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# Chapter 03 Physical Quantities, Units, Symbols and
Prefixes

Many experiments carried out in physics and chemistry are quantitative. They involve assigning numbers to length, mass, time, current, density etc. The thing which can be measured is called the physical quantity. Each quantity has its own symbol, a letter. Most quantities have units and each unit has a name and its own symbol. The symbol for a unit is either a letter or a series of letters. 

Some quantities are unitless, i.e. they are represented only by a number. The number, in turn, represents how much the quantity is greater or smaller than a reference or standard quantity. Examples of unitless quantities: magnification, relative density, efficiency.

Units, especially SI units, are often used with prefixes indicating multiples or submultiples. A prefix has its own symbol too. The list of prefixes used with SI units is presented in Table 1.

 
 

Do you like anecdotes, interesting and challenging problems, fun facts, puzzles, jokes related to metric system and measurement? Read them in the 2006 on-line edition of "SI Units, Conversion and Measurement Skills",186 pp.