Talk:SI base unit

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1 Removed table
2 Amount of substance?
3 WTF?
4 kilograms vs. grams
5 ampere:newton
6 time?
7 circular references
8 Candela
9 years?
10 Where is volume in the chart and why is the chart hidden
11 Different information in different places

[edit] Removed table

here's the table I took out in case eveyone wants it back:

Physical quantity	Symbol	Name of SI base unit	Symbol for SI unit	Remarks
length	$l$	metre	m	One metre is defined as the distance light travels in a vacuum in 1/299792458 second. This standard was adopted in 1983, when the speed of light in vacuum was defined to be precisely 299792458 m/s.
mass	$m$	kilogram	kg	One kilogram is defined to be the mass of a specific cylinder of platinum-iridium alloy, kept at the International Bureau of Weights and Measures (near Paris).
time	$t$	second	s	One second is defined as the time required for 9192631770 cycles of a hyperfine transition in cesium 133. This definition was adopted in 1967.
electric current	$I$	ampere	A
thermodynamic temperature	$T$	kelvin	K
amount of substance	$n$	mole	mol
luminous intensity	$I v$	candela	cd

[edit] Amount of substance?

Is 'amount of substance' the technical name for what a mole is a unit of? It's not really an amount; it's a number. That is, you apply it to things you can have fewer of, not stuff you can have less of. It's definately the oddball of the bunch, being a 'dimentionless unit'. I don't really understand why it needs to be a base unit at all; it seems lie a better base unit would be 1. --Spikey 01:10, 19 Feb 2004 (UTC)

I agree that it might not be entirely accurate or completely sensible, but since the BIPM calls it "amount of substance" [1], and the NIST agrees [2], it's probably the Right Thing. - Plutor 18:58, 13 May 2004 (UTC)

Yeah, seems to me that the mole is as dimensionless as the radian. I wonder what these guys were thinging. --P3d0 21:50, Sep 18, 2004 (UTC)

[edit] WTF?

Ampere, a base unit, is defined in terms of a Newton, which is not a base unit?

Yes. I imagine that it is for convenience. If you like, you can mentally substitute "amount of force required to accelerate a one-kilogram mass at a rate of one meter per second per second" for "newton". It's also "equal to a flow of one coulomb per second" (ampere). I am not a physicist, but I don't really see any problem with defining base units in terms of non-base units — as long as there are no circular references — since those non-base units are in turn defined in terms of base units. See also: http://physics.nist.gov/cuu/Units/current.html. --Timc 19:43, 15 Jun 2004 (UTC)

[edit] kilograms vs. grams

Why list the fundamental unit as 1000 grams instead of 1 gram? 02:53, 27 Jul 2004 (UTC)

Because the kilogram is the base unit. Timc 19:07, 27 Jul 2004 (UTC)

Pascals are small enough as it is. Pascal = Force/Area = (kg m/s^2)/(m^2) = kg/m/s^2. An atmosphere (standard ground level pressure) is already 1.01325 x 10^5 Pa. -ub3rm4th 17:42, 29 Jul 2004 (UTC)

[edit] ampere:newton

"Yes. I imagine that it is for convenience. If you like, you can mentally substitute "amount of force required to accelerate a one-kilogram mass at a rate of one meter per second per second" for "newton""

Please edit this into the article 02:55, 27 Jul 2004 (UTC) what it means the MASS?

[edit] time?

This article says that 1s is the amount of time it takes for a caesium-133 to shift however many times at 0K, but there's no way of reaching 0K, I thought? I don't know for sure, but might it mean 0C, or 273K, or even 25C or something? 18:45, 25 Mar 2005

No, 0 K is correct. Although there's no way to reach absolute zero, what this simply means is that the transitions should be taken as occurring on an atom with no temperature. To relate the abstract number of transitions to an actual measurement, the temperature has to be taken into account. This is just to simplify the definition. 82.92.119.11 22:55, 13 December 2005 (UTC)

So the length of a second varies with temperature?--Eddwilson 10:52, 21 December 2006 (UTC)

The time it takes for a cesium atom to shift 9,192,631,770 times varies with temperature, and how long it would take if it were at absolute zero is defined as 1 second. Someguy1221 12:14, 24 February 2007 (UTC)

[edit] circular references

I do not understand the section titled circular references. It says there are two base units above which do not appear to be defined purely in terms of other base units. Surely that should be do appear instead of do not appear. Can anyone explain? Bobblewik 01:03, 28 September 2005 (UTC)

[edit] Candela

From Candela talk page: It is base unit, because it is connected with waveleight, and sensitivity of human eye at that waveleight. Corect formula is $1Cd= \frac {1W/683}{V(l)}$ where V(l) is the relative sensitivity of the eye at wavelength l. Values of V(l), defined by the International Commission on Illumination (CIE)[3], are available online from the Color and Vision Research Laboratories of the University of California [4] at San Diego and the University of Tübingen, Germany. read [5] Stijak 18:59, 13 March 2006 (UTC)

~~I think you mean [DELETED]~~. --Srleffler 23:36, 13 March 2006 (UTC)

I deleted the formula I had put here, because it was confusing and perhaps incorrect. A better statement would be that a 1 cd source of wavelength

λ

must have a radiant intensity given by $I(\lambda)= \frac {1}{683 V(\lambda)} \ \mathrm{W/sr}$ --Srleffler 17:15, 14 March 2006 (UTC)

The candela is a base unit by definition, but is in fact completely defined in terms of other units and an arbitrary mathematical curve (the luminosity function). One can construct an experiment to measure a light source in candelas purely using instruments calibrated to other SI base units, with no reference to a standard SI luminous intensity reference. This makes the candela definitely not a fundamental unit.--Srleffler 03:50, 14 March 2006 (UTC)

Further discussion of this issue can be found at Talk:Candela.--Srleffler 04:04, 14 March 2006 (UTC)

[edit] years?

It seems like Annum should be included as well. Although only some variations are used, Kilo-annum and Mega-annum, they do seem to be SI units with Annum as the base unit 24.35.112.221 02:48, 16 September 2006 (UTC)

[edit] Where is volume in the chart and why is the chart hidden

the chart does not contain The cubic metre (symbol m³) which is the SI derived unit of volume. Why? hey could you put Volum on the Chart

and why is the chart hidden --Antiedman 13:19, 14 October 2007 (UTC)

Most units are derived from other units (cubic meter, millitre, volt, newton,…). A few units must have arbitrary definitions so that all the other units can be derived from them. That is what the base units are. Once the metre is defined, the cubic metre can be defined from it, so cubic metre is not a base unit. --Gerry Ashton 15:36, 14 October 2007 (UTC)

As for why the chart is "hidden", it is a template, so that the same chart can easily be presented in more than one article. If you enter Template:SI_base_units in the search box you will be taken to the template, and you can even edit it. You can also click the toolbox command "What links here" (on the left edge of the screen) and discover that this is the only article using the template, although it is linked in a few things that are not articles. --Gerry Ashton 15:41, 14 October 2007 (UTC)

[edit] Different information in different places

--non registered user 12:34 16 December 2007 (GMT) "The fundamental units of the SI system are kilogram, second, ampere, kelvin, and mole. The metre and candela were formerly fundamental units but have been redefined in terms of the other units." Fundamental units I don't know which is correct but it should say the same thing on both places.

Fundamental units and base units are different concepts. You will notice, for example, that the SI base unit metre is defined in terms of the second, by making measurements that depend on the speed of light. The candella is defined in terms of the watt and the inverse of the second (Hz). The watt is further defined in terms of the kg, m, and second, or, in terms of the kg, second, and the speed of light.

While the metre and candella are not fundamental units, references to the speed of light in the definition of the metre would make dimensional analysis difficult. I've never worked with the candella, but working directly with its definition might be difficult as well. Thus, the metre and candella are retained for the purpose of defining other units and dimensional analysis. --Gerry Ashton (talk) 18:52, 16 December 2007 (UTC)