|
A physical quantity (or "physical magnitude") is a physical property of a phenomenon, body, or substance, that can be quantified by measurement.〔Joint Committee for Guides in Metrology (JCGM), ''International Vocabulary of Metrology, Basic and General Concepts and Associated Terms'' (''VIM''), III ed., Pavillon de Breteuil : JCGM 200:2012 ((on-line ))〕 A physical quantity can be expressed as the combination of a number – usually a real number – and a unit or combination of units; for example, (the mass of the neutron), or (the speed of light). ==Symbols, nomenclature== ''General'': Symbols for quantities should be chosen according to the international recommendations from ISO/IEC 80000, 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''. ''Subscripts and indices'' Subscripts are used for two reasons, to simply attach a name to the quantity or associate it with another quantity, or represent a specific vector, matrix, or tensor component. :''Name reference'': The quantity has a subscripted or superscripted single letter, a number of letters, or an entire word, to specify what concept or entity they refer to, and tend to be written in upright roman typeface rather than italic while the quantity is in italic. For instance ''E''k or ''E''kinetic is usually used to denote kinetic energy and ''E''p or ''E''potential is usually used to denote potential energy. :''Quantity reference'': The quantity has a subscripted or superscripted single letter, a number of letters, or an entire word, to specify what measurement/s they refer to, and tend to be written in italic rather than upright roman typeface while the quantity is also in italic. For example ''cp'' or ''cisobaric'' is heat capacity at constant pressure. :Note the difference in the style of the subscripts: k and p are abbreviations of the words kinetic and potential, whereas ''p'' (italic) is the symbol for the physical quantity ''pressure'' rather than an abbreviation of the word "pressure". :''Indices'': These are quite apart from the above, their use is for mathematical formalism, see Index notation. ''Scalars'': Symbols for physical quantities are usually chosen to be a single letter of the Latin or Greek alphabet, and are printed in italic type. ''Vectors'': Symbols for physical quantities that are vectors are in bold type, underlined or with an arrow above. If, e.g., ''u'' is the speed of a particle, then the straightforward notation for its velocity is u, u, or . ''Numbers and elementary functions'' Numerical quantities, even those denoted by letters, are usually printed in roman (upright) type, though sometimes can be italic. Symbols for elementary functions (circular trigonometric, hyperbolic, logarithmic etc.), changes in a quantity like Δ in Δ''y'' or operators like d in d''x'', are also recommended to be printed in roman type. :Examples :real numbers are as usual, such as 1 or √2, :e for the base of natural logarithm, :i for the imaginary unit, :π for 3.14159265358979323846264338327950288... : δ''x'', Δ''y'', d''z'', :sin ''α'', sinh ''γ'', log ''x'' 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「physical quantity」の詳細全文を読む スポンサード リンク
|