kg definition calculated using constants not standard weight comparison.

[DrP]

I used to think (because we were told this in school) that the standard definition of a kilogram was the mass of 1 litre of water at stp.  I guess this definition could be ambiguous anyway what with isotopes in the water and the accuracy of measuring the litre exactly.

They have apparently got a way to calculate 1 kilogram and to define it with that calculation from Plank's constant. I did not see the derivation in this New Scientist article. I'll try to look it up sometime unless anyone knows what the relationship is between Plank and the kilo and can post it?

Here is their headline anyway:

https://www.newscientist.com/article/2203686-the-kilogram-has-been-transformed-today-as-new-definition-takes-hold/?utm_medium=SOC&utm_source=Facebook&fbclid=IwAR0fnJFJ66VpfxvCjh07GfXj1rkRus37xmv3d9zKH4I_DJVCSfAF-L04Cwg#Echobox=1558353102

 

 

[Strange]

We had a thread on this a while ago. I'll see if I can find it ...

[Later that day]

Here: https://www.scienceforums.net/topic/117010-new-defintion-of-kilogram

But the useful bit was the link to the description of how it works: https://www.nist.gov/si-redefinition/kilogram-kibble-balance

 

[DrP]

18 minutes ago, Strange said:

We had a thread on this a while ago

yea - I thought there might have been...  I did a quick search to try to find it because I thought I'd read something... but couldn't find it.

Please feel free to delete this or merge with the other.  My question still stands though.  Does anyone know the mathematical relationship between the kilogram and plank's constant?  I might try to look for it later when not at work....

 

....aaaaand it's in your link already  -  thanks.

Edited May 21, 2019 by DrP

[tuco]

CODATA recommended values of the fundamental physical constants: 2014* - https://ws680.nist.gov/publication/get_pdf.cfm?pub_id=920687

----

The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 x 10–34 when expressed in the unit J s, which is equal to kg m2 s–1, where the metre and the second are defined in terms of c and Cs.     This definition implies the exact relation h = 6.626 070 15 x 10–34 kg m2 s–1. Inverting this relation gives an exact expression for the kilogram in terms of the three defining constants h, Cs and c: which is equal to The effect of this definition is to define the unit kg m2 s–1 (the unit of both the physical quantities action and angular momentum). Together with the definitions of the second and the metre this leads to a definition of the unit of mass expressed in terms of the Planck constant h.

https://www.bipm.org/en/measurement-units/

Edited May 21, 2019 by tuco