# 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 oC

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 109 mega M million 106 kilo k thousand 103

 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:

• large unit to small unit = multiply by the conversion

move the decimal to the right

• small unit to large unit = divide by the conversion

move the decimal to the left

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