Symbols, SI units, and definitions for

Section 2 combines the former separate section on Mathematics with the material involving General Information and Conversion Tables. The fundamental physical constants reflect values recommended in 1986. Physical and chemical symbols and definitions have undergone extensive revision and expansion. Presented in 14 categories, the entries follow recommendations published in 1988 by the lUPAC. The table of abbreviations and standard letter symbols provides, in a sense, an alphabetical index to the foregoing tables. The table of conversion factors has been modified in view of recent data and inclusion of SI units cross-entries for archaic or unusual entries have been curtailed. 

A substantial number of definitions in the terminology section are either of physical quantities or are expressed mathematically. In such cases, there are recommended symbols for the quantities and, when appropriate, corresponding SI units. Other terms have eommon abbreviations. The following format is used to indicate these essential eharaeteristics name of term (abbreviation), symbol, SI unit unit. Typical examples are tensile stress, interpenetrating polymer network (IPN). If there are any, alternative names or synonyms follow on the next line, and the definition on the sueeeeding lines. 

For historic reasons a number of different units of measurement have evolved to express quantity of the same thing. In the 1960s, many international scientific bodies recommended the standardisation of names and symbols and the adoption universally of a coherent set of units—the SI units (Systeme Internationale d Unites)— based on the definition of five basic units metre (m) kilogram (kg) second (s) ampere (A) mole (mol) and candela (cd). 

SI units should be used throughout the report. The symbols for SI units are not abbreviations and do not require definition in the report. These symbols are defined in the official SI brochure (3). 

The tables are arranged by subject. The five columns in each table give the name of the quantity, the recommended symbol(s), a brief definition, the symbol for the coherent SI unit (without multiple or submultiple prefixes, see p.74), and footnote references. When two or more symbols are recommended, commas are used to separate symbols that are equally acceptable, and symbols of second choice are put in parentheses. A semicolon is used to separate symbols of slightly different quantities. The definitions are given primarily for identification purposes and are not necessarily complete they should be regarded as useful relations rather than formal definitions. For dimensionless quantities a 1 is entered in the SI unit column. Further information is added in footnotes, and in text inserts between the tables, as appropriate. 

The international unit itself may eventually be replaced by the SI unit termed the katal, the SI derived unit for catalytic activity (see Chapter 1). It is defined as moles per second. The name katal had been used for this unit for decades, but did not become an official SI derived unit until 1999 with Resolution 12 of the 21st CGPM, on the recommendation of the International Federation of Cfinical chemistry and Laboratory Medicine. Both the International Union of Pure and Applied Chemistry and the lUB now recommend that enzyme activity be expressed in moles per second and that the enzyme concentration be expressed in terms of katals per liter (kat/L). Thus, lU = lO mol/fiOs = 16.7 X 10 mol/s, or l.Onkat/L - 0.06U/L. The formal adoption of the katal is hoped to discourage the use of a non-SI unit called unit, symbol U, defined as micromoles per minute. Units are more commonly used than the katal in practice at present, but their definition lacks coherence with the SI system. 

No attempt has been made to define every symbol and, in particular, symbols used only locally and that are not referred to again are generally not included. An abbreviated verbal definition and, where appropriate, an equation number or symbolic definition are given. Illustrative SI base or derived units are given for dimensional quantities. 

The following tables give the recommended symbols for the major classes of physical and chemical quantities. The expression in the Definition column is given as an aid in identifying the quantity but is not necessarily the complete or unique definition. The SI Unit gives one (not necessarily unique) expression for the coherent SI unit for the quantity. Other equivalent unit expressions, including those that involve SI prefixes, may be used. 

This table lists some abbreviations, acronyms, and symbols encountered in the physical sciences. Most entries in italic type are symbols for physical quantities for more details on these, see the table Symbols and Terminology for Physical and Chemical Quantities in this section. Additional information on units may be found in the table International System of Units (SI) in Section 1. Many of the terms to which these abbreviations refer are included in the tables Definitions of Scientific Terms in Section 2 and Techniques for Materials Characterization in Section 12. Useful references for further information are given below. 

SYMBOLS AND TERMINOLOGY FOR PHYSICAL AND CHEMICAL QUANTITIES (continued) Name Symbol Definition SI unit... 

The surface energy is a variety in the same family as the varieties of translation mechanics and hydrodynamics. Under its capacitive subvariety, it possesses as basic quantity the area A (in m ) and as effort, the surface tension y (in kg s or J m ). The flow is called rate of surface expansion, often notated with the A surmounted by a point for indicating a temporal derivative. As it cannot be a definition for a flow, the adopted symbol in the Formal Graph theory is the generic symbol with SI units in m s. All these state variables are vectors because they are oriented with respect to the surface. 

The SI unit of enzyme activity, 1 mol s", may also be called the katal, with symbol kat, but this term has not been widely adopted. It is a very large unit for most purposes, but its submultiples the nanokatal (nkat) and microkatal (jxkat) are of a convenient size for measuring enzyme activities in the laboratory. In the past it was usual to define a unit of enzyme activity measured in ixmol min" (= 16.67 nkat), and this is still used, though it is not very satisfactory because the name is uninformative and fails to convey the incorporation of a factor of 10" and a non-SI unit (the minute) in the definition. 

Quantity Symbol and definition Dimensions SI unit Symbol for unit... 

Units of pressure come from the definition. In the United States Customary System, if force is measured in pounds and area in square inches, the pressure unit is pounds per square inch (psi). The SI unit of pressure is the pascal (Pa), which is one newton per square meter. (The newton is the SI unit of force.) Although chemists generally follow SI guidelines, one pascal is a very small pressure the kilopascal (kPa) is a more practical unit. Many other pressure units are commonly used. One is the bar, which is 1 X 10 Pa or 100 kPa. The millimeter of mercury, or its equivalent, the torr, and the atmosphere are other common units of pressure. The millimeter of mercury is usually abbreviated mm Hg. (Hg is the elemental symbol for mercury.)... 

Quantity II Symbol and definition 1 Dimeiisiunit Symbol for imit... 

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