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PowerPedia:Kilogram

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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 There was an error working with the wiki: Code[7] of pure Water at a Temperature of 3.98 degrees There was an error working with the wiki: Code[8] and There was an error working with the wiki: Code[9].

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 There was an error working with the wiki: Code[1] in the definition of the kilogram.

To avoid these problems, the kilogram was redefined as precisely the mass of a particular There was an error working with the wiki: Code[2] of There was an error working with the wiki: Code[3] height and diameter, and is kept at the There was an error working with the wiki: Code[4]. The international prototype kilogram was made in the There was an error working with the wiki: Code[10].

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 There was an error working with the wiki: Code[11]s. 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 There was an error working with the wiki: Code[12], 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 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 There was an error working with the wiki: Code[13]es are applied.

The gram was the base unit of the older There was an error working with the wiki: Code[14] 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 There was an error working with the wiki: Code[15] approach attempts to define the kilogram as a fixed number of There was an error working with the wiki: Code[16] atoms. As a practical realization, a There was an error working with the wiki: Code[17] would be used and its size would be measured by There was an error working with the wiki: Code[18].

The Ion accumulation approach involves accumulation of There was an error working with the wiki: Code[19] atoms and measuring the electrical current required to neutralise them.

Fundamental-constant approaches

The There was an error working with the wiki: Code[20] uses the There was an error working with the wiki: Code[21] that was formerly used to define the Ampere to relate the kilogram to a value for There was an error working with the wiki: Code[22], based on the definitions of the Volt and the There was an error working with the wiki: Code[23].

The levitated There was an error working with the wiki: Code[24] 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 There was an error working with the wiki: Code[5] (CIPM (1988) Recommendation 1, PV 56 19) and There was an error working with the wiki: Code[6] (CIPM (1988), Recommendation 2, PV 56 20) constants have been given conventional values, it is possible to combine these values (KJ &#8801 4.835 979×1014 Hz/V and RK &#8801 2.581 280 7×104 &#937) with the definition of the Ampere to define the kilogram as follows:

:The kilogram is the mass which would be accelerated at precisely 2×10-7 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 × 1018 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 There was an error working with the wiki: Code[25] (kp). An object of mass 1 kg at the surface of the There was an error working with the wiki: Code[26] will be subjected to a gravitational force of approximately 9.80665 There was an error working with the wiki: Code[27]s (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 There was an error working with the wiki: Code[28], a standard measure of gravitational acceleration.)

Examples

Attogram: a research team at There was an error working with the wiki: Code[29] made a detector using There was an error working with the wiki: Code[30] There was an error working with the wiki: Code[31]s with sub-attogram sensitivity.

Yoctogram: can be used for masses of There was an error working with the wiki: Code[32]s, Atoms and There was an error working with the wiki: Code[33]s. 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 There was an error working with the wiki: Code[34]: 1 There was an error working with the wiki: Code[35] = 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 an Electron: 0.000 91 yg

Mass of a There was an error working with the wiki: Code[36] : 1.672 6 yg

Mass of a There was an error working with the wiki: Code[37]: 1.674 9 yg

SI multiples

{|class="wikitable"

! Multiple

! Name

! Symbol

!

! Multiple

! Name

! Symbol

|-

|100

| gram

|g

|

|&nbsp

|&nbsp

|&nbsp

|-

|101

| decagram

|dag

|

|10–1

| decigram

|dg

|-

|102

| hectogram

| hg

|

|10–2

| centigram

| cg

|-

|103

| kilogram

|kg

|

|10–3

| milligram

| mg

|-

|106

| megagram

|Mg

|

|10–6

| microgram

| µg

|-

|109

| gigagram

|Gg

|

|10–9

| nanogram

|ng

|-

|1012

| teragram

|Tg

|

|10–12

| picogram

|pg

|-

|1015

| petagram

|Pg

|

|10–15

| femtogram

|fg

|-

|1018

| exagram

|Eg

|

|10–18

| attogram

|ag

|-

|1021

| zettagram

| Zg

|

|10–21

| zeptogram

| zg

|-

|1024

|yottagram

|Yg

|

|10–24

| yoctogram

| yg

|-

|}

External links

National Physical Laboratory FAQ on kilogram definition, the need for a new definition, and some alternatives

Conversion Calculator for Units of MASS (& Weight)

More on the NIST Watt Balance

More on the Avogadro project

Conversion: Units of Weight

Le Bureau International des Poids et Mesures

Attogram Detection

World's most sensitive scales weigh a zeptogram, by New Scientist.com

Scales tip with tiniest mass yet, by BBC News Online

There was an error working with the wiki: Code[38]

There was an error working with the wiki: Code[39]

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