A physical quantity is a physical property that can be quantified by measurement.
Formally, the International Vocabulary of Metrology', 3rd edition (VIM) defines quantity as:
Hence the value of a physical quantity Q is expressed as the product of a numerical value {Q} and a unit of measurement [Q].
Quantity calculus describes how to do math with quantities.
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Examples
If the temperature T of a body is quantified (measured) as 300 degrees Kelvin this is written
where T is the symbol of the physical quantity (NB degrees Celsius cannot be treated in this way)
If a person weighs 120 pounds, then "120" is the numerical value and "pound" is the unit. This physical quantity mass would be written as "120 lbs", or
An example employing SI units and scientific notation for the number, might be a measurement of power written as
Here, P represents the physical quantity of power, 42.3 x 10^{3} is the numerical value {P}, and W is the symbol for the unit of power [P], the watt
Symbols for physical quantities
Usually, the symbols for physical quantities are chosen to be a single letter of the Latin or Greek alphabet, and are often printed in italic type. Often, the symbols are modified by subscripts and superscripts, to specify what they refer to — for instance E_{k} is usually used to denote kinetic energy and c_{p} heat capacity at constant pressure. (Note the difference in the style of the subscripts: k is the abbreviation of the word kinetic, whereas p is the symbol for the physical quantity pressure rather than an abbreviation of the word "pressure".)
Symbols for quantities should be chosen according to the international recommendations from ISO 31, the IUPAP red book and the IUPAC green book. For example, the recommended symbol for the physical quantity 'mass' is m, and the recommended symbol for the quantity 'charge' is Q.
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