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SI Base Units and SI Prefixes

In 1960 the General Conference on Weights and Measures adopted the International System of units (or SI, after the French le Système International d' Unitès), which is a particular choice of metric units. This system has nine SI base units, the SI units from which all others can be derived.

 

One of the advantages of any metric system is that it is a decimal system. In SI, a larger or smaller unit for a physical quantity is indicated by an SI prefix, which is a prefix used in the International System to indicate a power of ten. For example, the base unit of length in SI is the meter, and 10-2 meter is called a centimeter. Thus 2.80 centimeters equal 2.80 x 10-2 meter. The base units used in chemistry are: length, mass, time, and temperature.

Length

The meter (m) is the SI base unit of length. By combining it with one of the SI prefixes, you can get a unit of appropriate size for any length measurement.

Mass

The kilogram (kg) is the SI base unit of mass. This is a somewhat unusual base unit in that it contains a prefix. To form other SI mass units, prefixes are added to the word gram (g) to give an appropriate unit.

Time

The second (s) is the SI base unit of time. Using prefixes such as milli-, micro-, nano-, we can create units appropriate for measuring rapid events. If we are measuring times for slower events, we use minutes and hours which are not SI units.

Temperature

Three temperature scales are currently in use. Their units are oF (degrees Fahrenheit), oC (degrees Celsius), and K (kelvin). The Fahrenheit scale is commonly used in the united States outside the laboratory. The Celsius scale is the temperature scale in general scientific use. On this scale, the freezing point of water is 0oC and the boiling point of water at normal atmospheric pressure is 100oC. The SI base unit of temperature is the kelvin (K). The kelvin scale is an absolute temperature scale, that is a scale on which the lowest temperature possible is zero. The Celsius and the Kelvin scales have equal-sized units, but 0oC is equal to 273.15 K. This makes it easy to convert from one scale to the other.

 

Converting Units

Suppose we wish to convert 23 mm to an equivalent length expressed as cm. First we construct a conversion factor.

1000 mm = 1 m = 100 cm

1000 mm = 100 cm

10 mm = 1 cm

We can make two conversion factors from this. First divide each side by 10 mm.

10 mm / 10 mm = 1 = 1 cm / 10 mm   ( this factor will convert from mm to cm  --- it converts from unit on bottom to unit on top)

We could have divided each side by 1 cm and obtained this conversion factor.

10 mm / 1 cm = 1 cm / 1cm = 1  ( this factor will convert from cm to mm)

Next we take the quantity to be converted and multiply it times the conversion factor which will cause all units except the desired one to cancel.

    1 cm    
23 mm x ----- = 2.3 cm
    10 mm    
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