Witty Rejoinder said:
dv said:
In principle, I approve
You’re not the boss of mass!
Dare I say it. I was hoping for a different result. I was hoping that the kilogram would be defined to be an exact number of atoms of a given isotope. That’s the most direct measurement of mass.
For more on the new system, see:
https://en.wikipedia.org/wiki/Proposed_redefinition_of_SI_base_units
There will still be the same seven base units (second, metre, kilogram, ampere, kelvin, mole, and candela). Of these, the kilogram, ampere, kelvin and mole will be redefined by choosing exact numerical values for the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively.
Kilogram
Current definition: The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.
Proposed definition: 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 6.62607015×10−34 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 ΔνCs.
A consequence of this change is that the new definition of the kilogram is dependent on the definitions of the second and the metre.
Ampere
The definition of the ampere is due to undergo a major revision.
Current definition: The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2×10−7 newton per metre of length.
Proposed definition: The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602176634×10−19 when expressed in the unit C, which is equal to A⋅s, where the second is defined in terms of ΔνCs.
Since the current definition contains a reference to force, which has the dimensions MLT−2, it follows that in SI the kilogram, metre, and second, the base units representing these dimensions, must be defined before the ampere can be defined. Other consequences of the current definition are that in SI the value of vacuum permeability (μ0) is fixed at exactly 4π×10−7 H⋅m−1. Since the speed of light in vacuum © is also fixed, it follows from the relationship …
Kelvin
The definition of the kelvin is due to undergo a fundamental change. Rather than using the triple point of water to fix the temperature scale, the proposal recommends that the energy equivalent as given by Boltzmann’s equation be used.
Current definition: The kelvin, unit of thermodynamic temperature, is 1/273.16 of the thermodynamic temperature of the triple point of water.
Proposed definition: The kelvin is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380649×10−23 when expressed in the unit J⋅K−1, which is equal to kg⋅m2⋅s−2⋅K−1.
Mole
A near-perfect sphere of ultra-pure silicon – part of the Avogadro project, an International Avogadro Coordination project to determine the Avogadro number.
Current definition: The mole is the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
Proposed definition: One mole contains exactly 6.02214076×10^23 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol−1 and is called the Avogadro number.
The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.
Candela
The proposed definition of the candela is effectively the same as the current definition, with the only difference being that the additional rigour in the definition of the second and metre will propagate to the candela.
