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Kilogram

Kilogram

:For other uses of 'kg' see kg (disambiguation) kg (disambiguation) The kilogram or kilogramme, (symbol: kg) is the SI base unit of mass. It is defined as being equal to the mass of the international prototype of the kilogram. It is the only SI base unit that employs a prefix, and the only SI unit that is still defined in relation to an artifact rather than to a fundamental physical property.

History

The kilogram was originally defined as the mass of one litre of pure water at a temperature of 3.98 degrees Celsius and standard atmospheric pressure. This definition was hard to realize accurately, partially because the density of water depends ever-so-slightly on the pressure, and pressure units include mass as a factor, introducing a circular dependency in the definition of the kilogram. To avoid these problems, the kilogram was redefined as precisely the mass of a particular standard mass created to approximate the original definition. Since 1889, the SI system defines the unit to be equal to the mass of the international prototype of the kilogram, which is made from an alloy of platinum and iridium of 39 mm height and diameter, and is kept at the Bureau International des Poids et Mesures (International Bureau of Weights and Measures). Official copies of the prototype kilogram are made available as national prototypes, which are compared to the Paris prototype ("Le Grand Kilo") roughly every 10 years. The international prototype kilogram was made in the 1880s. By definition, the error in the repeatability of the current definition is exactly zero; however, in the usual sense of the word, it can be regarded as of the order of 2 micrograms. This is found by comparing the official standard with its official copies, which are made of roughly the same materials and kept under the same conditions. There is no reason to believe that the official standard is any more or less stable than its official copies, thus giving a way to estimate its stability. This procedure is performed roughly once every forty years. The international prototype of the kilogram seems to have lost about 50 micrograms in the last 100 years, and the reason for the loss is still unknown (reported in Der Spiegel, 2003 #26). The observed variation in the prototype has intensified the search for a new definition of the kilogram. It is accurate to state that any object in the universe (other than the reference metal in France) that had a mass of 1 kilogram 100 years ago, and has not changed since then, now has a mass of 1.000 000 05 kg. This perspective is counterintuitive and defeats the purpose of a standard unit of mass, since the standard should not change arbitrarily over time.

The gram

The gram or gramme is the term to which SI prefixes are applied. The gram was the base unit of the older cgs system of measurement, a system which is no longer widely used.

Proposed future definitions

There is an ongoing effort to introduce a new definition for the kilogram by way of fundamental or atomic constants. The proposals being worked on are:

Atom-counting approaches

- The Avogadro approach attempts to define the kilogram as a fixed number of silicon atoms. As a practical realization, a sphere would be used and its size would be measured by interferometry.
- The ion accumulation approach involves accumulation of gold atoms and measuring the electrical current required to neutralise them.

Fundamental-constant approaches

- The Watt balance uses the current balance that was formerly used to define the ampere to relate the kilogram to a value for Planck's constant, based on the definitions of the volt and the ohm.
- The levitated superconductor approach relates the kilogram to electrical quantities by levitating a superconducting body in a magnetic field generated by a superconducting coil, and measuring the electrical current required in the coil.
- Since the values of the Josephson (CIPM (1988) Recommendation 1, PV 56; 19) and von Klitzing (CIPM (1988), Recommendation 2, PV 56; 20) constants have been given conventional values, it is possible to combine these values (K_J ≡ 4.835 979 Hz/V and R_K ≡ 2.581 280 7 Ω) with the definition of the ampere to define the kilogram as follows: :The kilogram is the mass which would be accelerated at precisely 2 m/s² if subjected to the per metre force between two straight parallel conductors of infinite length, of negligible circular cross section, placed 1 metre apart in vacuum, through which flow a constant current of exactly 6.241 509 629 152 65 elementary charges per second.

Link with weight

When the weight of an object is given in kilograms, the property intended is almost always mass. Occasionally the gravitational force on an object is given in "kilograms", but the unit used is not a true kilogram: it is the deprecated kilogram-force (kgf), also known as the kilopond (kp). An object of mass 1 kg at the surface of the Earth will be subjected to a gravitational force of approximately 9.80665 newtons (the SI unit of force). Note that the factor of 980.665 cm/s² (as the CGPM defined it, when cgs systems were the primary systems used) is only an agreed-upon conventional value (3rd CGPM (1901), CR 70) whose purpose is to define grams force. The local gravitational acceleration g varies with latitude and altitude and location on the Earth, so before this conventional value was agreed upon, the gram-force was only an ill-defined unit. (See also gee, a standard measure of gravitational acceleration.)

Examples

- Attogram: a research team at Cornell University made a detector using NEMS cantilevers with sub-attogram sensitivity.
- Yoctogram: can be used for masses of nucleons, atoms and molecules. It is a little large for light particles, but yocto- is the last official prefix in the sequence.
- The coefficient is close to the reciprocal of Avogadro's number: 1 unified atomic mass unit = 1.660 54 yg
- Although the unified atomic mass unit is often convenient as a unit, one may sometimes want to use yoctograms to relate easily to other SI values.
- Mass of a free electron: 0.000 91 yg
- Mass of a free proton : 1.672 6 yg
- Mass of a free neutron: 1.674 9 yg

SI multiples

External links

- [http://www.npl.co.uk/mass/faqs/kilogram.html National Physical Laboratory FAQ on kilogram definition, the need for a new definition, and some alternatives]
- [http://www.ex.ac.uk/trol/scol/index.htm Conversion Calculator for Units of MASS (& Weight)]
- [http://nvl.nist.gov/pub/nistpubs/jres/106/4/j64schw.pdf More on the NIST Watt Balance]
- [http://www.npl.co.uk/mass/avogadro.html More on the Avogadro project]
- [http://www.ex.ac.uk/trol/scol/ccmass.htm Conversion: Units of Weight]
- [http://www.bipm.fr Le Bureau International des Poids et Mesures]
- [http://www.hgc.cornell.edu/Nems%20Folder/Attogram%20Sensitivity%20Using%20Nanoelectromechanical.html Attogram Detection]
- [http://www.newscientist.com/article.ns?id=dn7208&feedId=online-news_rss20 World's most sensitive scales weigh a zeptogram, by New Scientist.com]
- [http://news.bbc.co.uk/1/hi/sci/tech/4394947.stm Scales tip with tiniest mass yet, by BBC News Online] Category:SI base units Category:Units of mass zh-min-nan:Kong-kin ko:킬로그램 ja:キログラム simple:Kilogram th:กิโลกรัม

Kg (disambiguation)

KG, Kg or kg may indicate:
- A Kampfgeschwader, a bomber squadron of the former German Luftwaffe
- Basketball Player Kevin Garnett
- An abbreviation for kilogram (always kg)
- Knight of the Garter, a British decoration
- Kommanditgesellschaft, German version of a limited partnership
- Kongo language (ISO 639 alpha-2)
- An abbreviation for "konig" or king
- Kwansei Gakuin University (Japan) [http://www.kwansei.ac.jp/english/index_english.html]
- Kyrgyzstan (ISO 3166-1 alpha-2 country code)
- Tenacious D band member Kyle Gass
- [http://www.blogger.com/profile/8321753 KG] creator-editor-admin of the [http://kgbee.blogspot.com KG Bee online] news resource
- King George
- Krazygamers.com An anime/gaming website [http://www.krazygamers.com Krazygamers]
- Volkswagen Abbreviation in the VW community for "Karmann Ghia" ko:KG ja:KG

SI base unit

The SI system of units defines seven SI base units: fundamental physical units defined by an operational definition. All other physical units can be derived from these base units: these are known as SI derived units. Derivation is by dimensional analysis. Use SI prefixes to abbreviate long numbers. The following are the fundamental units from which all others are derived, they are dimensionally independent. The definitions stated below are widely accepted.

External links

- [http://www1.bipm.org/en/si/base_units/ BIPM]
- [http://www.npl.co.uk/mass/faqs/kilogram.html NPL - Kilogram]
- [http://physics.nist.gov/cuu/Units/index.html NIST -SI] Category:SI units Category:Dimensional analysis ja:SI基本単位 ko:SI 기본 단위 simple:SI base unit zh-cn:国际标准基准单位 zh-tw:國際標準基準單位

Mass

:For other senses of this word, see mass (disambiguation). Mass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. Unlike weight, the mass of something stays the same regardless of location. It is a central concept of classical mechanics and related subjects. Strictly speaking, there are three different quantities called mass:
- Inertial mass is a measure of an object's inertia: its resistance to changing its state of motion when a force is applied. An object with small inertial mass changes its motion more readily, and an object with large inertial mass does so less readily.
- Passive gravitational mass is a measure of the strength of an object's interaction with the gravitational field. Within the same gravitational field, an object with a smaller passive gravitational mass experiences a smaller force than an object with a larger passive gravitational mass. (This force is called the weight of the object. In informal usage, the word "weight" is often used synonymously with "mass", because the strength of the gravitational field is roughly constant everywhere on the surface of the Earth. In physics, the two terms are distinct: an object will have a larger weight if it is placed in a stronger gravitational field, but its passive gravitational mass remains unchanged.)
- Active gravitational mass is a measure of the strength of the gravitational field due to a particular object. For example, the gravitational field that one experiences on the Moon is weaker than that of the Earth because the Moon has less active gravitational mass.

Introduction

Although inertial mass, passive gravitational mass and active gravitational mass are conceptually distinct, no experiment has ever unambiguously demonstrated any difference between them. One of the consequences of the equivalence of inertial mass and passive gravitational mass is the fact, famously demonstrated by Galileo Galilei, that objects with different masses fall at the same rate, assuming factors like air resistance are negligible. The theory of general relativity, the most accurate theory of gravitation known to physicists to date, rests on the assumption that inertial and passive gravitational mass are completely equivalent. This is known as the weak equivalence principle. Classically, active and passive gravitational mass were equivalent as a consequence of Newton's third law, but a new axiom is required in the context of relativity's reformulation of gravity and mechanics. Thus, standard general relativity also assumes the equivalence of inertial mass and active gravitational mass; this equivalence is sometimes called the strong equivalence principle. If one were to treat inertial mass m_i, passive gravitational mass m_p, and active gravitational mass m_a distinctly, Newton's law of universal gravitation would give as force on the second mass due to the first mass :

m_a_2=\frac.

Newton's third law, of reciprocal actions, shows that active and passive mass are proportional. As a result they can be defined to be equal.

Units of mass

In the SI system of units, mass is measured in kilograms (kg). Many other units of mass are also employed, such as: grams (g), metric tons, pounds, ounces, long and short tons, quintals, slugs, atomic mass units, Planck masses, solar masses, and eV/c². The eV/c² unit is based on the electron volt (eV), which is normally used as a unit of energy. However, because of the relativistic connection between (rest) mass and energy, E = mc² (see below), it is possible to use any unit of energy as a unit of mass instead. Thus, in particle physics where mass and energy are often interchanged, it is common to use not only eV/c² but even simply eV as a unit of mass (roughly 1.783 × 10^-36 kg). Because the gravitational acceleration is approximately constant on the surface of the Earth, a unit like the pound is often used to measure either mass or force (e.g. weight), although the pound is officially defined as a unit of mass. For more information on the different units of mass, see Orders of magnitude (mass).

Inertial mass

Inertial mass is the mass of an object measured by its resistance to acceleration. To understand what the inertial mass of a body is, one begins with classical mechanics and Newton's Laws of Motion. Later on, we will see how our classical definition of mass must be altered if we take into consideration the theory of special relativity, which is more accurate than classical mechanics. However, the implications of special relativity will not change the meaning of "mass" in any essential way. According to Newton's second law, we say that a body has a mass m if, at any instant of time, it obeys the equation of motion :

F = \frac (mv)

where F is the force acting on the body and v is its velocity. For the moment, we will put aside the question of what "force acting on the body" actually means. Now, suppose that the mass of the body in question is a constant. This assumption, known as the conservation of mass, rests on the ideas that (i) mass is a measure of the amount of matter contained in a body, and (ii) matter can never be created or destroyed, only split up or recombined. These are very reasonable assumptions for everyday objects, though, as we will see, the situation gets more complicated when we take special relativity into account. Another point to note is that, even in classical mechanics, it is sometimes useful to treat the mass of an object as changing with time. For example, the mass of a rocket decreases as the rocket fires. However, this is an approximation, based on ignoring pieces of matter which enter or leave the system. In the case of the rocket, these pieces correspond to the ejected propellent; if we were to measure the total mass of the rocket and its propellent, we would find that it is conserved. When the mass of a body is constant, Newton's second law becomes :

F = m \frac = m a

where a denotes the acceleration of the body. This equation illustrates how mass relates to the inertia of a body. Consider two objects with different masses. If we apply an identical force to each, the object with a bigger mass will experience a smaller acceleration, and the object with a smaller mass will experience a bigger acceleration. We might say that the larger mass exerts a greater "resistance" to changing its state of motion in response to the force. However, this notion of applying "identical" forces to different objects brings us back to the fact that we have not really defined what a force is. We can sidestep this difficulty with the help of Newton's third law, which states that if one object exerts a force on a second object, it will experience an equal and opposite force. To be precise, suppose we have two objects A and B, with constant inertial masses m_A and m_B. We isolate the two objects from all other physical influences, so that the only forces present are the force exerted on A by B, which we denote F_AB, and the force exerted on B by A, which we denote F_BA. As we have seen, Newton's second law states that :

F_ = m_A a_A \,

and

F_ = m_B a_B \,

where a_A and a_B are the accelerations of A and B respectively. Suppose that these accelerations are non-zero, so that the forces between the two objects are non-zero. This occurs, for example, if the two objects are in the process of colliding with one another. Newton's third law then states that :

F_ = - F_ \,

Substituting this into the previous equations, we obtain :

m_A = - \frac \, m_B

Note that our requirement that a_A be non-zero ensures that the fraction is well-defined. This is, in principle, how we would measure the inertial mass of an object. We choose a "reference" object and define its mass m_B as (say) 1 kilogram. Then we can measure the mass of every other object in the universe by colliding it with the reference object and measuring the accelerations.

Gravitational mass

Gravitational mass is the mass of an object measured using the effect of a gravitational field on the object. The concept of gravitational mass rests on Newton's law of gravitation. Let us suppose we have two objects A and B, separated by a distance |r_AB|. The law of gravitation states that if A and B have gravitational masses M_A and M_B respectively, then each object exerts a gravitational force on the other, of magnitude :

|F| =

where G is the universal gravitational constant. The above statement may be reformulated in the following way: if g is the acceleration of a reference mass at a given location in a gravitational field, then the gravitational force on an object with gravitational mass M is :

F = Mg \,

This is the basis by which masses are determined by weighing. In simple bathroom scales, for example, the force F is proportionate to the displacement of the spring beneath the weighing pan (see Hooke's law), and the scales are calibrated to take g into account, allowing the mass M to be read off.

Equivalence of inertial and gravitational masses

The equivalence of inertial and gravitational masses is sometimes referred to as the Galilean equivalence principle or weak equivalence principle. The most important consequence of this equivalence principle applies to freely falling objects. Suppose we have an object with inertial and gravitational masses m and M respectively. If the only force acting on the object comes from a gravitational field g, combining Newton's second law and the gravitational law yields the acceleration :

K = \frac g

This says that the ratio of gravitational to inertial mass of any object is equal to some constant K if and only if all objects fall at the same rate in a given gravitational field. This phenomenon is referred to as the universality of free-fall. (In addition, the constant K can be taken to be 1 by defining our units appropriately.) The first experiments demonstrating the universality of free-fall were conducted by Galileo. It is commonly stated that Galileo obtained his results by dropping objects from the Leaning Tower of Pisa, but this is unlikely to be true; actually, he performed his experiments with balls rolling down inclined planes. Increasingly precise experiments have been performed, such as those performed by Roland Eötvös, using the torsion balance pendulum, in 1889. To date, no deviation from universality, and thus from Galilean equivalence, has ever been found. More precise experimental efforts are still being carried out. It should be noted that the universality of free fall only applies to systems in which gravity is the only force acting. All other forces, especially friction and air resistance, must be absent or at least negligible. For example, if a hammer and a feather are dropped from the same height, we all know that the feather will take much longer to reach the ground. This happens because the feather is not really in free fall: the force of air resistance on it is about as strong as the force of gravity. On the other hand, if the experiment is performed in a vacuum, where there is no air resistance, the hammer and the feather should fall at the same rate and reach the ground together. This demonstration was, in fact, carried out in 1971 during the Apollo 15 Moon walk, by Commander David Scott. A stronger version of the equivalence principle, known as the Einstein equivalence principle or the strong equivalence principle, lies at the heart of the general theory of relativity. Einstein's equivalence principle states that it is impossible to distinguish between a uniform acceleration and a uniform gravitational field. Thus, the theory postulates that inertial and gravitational masses are fundamentally the same thing. All of the predictions of general relativity, such as the curvature of spacetime, are ultimately derived from this principle.

Relativistic relation among mass, energy and momentum

Special relativity is a necessary extension of classical physics. In particular, special relativity succeeds where classical mechanics fails badly in describing objects moving at speeds close to the speed of light. In relativistic mechanics, the mass (m) of a free particle is related to its energy (E) and momentum (p) by the equation :

\frac = m^2 c^2 + p^2

. where c is the speed of light. This is sometimes referred to as the mass-energy-momentum relation. The first thing to notice about this equation is that it can cope with massless objects (m = 0), for which it reduces to :

E = pc \,

In classical mechanics, massless objects are an ill-defined concept, since applying any force to one would produce, via Newton's second law, an infinite acceleration - a nonsensical result. In relativistic mechanics, they are objects that are always traveling at the speed of light; an example being light itself, in the form of photons. The above equation says that the energy carried by a massless object is directly proportional to its momentum. Let us now consider objects with non-zero mass. For these, the quantity m has a simple physical meaning: it is the inertial mass of the object as measured in its rest frame, the frame of reference in which its velocity is zero. (Note: massless objects do not possess a rest frame; they are moving at the speed of light in any frame of reference.) The way we would measure m is exactly the same as in classical mechanics, which we described above: bouncing it off a reference object and measuring the accelerations. As long as the velocity of each object remains much smaller than the speed of light during this procedure, relativistic corrections to classical mechanics will be utterly negligible. In the rest frame, the velocity is zero, and thus so is the momentum p. The mass-energy-momentum relation thus reduces to :

E = mc^2 \,

which states that the energy of an object as measured in its rest frame - its "rest energy" - is equal to its mass times the square of the speed of light. Some books follow this up by stating that "mass and energy are equivalent", but this is somewhat misleading. The mass of an object, as we have defined it, is a quantity intrinsic to the object, and independent of our current frame of reference. The energy E, on the other hand, varies with the frame of reference; if the frame is moving at a high velocity relative to the object, E will be very large, simply because the object has a lot of kinetic energy in that frame. Thus, E = mc² is not a "good" relativistic statement; it is true only in the rest frame of the object. Some authors define a quantity known as the relativistic mass, which is basically the quantity E/c². This makes the "equivalence" of "mass" and energy true by definition, though neither quantity is frame-independent! "Relativistic mass" was used in many early writings on relativity, and it is still used in books for laymen as well as introductory physics classes. However, the concept is downplayed or discouraged by many physicists nowadays, for reasons explained in the article on relativistic mass. Following the modern usage, whenever we refer to "mass" in this article we always mean the rest mass, unless otherwise identified. Having defined the mass of an object, let us look at how it behaves when not at rest. We can arrange the mass-energy-momentum relation in the following way: :

E = mc^2 \sqrt

When the momentum p is much smaller than mc, we can Taylor expand the square root, with the result :

E = mc^2 +  + \cdots

The leading term, which is the largest, is of course the rest energy. The object always has this minimum amount of energy, regardless of its momentum. The second term is the classical expression for the kinetic energy of the particle, and the higher-order terms are basically relativistic corrections for the kinetic energy. Under normal circumstances, the rest energy of an object is inaccessible, in the sense that it cannot be used to do mechanical work. When the object hits something, it can do work by transferring its momentum, and thus its kinetic energy, to whatever it hit. However, the rest energy depends only on the mass of the object, which does not change during collisions, so it cannot be transferred along with the kinetic energy. On the other hand, it is possible to access the rest energy using processes that split or combine particles. The reason is that mass, as we have defined it, is not conserved during such processes. The simplest example is the process of electron-positron annihilation, in which an electron and a positron annihilate each other to produce a pair of photons: the electron and positron both have non-zero mass, but the photons are massless. Other examples include nuclear fusion and nuclear fission. Metabolism, fire and other exothermic chemical processes also convert mass to energy, however the mass change from these is negligible. Energy, unlike mass, is always conserved in special relativity, so, roughly speaking, what is happening in these reactions is that the rest energy of the reactants is being transformed into the kinetic energy of the reaction products. The fact that rest energy can be liberated in this way is one of the most important predictions of special relativity.

References

- R.V. Eötvös et al, Ann. Phys. (Leipzig) 68 11 (1922)

External links

- [http://math.ucr.edu/home/baez/physics Usenet Physics FAQ]
- [http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html Does mass change with velocity?]
- [http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html Does light have mass?]
- [http://www.teleles.nl/pdf/total_artikel.pdf Mass & energy]
- [http://www.geocities.com/physic1525/inertiaenergy.html The law of the inertia of the energy and the speed of the gravity. See chapter 3 The energy has mass ]
- [http://www.geocities.com/physics_world/stp/title.htm Dialog: Use and abuse of the concept of mass (from Spacetime physics by Edwin F. Taylor and John A. Wheeler)]
- [http://arxiv.org/PS_cache/physics/pdf/0111/0111134.pdf Photons, Clocks, Gravity and the Concept of Mass by L.B.Okun]
- [http://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_15_feather_drop.html The Apollo 15 Hammer-Feather Drop]
- [http://calc.skyrocket.de/en/ Online Unit Converter - Conversion of many different units] Category:Physical chemistry Category:Classical mechanics
- ko:질량 ms:Jisim ja:質量 simple:Mass th:มวล

Litre

The litre (spelled litre in Commonwealth English and liter in American English) is a unit of capacity. There are two official symbols: lowercase l and uppercase L. The litre is not an SI unit but is accepted for use with the SI. The SI unit of volume is the cubic metre (m³).

Definitions and equivalents

A litre is defined as a special name for a cubic decimetre (1 L = 1 dm³).
- 1 L = 1000 cm³ (exactly)
- 1000 L = 1 m³ (exactly)

SI prefixes applied to the litre

The litre may be used with some SI prefixes.

Name origin

The word "litre" is derived from an older French unit, the litron, whose name came from Greek via Latin.

Other common metric equivalencies

- 1 µL (microlitre) = 1 mm³ (cubic millimetre)

Conversions

One litre :≈ 0.87987699 Imperial quart ::Inverse: One Imperial quart ≡ 1.1365225 litres :≈ 1.056688 US fluid quarts ::Inverse: One US fluid quart ≡ 0.946352946 litres :≈ 0.0353146667 cubic foot ::Inverse: One cubic foot ≡ 28.316846592 litres One millilitre :≈ 0.03519507972785404600 Imperial fluid ounce ::Inverse: One Imperial fluid ounce ≡ 28.4130625 mL :≈ 0.0338140227018429971686 US fluid ounce ::Inverse: One US fluid ounce ≡ 29.5735295625 mL :≈ 0.0000353146667 cubic foot ::Inverse: One cubic foot ≡ 28,316.846592 mL

Explanation

Litres are most commonly used for items measured by the capacity or size of their container (such as fluids and berries), whereas cubic metres (and derived units) are most commonly used for items measured either by their dimensions or their displacements. The litre is often also used in some calculated measurements, such as density (kg/L), allowing an easy comparison with the density of water. One litre of water weighs almost exactly one kilogram. Similarly: 1 ml of water weighs about 1 g; 1000 litres of water weighs about 1000 kg (1 tonne). This relationship is due to the history of the unit but since 1964 has not been part of the definition.

Symbol

Originally, the only symbol for the litre was l (lowercase letter l), following the SI convention that only those unit symbols that abbreviate the name of a person start with a capital letter. In many English-speaking countries, the most common shape of a handwritten Arabic digit 1 is just a vertical stroke, that is it lacks the upstroke added in many other cultures. Therefore, the digit 1 may easily be confused with the letter l. On some typewriters, particularly older ones, the l key had to be used to type the numeral 1. Further, in some typefaces the two characters are nearly indistinguishable. This caused some concern, especially in the medical community. As a result, L (uppercase letter L) was accepted as an alternative symbol for litre in 1979. The United States National Institute of Standards and Technology now recommends the use of the uppercase letter L, a practice that is also widely followed in Canada and Australia. In these countries, the symbol L is also used with prefixes, as in mL and µL, instead of the traditional ml and µl used in Europe. Prior to 1979, the symbol ℓ (script small l, U+2113), came into common use in some countries; for example, it was recommended by South African Bureau of Standards publication M33 in the 1970s. This symbol can still be encountered occasionally in some English-speaking countries, but it is not used in most countries and not officially recognised by the BIPM, the International Organization for Standardization, or any national standards body.

History

In 1793, the litre was introduced in France as one of the new "Republican Measures", and defined as one cubic decimetre. In 1879, the CIPM adopted the definition of the litre, and the symbol l (lowercase letter l). In 1901, at the 3rd CGPM conference, the litre was redefined as the space occupied by 1 kg of pure water at the temperature of its maximum density (3.98 °C) under a pressure of 1 atm. This made the litre equal to about 1.000 028 dm³ (earlier reference works usually put it at 1.000 027 dm³). In 1964, at the 12th CGPM conference, the litre was once again defined in exact relation to the metre, as another name for the cubic decimetre, that is, exactly 1 dm³. [http://ts.nist.gov/ts/htdocs/230/235/appxc/appxc.htm#footnote1 NIST Reference] In 1979, at the 16th CGPM conference, the alternative symbol L (uppercase letter L) was adopted. It also expressed a preference that in the future only one of these two symbols should be retained, but in 1990 said it was still too early to do so.

External links

- [http://www.bipm.org/en/si/si_brochure/ BIPM's "SI Brochure"]
- [http://www.bipm.org/en/si/si_brochure/chapter4/table6.html BIPM's "(Table 6 -) Non-SI units accepted for use with the International System"]
- [http://physics.nist.gov/cuu/Units/units.html NIST note on SI units]
- [http://physics.nist.gov/cuu/Units/outside.html NIST recommends uppercase letter L]
- [http://www.npl.co.uk/npl/reference/international.html UK National physical laboratory's "Internationally recognised non SI units" page] Category:Units of volume ko:리터 ja:リットル simple:Litre th:ลิตร

Temperature

Temperature is the physical property of a system which underlies the common notions of "hot" and "cold"; the material with the higher temperature is said to be hotter. Physically, temperature is a measure of the random agitation of matter and ambiant photons, under the effect of thermal fluctuations. It is a fundamental parameter in thermodynamics and it is conjugate to entropy. More quantitatively, the order of magnitude of the fluctuations of the energy associated with an atom, molecule or another elementary constituant of a physical system is

k_B T

, where

k_B

is Boltzmann's constant, and T is temperature, expressed in Kelvins.

Overview

The formal properties of temperature are studied in thermodynamics and statistical mechanics. The temperature of a system at thermodynamic equilibrium is defined by a relation between the amount of heat

\delta Q

incident on the system during an infinitesimal quasistatic transformation, and the variation

\delta S

of its entropy during this transformation. :

\delta S = \frac

Contrarly to entropy and heat, whose microscopic definitions are valid even far away from thermodynamic equilibrium temperature can only be defined at thermodynamic equilibrium, or local thermodynamic equilibrium (see below). As a system receives heat its temperature rises, similarly a loss of heat from the system tends to decrease its temperature (at the - uncommon - exception of negative temperature, see below). When two systems are at the same temperature, no heat transfer occurs between them. When a temperature difference does exist, heat will tend to move from the higher-temperature system to the lower-temperature system, until they are at thermal equilibrium. This heat transfer may occur via conduction, convection or radiation (see heat for additional discussion of the various mechanisms of heat transfer). Temperature is also related to the amount of internal energy and enthalpy of a system. The higher the temperature of a system, the higher its internal energy and enthalpy are. Temperature is an intensive property of a system, meaning that it does not depend on the system size or the amount of material in the system. Other intensive properties include pressure and density. By contrast, mass and volume are extensive properties, and depend on the amount of material in the system.

Role of temperature in nature

Temperature plays an important role in almost all fields of science, including physics, chemistry, and biology. Many physical properties of materials including the phase (solid, liquid, gaseous or plasma), density, solubility, vapor pressure, and electrical conductivity depend on the temperature. Temperature also plays an important role in determining the rate and extent to which chemical reactions occur. This is one reason why the human body has several elaborate mechanisms for maintaining the temperature at 37 °C, since temperatures only a few degrees higher can result in harmful reactions with serious consequences. Temperature also controls the type and quantity of thermal radiation emitted from a surface. One application of this effect is the incandescent light bulb, in which a tungsten filament is electrically heated to a temperature at which significant quantities of visible light are emitted. Temperature-dependence of the speed of sound in air c, density of air ρ and acoustic impedance Z vs. temperature °C

Temperature measurement

Main article: Temperature measurement Temperature measurement using modern scientific thermometers and temperature scales goes back at least as far as the early 18th century, when Gabriel Fahrenheit adapted a thermometer (switching to mercury) and a scale both developed by Ole Christensen Rømer. Fahrenheit's scale is still in use, alongside the Celsius scale and the Kelvin scale.

Units of temperature

The basic unit of temperature (symbol: T) in the International System of Units (SI) is the kelvin (K). One kelvin is formally defined as 1/273.16 of the temperature of the triple point of water (the point at which water, ice and water vapor exist in equilibrium). The temperature 0 K is called absolute zero and corresponds to the point at which the molecules and atoms have the least possible thermal energy. An important unit of temperature in theoretical physics is the Planck temperature (1.4 × 10³² K). In the field of plasma physics, because of the high temperatures encountered and the electromagnetic nature of the phenomena involved, it is customary to express temperature in electronvolts (eV) or kiloelectronvolts (keV), where 1 eV = 11,605 K. In the study of QCD matter one routinely meets temperatures of the order of a few hundred MeV, equivalent to about 10¹² K. For everyday applications, it is often convenient to use the Celsius scale, in which 0 °C corresponds to the temperature at which water freezes and 100 °C corresponds to the boiling point of water at sea level. In this scale a temperature difference of 1 degree is the same as a 1 K temperature difference, so the scale is essentially the same as the kelvin scale, but offset by the temperature at which water freezes (273.15 K). Thus the following equation can be used to convert from degrees Celsius to kelvins. :

\mathrm

In the United States, the Fahrenheit scale is widely used. On this scale the freezing point of water corresponds to 32 °F and the boiling point to 212 °F. The following formula can be used to convert from Fahrenheit to Celsius: :

\mathrm

See temperature conversion formulas for conversions between most temperature scales. ¹ Only the kelvin, Celsius, Fahrenheit, and Rankine scales are in use today.
² Some numbers in this table have been rounded off.
³ Normal human body temperature is 36.8 °C ±0.7 °C, or 98.2 °F ±1.3 °F.

Negative temperatures

:See main article: Negative temperature. For some systems and specific definitions of temperature, it is possible to obtain a negative temperature. A system with a negative temperature is not colder than absolute zero, but rather it is, in a sense, hotter than infinite temperature (sic).

Articles about temperature ranges:

- 10⁻¹² K = 1 picokelvin (pK)
- 10⁻⁹ K = 1 nanokelvin (nK)
- 10⁻⁶ K = 1 microkelvin (µK)
- 10⁻³ K = 1 millikelvin (mK)
- 10⁰ K = 1 kelvin
- 10¹ K = 10 kelvins
- 10² K = 100 kelvins
- 10³ K = 1,000 kelvin = 1 kilokelvin (kK)
- 10⁴ K = 10,000 kelvins = 10 kK
- 10⁵ K = 100,000 kelvins = 100 kK
- 10⁶ K = 1 megakelvin (MK)
- 10⁹ K = 1 gigakelvin (GK)
- 10¹² K = 1 terakelvin (TK) See Orders of magnitude (temperature).

Theoretical foundation of temperature

Zeroth-law definition of temperature

While most people have a basic understanding of the concept of temperature, its formal definition is rather complicated. Before jumping to a formal definition, let us consider the concept of thermal equilibrium. If two closed systems with fixed volumes are brought together, so that they are in thermal contact, changes may take place in the properties of both systems. These changes are due to the transfer of heat between the systems. When a state is reached in which no further changes occur, the systems are in thermal equilibrium. Now a basis for the definition of temperature can be obtained from the so-called zeroth law of thermodynamics which states that if two systems, A and B, are in thermal equilibrium and a third system C is in thermal equilibrium with system A then systems B and C will also be in thermal equilibrium (being in thermal equilibrium is a transitive relation; moreover, it is an equivalence relation). This is an empirical fact, based on observation rather than theory. Since A, B, and C are all in thermal equilibrium, it is reasonable to say each of these systems shares a common value of some property. We call this property temperature. Generally, it is not convenient to place any two arbitrary systems in thermal contact to see if they are in thermal equilibrium and thus have the same temperature. Also, it would only provide an ordinal scale. Therefore, it is useful to establish a temperature scale based on the properties of some reference system. Then, a measuring device can be calibrated based on the properties of the reference system and used to measure the temperature of other systems. One such reference system is a fixed quantity of gas. The ideal gas law indicates that the product of the pressure and volume (P · V) of a gas is directly proportional to the temperature: :

P \cdot V = n \cdot R \cdot T

(1) where 'T is temperature, n is the number of moles of gas and R is the gas constant. Thus, one can define a scale for temperature based on the corresponding pressure and volume of the gas: the temperature in kelvins is the pressure in pascals of one mole of gas in a container of one cubic metre, divided by 8.31... In practice, such a gas thermometer is not very convenient, but other measuring instruments can be calibrated to this scale. Equation 1 indicates that for a fixed volume of gas, the pressure increases with increasing temperature. Pressure is just a measure of the force applied by the gas on the walls of the container and is related to the energy of the system. Thus, we can see that an increase in temperature corresponds to an increase in the thermal energy of the system. When two systems of differing temperature are placed in thermal contact, the temperature of the hotter system decreases, indicating that heat is leaving that system, while the cooler system is gaining heat and increasing in temperature. Thus heat always moves from a region of high temperature to a region of lower temperature and it is the temperature difference that drives the heat transfer between the two systems.

Temperature in gases

As mentioned previously for a monatomic ideal gas the temperature is related to the translational motion or average speed of the atoms. The kinetic theory of gases uses statistical mechanics to relate this motion to the average kinetic energy of atoms and molecules in the system. For this case 7736 K = 7463 degrees Celsius corresponds to an average kinetic energy of one electronvolt; to take room temperature (300 K) as an example, the average energy of air molecules is 300/7736 eV, or 0.0388 electronvolt. This average energy is independent of particle mass, which seems counterintuitive to many people. Although the temperature is related to the average kinetic energy of the particles in a gas, each particle has its own energy which may or may not correspond to the average. However, after an examination of some basic physics equations it makes perfect sense. The second law of thermodynamics states that any two given systems when interacting with each other will later reach the same average energy. Temperature is a measure of the average kinetic energy of a system. The formula for the kinetic energy of an atom is:
:

K_t = \begin \frac \end mv^2

(Note that a calculation of the kinetic energy of a more complicated object, such as a molecule, is slightly more involved. Additional degrees of freedom are available, so molecular rotation or vibration must be included.)

Thus, particles of greater mass (say a neon atom relative to a hydrogen molecule) will move slower than lighter counterparts, but will have the same average energy. This average energy is independent of the mass because of the nature of a gas, all particles are in random motion with collisions with other gas molecules, solid objects that may be in the area and the container itself (if there is one). A visual illustration of this [http://intro.chem.okstate.edu/1314F00/Laboratory/GLP.htm from Oklahoma State University] makes the point more clear. Not all the particles in the container have different velocities, regardless of whether there are particles of more than one mass in the container, but the average kinetic energy is the same because of the ideal gas law. In a gas the distribution of energy (and thus speeds) of the particles corresponds to the Boltzmann distribution. An electronvolt is a very small unit of energy, approximately 1.602×10^-19 joule.

Temperature of the vacuum

When a satellite in empty space is heated by sunshine and cooled by radiating energy away it is not in thermodynamic equilibrium and has no well-defined temperature. A system in a vacuum will radiate its thermal energy, i.e. convert heat into electromagnetic waves. If vacuum is filled with electromagnetic waves (say, radiation from walls of vacuum chamber, or relic microwave radiation in space) then the system will exchange by energy with these waves and thermally equilibrates at some finite (non zero) temperature. Cosmic microwave background radiation being remnant of radiation of hot early universe when radiation was in thermal equilibrium with matter has Planck spectrum (black body spectrum) with the temperature (at present) of about 2.7 K.

Second-law definition of temperature

In the previous section temperature was defined in terms of the Zeroth Law of thermodynamics. It is also possible to define temperature in terms of the second law of thermodynamics, which deals with entropy. Entropy is a measure of the disorder in a system. The second law states that any process will result in either no change or a net increase in the entropy of the universe. This can be understood in terms of probability. Consider a series of coin tosses. A perfectly ordered system would be one in which every coin toss would come up either heads or tails. For any number of coin tosses, there is only one combination of outcomes corresponding to this situation. On the other hand, there are multiple combinations that can result in disordered or mixed systems, where some fraction are heads and the rest tails. As the number of coin tosses increases, the number of combinations corresponding to imperfectly ordered systems increases. For a very large number of coin tosses, the number of combinations corresponding to ~50% heads and ~50% tails dominates and obtaining an outcome significantly different from 50/50 becomes extremely unlikely. Thus the system naturally progresses to a state of maximum disorder or entropy. Now, we have stated previously that temperature controls the flow of heat between two systems and we have just shown that the universe, and we would expect any natural system, tends to progress so as to maximize entropy. Thus, we would expect there to be some relationship between temperature and entropy. In order to find this relationship let's first consider the relationship between heat, work and temperature. A heat engine is a device for converting heat into mechanical work and analysis of the Carnot heat engine provides the necessary relationships we seek. The work from a heat engine corresponds to the difference between the heat put into the system at the high temperature, q_H and the heat ejected at the low temperature, q_C. The efficiency is the work divided by the heat put into the system or: :

\textrm = \frac  = \frac = 1 - \frac

(2) where w_cy is the work done per cycle. We see that the efficiency depends only on q_C/q_H. Because q_C and q_H correspond to heat transfer at the temperatures T_C and T_H, respectively, q_C/q_H should be some function of these temperatures: :

\frac = f(T_H,T_C)

(3) Carnot's theorem states that all reversible engines operating between the same heat reservoirs are equally efficient. Thus, a heat engine operating between T₁ and T₃ must have the same efficiency as one consisting of two cycles, one between T₁ and T₂, and the second between T₂ and T₃. This can only be the case if: :

q_ = \frac

which implies: :

q_13 = f(T_1,T_3) = f(T_1,T_2)f(T_2,T_3)

Since the first function is independent of T₂, this temperature must cancel on the right side, meaning f(T₁,T₃) is of the form g(T₁)/g(T₃) (i.e. f(T₁,T₃) = f(T₁,T₂)f(T₂,T₃) = g(T₁)/g(T₂)· g(T₂)/g(T₃) = g(T₁)/g(T₃)), where g is a function of a single temperature. We can now choose a temperature scale with the property that: :

\frac = \frac

(4) Substituting Equation 4 back into Equation 2 gives a relationship for the efficiency in terms of temperature: :

\textrm = 1 - \frac = 1 - \frac

(5) Notice that for T_C = 0 K the efficiency is 100% and that efficiency becomes greater than 100% below 0 K. Since an efficiency greater than 100% violates the first law of thermodynamics, this implies that 0 K is the minimum possible temperature. In fact the lowest temperature ever obtained in a macroscopic system was 20 nK, which was achieved in 1995 at NIST. Subtracting the right hand side of Equation 5 from the middle portion and rearranging gives: :

\frac  - \frac = 0

where the negative sign indicates heat ejected from the system. This relationship suggests the existence of a state function, S, defined by: :

dS = \frac

(6) where the subscript indicates a reversible process. The change of this state function around any cycle is zero, as is necessary for any state function. This function corresponds to the entropy of the system, which we described previously. We can rearranging Equation 6 to get a new definition for temperature in terms of entropy and heat: :

T = \frac

(7) For a system, where entropy S may be a function S(E) of its energy E, the temperature T is given by: :

\frac = \frac

(8) The reciprocal of the temperature is the rate of increase of entropy with energy.

References

External links

- [http://www.unitconversion.org/unit_converter/temperature.html Online Temperature Converter] - convert between various units of temperature, such as kelvin, Celsius, Fahrenheit, Rankine, Reaumur, and even Triple point of water
- [http://www.unitconversion.org/unit_converter/temperature-v.html Interactive Temperature Conversion Table] - convert selected unit to all other units of temperature
- [http://www.indiana.edu/~animal/fun/conversions/temperature.html Temperature Conversions: Celsius, Fahrenheit, Kelvin, Réaumur and Rankine]
- [http://www.unidata.ucar.edu/staff/blynds/tmp.html An elementary introduction to temperature aimed at a middle school audience]
- [http://www.straightdope.com/mailbag/mtempscales.html Why do we have so many temperature scales?]
- [http://thermodynamics-information.net A Brief History of Temperature Measurement] Category:Meteorology Category:Physical quantity Category:Thermodynamics Category:Heat ko:온도 ja:温度 th:อุณหภูมิ

Standard atmospheric pressure

Atmospheric pressure is the pressure above any area in the Earth's atmosphere caused by the weight of air. Standard atmospheric pressure (atm) is discussed in the next section. Air masses are affected by the general atmospheric pressure within the mass, creating areas of high pressure (anti-cyclones) and low pressure (depressions). As elevation increases, fewer air molecules are above. Therefore, atmospheric pressure decreases with increasing altitude. The following relationship is a first-order approximation: :

\log_ P \approx

, where P is the pressure in pascals and h the height in metres. This shows that the pressure at an altitude of 31 km is about 10^(5-2) Pa = 1000 Pa, or 1% of that at sea level¹. A column of air, 1 square inch in cross section, measured from sea level to the top of the atmosphere would weigh approximately 14.7 lbf. A 1 m² column of air would weigh about 100 kilonewtons. See density of air.

Standard atmospheric pressure

Standard atmospheric pressure or "the standard atmosphere" (1 atm) is defined as 101.325 kilopascals (kPa). (see also Standard temperature and pressure) This can also be stated as:
- 29 117/127 inches of mercury ≈ 29.92 inHg
- 760 millimetres of mercury (mmHg) or torrs (Torr)
- 1013.25 millibars (mbar, also mb) or hectopascals (hPa)
- 14.6959 psia or 0 psig (pounds-force per square inch, absolute or gauge) (lbf/in²)
- 2116.2 pounds-force per square foot (lbf/ft²)
- 1 6517/196133 technical atmospheres (at) ≈ 1.03322745 at This "standard pressure" is a purely arbitrary representative value for pressure at sea level, and real atmospheric pressures vary from place to place and moment to moment everywhere in the world. In the United States, compressed air flow is often measured in "standard cubic feet" per unit of time, where the "standard" means the equivalent quantity of air at standard temperature and pressure. However, this standard atmosphere is defined slightly differently: temperature = 68 °F (20 °C), air density = 0.075 lb/ft³ (1.20 kg/m³), altitude = sea level, and relative humidity = 0%. In the air conditioning industry, the standard is often temperature = 32 °F (0 °C) instead. For natural gas, the petroleum industry uses a standard temperature of 60 °F (15.6 °C).

Mean sea level pressure (MSLP or SLP)

Mean sea level pressure (MSLP or SLP) is the pressure at sea level or (when measured at a given height on land) the station pressure reduced to sea level by an appropriate altitude dependant formula. This is the pressure normally given in weather reports on radio, television, and newspapers. When barometers in the home are set to match the local weather reports, they measure pressure reduced to sea level, not the actual local atmospheric pressure. The reduction to sea level means that the normal range of fluctuations in pressure is the same for everyone. The pressures which are considered high pressure or low pressure do not depend on geographical location. This makes isobars on a weather map meaningful and useful tools. The altimeter setting in aviation, set either QNH or QFE, is another atmospheric pressure reduced to sea level, but the method of making this reduction differs slightly. See altimeter.
- QNH barometric altimeter setting which will cause the altimeter to read altitude above mean sea level in the vicinity of an airfield.
- QFE barometric altimeter setting which will cause an altimeter to read height above a particular runway threshold if set to the correct field elevation while on the ground. Average sea-level pressure is 1013.25 hPa (mbar) or 29.921 inches of mercury (inHg). In aviation weather reports (METAR), QNH is transmitted around the world in millibars or hectopascals, except in the United States and Canada where it is reported in inches of mercury. (The United States also reports sea level pressure SLP, which is reduced to sea level by a different method, in the remarks section, not an internationally transmitted part of the code, in hectopascals or millibars. In Canada's public weather reports, sea level pressure is reported in kilopascals, while Environment Canada's standard unit of pressure is hectopascal.) In the weather code, three digits are all that is needed, Decimal points and the one or two most significant digits are omitted: 1013.2 mbar or 101.32 kPa is transmitted as 132; 1000.0 mbar or 100.00 kPa is transmitted as 000; 998.7 mbar or 99.87 kPa is transmitted as 987; etc. The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above 1032.0 mbar. The lowest measurable sea-level pressure is found at the centers of hurricanes (typhoons, baguios).

Atmospheric pressure variation

Atmospheric pressure varies widely on the Earth, and these variations are important in studying weather and climate. See pressure system for the effects of air pressure variations on weather.. The highest recorded atmospheric pressure, 108.6 kPa (1086 mbar or 32.06 inches of mercury), occurred at Tosontsengel, Mongolia, 19 December, 2001². The lowest recorded non-tornadic atmospheric pressure, 87.0 kPa (870 mbar or 25.69 inHg), occurred in the Western Pacific during Typhoon Tip on 12 October, 1979². The record for the Atlantic ocean was 88.2 kPa (882 mbar or 26.04 inHg) during Hurricane Wilma on 19 October 2005. Atmospheric pressure shows a diurnal (daily) rhythm. This effect is very strong in tropical zones, and almost zero in polar areas. A graph shows these rhymic variations in northern Europe on the top of this page. In tropical zones it may reach above 5 mbar variation. These variations follow a circadian (24 h) and at the same time semi-circadian (12 h) rhythm.

Intuitive feeling for atmospheric pressure based on height of water

Atmospheric pressure is often measured with a mercury barometer, and a height of approximately 30 inches of mercury is often used to teach, make visible, and illustrate (and measure) atmospheric pressure. However, since mercury is not a substance that humans commonly come in contact with, water often provides a more intuitive way to conceptualize the amount of pressure in one atmosphere. 1 atmosphere (14.7 lbf/in²) is the amount of pressure that can lift water approximately 33.9 feet (10.3 m). Thus, a diver at a depth 10.3 meters under water in the ocean experiences a pressure of about 2 atmospheres (1 atm for the air and 1 atm for the water). In terms of city water pressure, one atmosphere is approximately one-half to one-fifth the pressure of typical city water mains (i.e., water pressure is around 2 to 5 atmospheres).

References

# US Department of Defense Military Standard 810E # Burt, Christopher C., (2004). Extreme Weather, A Guide & Record Book. W. W. Norton & Company # U.S. Standard Atmosphere, 1962, U.S. Government Printing Office, Washington, D.C., 1962. # U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, D.C., 1976.

External links

- [http://www.atmosculator.com/The%20Standard%20Atmosphere.html? A detailed mathematical model of the 1976 U.S. Standard Atmosphere]

Experiments

- [http://avc.comm.nsdlib.org/cgi-bin/wiki_print.pl? An exercise in air pressure]
- [http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/patm.html#atm Movies on atmospheric pressure experiments] from Georgia State University's HyperPhysics website. (requires Quicktime) Category:Atmosphere Category:Meteorology Category:Units of pressure Category:Diving ko:대기압 ja:気圧

Standard

The word standard has several meanings: ---- Originally, standard referred to a conspicuous object used as a rallying point in battle. The term is probably from Frankish
- standhart "steadfast" (literally, "stand hard"). In the High Middle Ages, a standard was a tapering flag or ensign flown by lower ranking knights, as opposed to the square banners flown by knights banneret; The modern primary meaning evolved through symbolism: "a quality or measure which is established by authority, custom, or general consent". In the phrase "light standard" it retains the older meaning of a vertical support. ---- In technical use, a standard is a concrete example of an item or a specification against which all others may be measured. In this meaning, the word related to Old French estendre "extend", and is attested in the meaning "unit of measure" in Anglo-French from 1327. For example, there are "primary standards" for length, mass (see Kilogram standard), and other units of measure, kept by laboratories and standards organizations. Officially certified measuring instruments must be checked for accuracy using such standards (or secondary standards made from the primary). See examples in chemistry below. Other technical standards define a set of properties that a product or service should have. There are "voluntary" standards to which the producers adhere voluntarily. Such standards are laid down by an organization gathering representatives of producers and users of the type of product or service. There are also "mandatory" or "regulatory" standards with which products and services have to comply by Law i.e. imposed by a regulator, their genesis is however similar to the voluntary ones. A "de facto" standard rather is a set of properties of an outstanding member of a product category, which the user community requires without that they have been laid down formally by a producer/user organization. ---- In analytical chemistry a standard is a preparation containing a known concentration of a specified substance. A simple standard may be a dilute solution of the substance; this serves as a reference to calibrate equipment used to measure a sample's composition in terms of compounds or elements. For accuracy, real samples are bracketed by known standards, that is, standards are analyzed that contain concentrations of the analyte that are less than and greater than the real sample's concentration. There are also certified reference materials available which contain independently verified concentrations of elements available in different matrices (a matrix is bulk material of the sample, for example blood). ---- Some of the succesors to the Standard Oil Trust formerly used Standard as a brand name. These included Amoco in the Midwest United States before their merger with BP, Chevron, Exxon, and Sohio. ---- The Standard Motor Company made cars in England from 1903 to 1963. ---- The Standard type battleship was a series of US Navy battleships with relatively homogenous handling characteristics including a 21 knot flank speed and a 700 yard tactical diameter at flank speed. There were five classes in the Standard program, plus a sixth which was canceled: Nevada class, Pennsylvania class, New Mexico class, Tennessee class (called in contemporary European publications the California class, as USS California (BB-44) was commissioned first) and Colorado class (called in contemporary European publications the Maryland class for the same reasons as above). The class which was canceled and broken up was the BB-49 South Dakota class. ---- The RIM-66 Standard is a widely used medium to long range surface to air missile in use with many navies around the world and (until 2003) with the United States Navy. In the USN it has been replaced with the RIM-67 Standard II (which has considerably enhanced compatibility with the Aegis combat system). ---- In linguistics, a standard can refer to either a written standard, an endorsed "proper" way of spelling words or even of constructing sentences, or to a spoken standard or pronunciation standard: an endorsed way of pronouncing the words of a language. In English, which has no legal or international standards, for instance, "The Queen's English" is widely regarded (especially outside the United States) as the "proper" pronuciation. Unofficial spelling standards for English also exist in various countries, especially where the language is dominant. For example, the "American standard" spelling for "harbor" and "defense" is at odds with the "standard" spellings of most other Anglophone countries, where these words are spelled "harbour" and "defence". For other languages, such as French or Spanish, there exist centralized authorities which determine the "proper" pronunciation and spelling of each and every word in the "standard" language (see List of language regulators).

SI

The International System of Units (abbreviated SI from the French language name Système International d'Unités) is the modern form of the metric system. It is the world's most widely used system of units, both in everyday commerce and in science. The older metric system included several groupings of units. The SI was developed in 1960 from one of these, the metre-kilogram-second (MKS) system, rather than the centimetre-gram-second (CGS) system, which, in turn, had many variants. The SI introduced several newly named units. The SI is not static; it is a living set of standards where units are created and definitions are modified with international agreement as measurement technology progresses. With few exceptions (such as draught beer sales in the United Kingdom), the system is legally being used in every country in the world, and many countries do not maintain official definitions of other units. In the United States, industrial use of SI is increasing, but popular use is still limited. In the United Kingdom, conversion to metric units is official policy but not yet complete. Those countries that still recognize non-SI units (e.g. the US and UK) have redefined most of their traditional, non-SI units in terms of SI units.

History

:See main articles: metre, kilogram, second, ampere, Kelvin, and candela. The metric system was officially adopted in France after the French Revolution. During the history of the metric system a number of variations have evolved and their use spread around the world replacing many traditional measurement systems. By the end of World War II a number of different systems of measurement were still in use throughout the world. Some of these systems were metric system variations whilst others were based on the Imperial and American systems. It was recognised that additional steps were needed to promote a worldwide measurement system. As a result the 9th General Conference on Weights and Measures (CGPM), in 1948, asked the International Committee for Weights and Measures (CIPM) to conduct an international study of the measurement needs of the scientific, technical, and educational communities. Based on the findings of this study, the 10th CGPM in 1954 decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and electromagnetic quantities. The six base units recommended were the metre, kilogram, second, ampere, Kelvin degree (later renamed the kelvin), and the candela. In 1960, the 11th CGPM named the system the International System of Units, abbreviated SI from the French name: Le Système International d'Unités. The seventh base unit, the mole, was added in 1970 by the 14th CGPM. The International System is now either obligatory or permissible throughout the world. It is administered by the standards organisation: the Bureau International des Poids et Mesures (International Bureau of Weights and Measures).
Units
:Main articles: SI base unit, SI derived unit, SI prefix The international system of units consists of a set of units together with a set of prefixes. The units of SI can be divided into two subsets. There are the seven base units. Each of these base units are dimensionally independent. From these seven base units several other units are derived. In addition to the SI units there are also a set of non-SI units accepted for use with SI. A prefix may be added to units to produce a multiple of the original unit. All multiples are integer powers of ten. For example, kilo- denotes a multiple of a thousand and milli- denotes a multiple of a thousandth hence there are one thousand millimetres to the metre and one thousand metres to the kilometre. The prefixes are never combined: a millionth of a kilogram is a milligram not a microkilogram.
SI writing style

- Symbols are written in lower case, except for symbols derived from the name of a person. For example, the unit of pressure is named after Blaise Pascal, so its symbol is written "Pa" whereas the unit itself is written "pascal". The one exception is the litre, whose original abbreviation "l" is dangerously similar to "1". The NIST recommends that "L" be used instead, a usage which is common in the U.S., Canada and Australia, and has been accepted as an alternative by the CGPM. The cursive "ℓ" is occasionally seen, especially in Japan, but this is not currently recommended by any standards body. For more information, see Litre.
- Symbols are written without grammatical markers when used with singular numerals: i.e. "25 kg", not "25 kgs". Pluralization would be language dependent; "s" plurals (as in French and English) are particularly undesirable since "s" is the symbol of the second. Other cases may be marked in a language-dependent manner, e.g. Finnish 25 kg:lla = 25 kilogrammalla "with 25 kg".
- Symbols do not have an appended period (.).
- It is preferable to write symbols in upright Roman type (m for metres, L for litres), so as to differentiate from the italic type used for mathematical variables (m for mass, l for length).
- A space should separate the number and the symbol, e.g. "2.21 kg", "7.3×10² m²", "22 °C" [http://physics.nist.gov/Pubs/SP811/sec07.html]. Exceptions are the symbols for plane angular degrees, minutes and seconds (°, ′ and ″), which are placed immediately after the number with no intervening space.
- Spaces should be used to group decimal digits in threes, e.g. 1 000 000 or 342 142 (in contrast to the commas or dots used in other systems, e.g. 1,000,000 or 1.000.000).
- The 10th resolution of CGPM in 2003 declared that "the symbol for the decimal marker shall be either the point on the line or the comma on the line". In practice, the full stop is used in English, and the comma in most other European languages.
- Symbols for derived units formed from multiple units by multiplication are joined with a space or centre dot (·), e.g. N m or N·m.
- Symbols formed by division of two units are joined with a solidus (/), or given as a negative exponent. For example, the "metre per second" can be written "m/s", "m s^-1", "m·s^-1" or $\frac$ . A solidus should not be used if the result is ambiguous, i.e. "kg·m^-1·s^-2" is preferable to "kg/m/s²".
Spelling variations

- Several nations, notably the United States, typically use the spellings 'meter' and 'liter' instead of 'metre' and 'litre' in keeping with standard American English spelling. In addition, the official US spelling for the SI prefix 'deca' is 'deka'.
- The unit 'gram' is also sometimes spelled 'gramme' in English-speaking countries other than the United States, though that is an older spelling and its use is declining.
Cultural issues
The swift worldwide adoption of the metric system as a tool of economy and everyday commerce was based mainly on the lack of customary systems in many countries to adequately describe some concepts, or as a result of an attempt to standardize the many regional variations in the customary system. International factors also affected the adoption of the metric system, as many countries increased their trade. Scientifically, it provides ease when dealing with very large and small quantities because it lines up so well with our decimal numeral system. Cultural differences can be represented in the local everyday uses of metric units. For example, bread is sold in one-half, one or two kilogram sizes in many countries, but you buy them by multiples of one hundred grams in the former USSR. In some countries, the informal cup measurement has become 250 mL, and prices for items are sometimes given per 100 g rather than per kilogram. A profound cultural difference between physicists and engineers, especially radio engineers, existed prior to the adoption of the metre-kilogram-second (MKS) system and hence its descendent, SI. Engineers work with volts, amperes, ohms, farads, and coulombs, which are of great practical utility, while the centimetre-gram-second (CGS) units, which, though appropriate for theoretical physics, can be inconvenient for electrical engineering usage and are largely unfamiliar to householders using appliances rated in volts and watts. People with diabetes test their plasma glucose level regularly. In the U.S., measurement are recorded in milligrams per deciliter (mg/dL); in Europe, the standard is millimole/liter (mmol/L). The fine-tuning that has happened to the metric base units over the past 200 years, as experts have tried periodically to refine the metric system to fit the best scientific research do not affect the everyday use of metric units. Since most non-SI units, such as the U.S. customary units, are nowadays defined in terms of SI units, any change in the definition of the SI units results in a change of the definition of the older units as well.
See also

- Units of measurement
- Weights and measures
- Mesures usuelles
- Metrified English unit
- History of measurement
- Other systems of measurement:
- Imperial units
- U.S. customary units
- Metre-tonne-second system of units
- Chinese system of units
- Planck units
- Atomic units
- Geometrized units
- CODATA
- Metrication
- Metric system in the United States
- Metrology
- UTC (Coordinated Universal Time)
- Binary prefixes - used to quantify large amounts of computer data
- Orders of magnitude
- ISO 31
External links
Official
- [http://www.bipm.fr/en/si/ BIPM (SI maintenance agency)] (home page)
- [http://www.bipm.org/en/si/si_brochure/ BIPM brochure] (SI reference)
- [http://www.iso.ch/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=5448&ICS1=1 ISO 1000:1992 SI units and recommendations for the use of their multiples and of certain other units], with its price tag of 99 Swiss francs for a 22 page, coverless pamphlet showing why the public is sometimes a little slow to pick up on their recommendations. Information
- [http://physics.nist.gov/cuu/Units/index.html US NIST reference on SI]
- [http://ts.nist.gov/ts/htdocs/200/202/pub814.htm#chart chart]
- [http://www.aticourses.com/international_system_units.htm SI - Its history and use in science and industry]
- [http://www.unc.edu/~rowlett/units/ A Dictionary of Units of Measurement]
- [http://www.unics.uni-hannover.de/ntr/russisch/si-einheiten.html5 Cyrillic transcription of SI symbols]
- Judson, Lewis B., Weights and Measures Standards of the United States: A brief history, NBS Special Publication 447, orig. iss. October 1963, updated March 1976 ([http://ts.nist.gov/ts/htdocs/200/202/SP%20447.pdf 46 page PDF file])
- [http://www.france-property-and-information.com/metric_conversion_table.htm Metric system and conversion tables (courtesy French property advice)]
- [http://www.metre.info metre-info - an encyclopaedia of all metric units] Pro-metric pressure groups
- [http://www.ukma.org.uk/ The UK Metric Association]
- [http://www.metric.org/ The US Metric Association] Pro-customary measures pressure groups
- [http://www.bwmaonline.com/ The British Weights and Measures Association]
Further reading

- I. Mills, Tomislav Cvitas, Klaus Homann, Nikola Kallay, IUPAC: Quantities, Units and Symbols in Physical Chemistry, 2nd ed., Blackwell Science Inc 1993, ISBN 0632035838. Category:SI units Category:Systems of units Category:International standards Category:Dimensional analysis ko:SI 단위계 ja:国際単位系 simple:SI th:หน่วยเอสไอ

Alloy

An alloy is a combination, either in solution or compound, of two or more elements, which has a combination of at least one metal, and where the resultant material has metallic properties. An alloy with two components is called a binary alloy; one with three is a ternary alloy; one with four is a quaternary alloy. The result is a metallic substance with properties different from those of its components. Alloys are usually designed to have properties that are more desirable than those of their components. For instance, steel is stronger than iron, one of its main elements, and brass is more durable than copper, but more attractive than zinc. Unlike pure metals, many alloys do not have a single melting point. Instead, they have a melting range in which the material is a mixture of solid and liquid phases. The temperature at which melting begins is called the solidus, and that at which melting is complete is called the liquidus. Special alloys can be designed with a single melting point, however, and these are called eutectic mixtures. Sometimes an alloy is just named for the base metal, as 14 karat (58%) gold is an alloy of gold with other elements. The same holds for silver used in jewellery, and aluminium used structurally. Alloys include:
- aluminium bronze
- alnico
- amalgam
- brass
- bronze
- duralumin
- electrum
- galinstan
- intermetallics
- Mu-metal
- Nichrome
- pewter
- phosphor bronze
-