SI, Metric and Derived Unit
It is important that scientist use standards in both their work and communication. Standards are necessary in
order to ensure everyone is on the same page and using the same units, definitions and words.
The SI and metric system are the standards used by scientists.
These standards allow scientists to make accurate and precise
measurements that are reproducible and unchanging.
The metric system is a 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) has 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.
| electric current
| luminous intensity
| amount of a substance
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
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
| Prefixes for Small