Talk:SI base unit
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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 |
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| length | <math>l<math> | 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 | <math>m<math> | 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 | <math>t<math> | 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 | <math>I<math> | ampere | A | |
| thermodynamic temperature | <math>T<math> | kelvin | K | |
| amount of substance | <math>n<math> | mole | mol | |
| luminous intensity | <math>I_v<math> | candela | cd |
| Contents |
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] (http://www1.bipm.org/en/si/base_units/), and the NIST agrees [2] (http://physics.nist.gov/cuu/Units/mole.html), 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)
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)
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)
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)
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