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Encyclopedia > Physical quantity

A physical quantity is either a quantity within physics that can be measured (e.g. mass, volume, etc.), or the result of a measurement. A physical quantity Q is usually expressed as the product of a numerical value {Q} and a physical unit [Q]. Quantity is a kind of property which exists as magnitude or multitude. ... Physics (from the Greek, (phúsis), nature and (phusiké), knowledge of nature) is the science concerned with the discovery and understanding of the fundamental laws which govern matter, energy, space and time. ... Unsolved problems in physics: What causes anything to have mass? Mass is a property of a physical object that quantifies the amount of matter and energy it is equivalent to. ... Volume is a quantification of how much space a certain region occupies. ... Various meters Measurement is the process of estimating the ratio of a magnitude of a quantity to a unit of the same type. ... A number is an abstract entity that represents a count or measurement. ... The definition, agreement and practical use of units of measurement have played a crucial role in human endeavour from early ages up to this day. ...

Q = {Q} x [Q]

(SI units are usually preferred today). The notion of physical dimension of a physical quantity was introduced by Fourier (1822). Cover of brochure The International System of Units. ... The definition, agreement and practical use of units of measurement have played a crucial role in human endeavour from early ages up to this day. ...

Contents

Example

If a certain value of power is written as In physics, power (symbol: P) is the rate at which work is performed. ...

P = 42.3 x 103 W = 42.3 kW,

then

P represents the physical quantity of power
42.3 x 103 is the numerical value {P}
k is the SI prefix kilo, representing 103
W is the symbol for the unit of power [P], the watt

An SI prefix is a prefix that can be applied to an SI unit to form a decimal multiple (supramultiple or submultiple). ... Kilo (symbol: k) is a prefix in the SI system denoting 103 or 1000. ... SI derived units are part of the SI system of measurement units and are derived from the seven SI base units. ... The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. ...

Symbols for physical quantities

Usually, the symbols for physical quantities are chosen to be a single lower case or capital letter of the Latin or Greek alphabet written in italic type. Often, the symbols are modified by subscripts and superscripts, in order to specify what they pertain to - for instance Ep is usually used to denote potential energy and cp heat capacity at constant pressure. The Latin alphabet, also called the Roman alphabet, is the most widely used alphabetic writing system in the world today. ... Because of technical limitations, some web browsers may not display some special characters in this article. ... A subscript is a number, figure, or indicator that appears below the normal line of type, typically used in a formula, mathematical expression, or description of a chemical compound. ... A superscript is a number, figure, or symbol that appears above the normal line of type, at the right or left of another symbol or text. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... The use of water pressure - the Captain Cook Memorial Jet in Lake Burley Griffin, Canberra. ...


Symbols for quantities should be chosen according to the international recommendations from ISO 31, the IUPAC red book and the IUPAC green book. For example, the recommended symbol for a physical quantity of mass is m, and the recommended symbol for a quantity of charge is Q. International Standard ISO 31 (Quantities and units, International Organization for Standardization, 1992) is the most widely respected style guide for the use of units of measurement, and formulas involving them, in scientific and educational documents worldwide. ... Title: Quantities, Units and Symbols in Physical Chemistry Content: the IUPAC green book establishes standards for nomenclature in chemistry. ...


Units of physical quantities

Most physical quantites Q include a unit [Q] (where [Q] means "unit of Q"). Neither the name of a physical quantity, nor the symbol used to denote it, implies a particular choice of unit. For example, a quantity of mass might be represented by the symbol m, and could be expressed in the units kilogram (kg), pounds (lbs), or dalton (Da). The definition, agreement and practical use of units of measurement have played a crucial role in human endeavour from early ages up to this day. ...


Base quantities, derived quantities and dimensions

By convention, physical quantities are organized in a dimensional system built upon base quantities, each of which is regarded as having its own dimension. In the SI system of units, there are seven base units, but other conventions may have a different number of fundamental units. The base quantities according to the International System of Quantities (ISQ) and their dimensions are listed in the following table: Cover of brochure The International System of Units. ... In the language of measurement, quantities are quantifiable aspects of the world, such as time, distance, velocity, mass, and weight, and units measure them. ...

ISQ base quantities
Name Symbol for quantity Symbol for dimension SI base unit
Length l L meter
Time t T second
Mass m M kilogram
Electrical current I I ampere
Thermodynamic temperature T θ kelvin
Amount of substance n N mole
Luminous intensity Iv J candela

All other quantities are derived quantities since their dimensions are derived from those of base quantities by multiplication and division. For example, a physical quantity velocity is derived from base quantities length and time and has dimension L/T. See Dimensional Analysis for further information. Length is the long dimension of any object. ... The metre, or meter (symbol: m) is the SI base unit of length. ... Two distinct views exist on the meaning of time. ... Look up second in Wiktionary, the free dictionary. ... Unsolved problems in physics: What causes anything to have mass? Mass is a property of a physical object that quantifies the amount of matter and energy it is equivalent to. ... The international prototype, made of platinum-iridium, which is kept at the BIPM under conditions specified by the 1st CGPM in 1889. ... In electricity, current refers to electric current, which is the flow of electric charge. ... A multimeter can be used to measure current The ampere (symbol: A) is the SI base unit of electric current. ... Fig. ... The Kelvin scale is a thermodynamic (absolute) temperature scale where absolute zero—the lowest possible temperature where nothing could be colder and no heat energy remains in a substance—is defined as zero kelvin (0 K). ... Look up substance in Wiktionary, the free dictionary. ... The mole and its simple conversions into different units of measurements. ... Luminous intensity is a measure of the energy emitted by a light source in a particular direction. ... The candela (symbol: cd) is the SI base unit of luminous intensity (that is, power emitted by a light source in a particular direction, with wavelengths weighted by the luminosity function, a standardized model of the sensitivity of the human eye). ... Dimensional analysis is a conceptual tool often applied in physics, chemistry, and engineering to understand physical situations involving a mix of different kinds of physical quantities. ...


Some derived physical quantities have no dimension and are said to be dimensionless quantities. In the physical sciences, a dimensionless number (or more precisely, a number with the dimensions of 1) is a quantity which describes a certain physical system and which is a pure number without any physical units; it does not change if one alters ones system of units of measurement...


Extensive and intensive quantities

A quantity is called:

  • extensive when its magnitude is additive for subsystems (volume, mass, etc.)
  • intensive when the magnitude is independent of the extent of the system (temperature, pressure, etc.)

Some extensive physical quantities may be prefixed in order to further qualify their meaning: In physics and chemistry, an extensive quantity (also referred to as an extensive variable) is a physical quantity whose value is proportional to the size of the system it describes. ... It has been suggested that this article or section be merged into intensive and extensive properties. ...

  • specific is added to refer to the quantity divided by its mass (such as specific volume)
  • molar is added to refer to the quantity divided by the amount of substance (such as molar volume)

There are also physical quantities that can be classified as neither extensive nor intensive, for example angular momentum, area, force, length, and time. Specific volume is the volume of a unit of mass of a material. ... In chemistry, the molar volume of a substance is the ratio of the volume of a sample of that substance to the amount of substance (usually in mole) in the sample. ... Gyroscope. ... Area is a physical quantity expressing the size of a part of a surface. ... In physics, force is an influence that may cause a body to accelerate. ... Length is the long dimension of any object. ... Two distinct views exist on the meaning of time. ...


Physical quantities as coordinates over spaces of physical qualities

The meaning of the term physical quantity is generally well understood (everyone understands what it is meant by the frequency of a periodic phenomenon, or the resistance of an electric wire). It is clear that behind a set of quantities like temperature − inverse temperature − logarithmic temperature, there is a qualitative notion: the cold−hot quality. Over this one-dimensional quality space, we may choose different coordinates: the temperature, the inverse temperature, etc. Other quality spaces are multidimensional. For instance, to represent the properties of an ideal elastic medium we need 21 coefficients, that can be the 21 components of the elastic stiffness tensor cijkl , or the 21 components of the elastic compliance tensor (inverse of the stiffness tensor), or the proper elements (six eigenvalues and 15 angles) of any of the two tensors, etc. Again, we are selecting coordinates over a 21-dimensional quality space. On this space, each point represents a particular elastic medium.


It is always possible to define the distance between two points of any quality space, and this distance is —inside a given theoretical context— uniquely defined. For instance, two periodic phenomena can be characterized by their periods, T1 and T2, or by their frequencies, ν1 and ν2 . The only definition of distance that respects some clearly defined invariances is D = | log(T2 / T1) | = | log2 / ν1) | .


These notions have implications in physics. As soon as we accept that behind the usual physical quantities there are quality spaces, that usual quantities are only special coordinates over these quality spaces, and that there is a metric in each space, the following question arises: Can we do physics intrinsically, i.e., can we develop physics using directly the notion of physical quality, and of metric, and without using particular coordinates (i.e., without any particular choice of physical quantities)? In fact, physics can (and must?) be developed independently of any particular choice of coordinates over the quality spaces, i.e., independently of any particular choice of physical quantities to represent the measurable physical qualities. This point of view has recently been developed (Tarantola, 2006 [1] ).


See also

To find related topics in a list, see List of physical quantities.

List of Physical Quantities some further Physical Quantities yet to be incorporated into the above table (list incomplete) Categories: Stub | Physics | Physical quantity ... International Standard ISO 31 (Quantities and units, International Organization for Standardization, 1992) is the most widely respected style guide for the use of units of measurement, and formulas involving them, in scientific and educational documents worldwide. ... Title: Quantities, Units and Symbols in Physical Chemistry Content: the IUPAC green book establishes standards for nomenclature in chemistry. ... The definition, agreement and practical use of units of measurement have played a crucial role in human endeavour from early ages up to this day. ... Cover of brochure The International System of Units. ...

Books

  • Cook, Alan H. The observational foundations of physics, Cambridge, 1994. ISBN 0-521-45597-9.
  • Fourier, Joseph. Théorie analytique de la chaleur, Firmin Didot, Paris, 1822. (In this book, Fourier introduces the concept of physical dimensions for the physical quantities.)
  • Tarantola, Albert. Elements for physics - Quantities, qualities and intrinsic theories, Springer, 2006. ISBN 3-540-25302-5. [2]

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Physical quantities of the same absolute dimensionality of the GCS-system express the different manifestation of the same property of a multiface process or an object of study.
Physical quantities belonging to the classes with the unit argument of contents D(1, l, m) form a set of dynamic physical quantities or, simply, the dynamic set.
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