31–34 of 34 posts
Why is the kilogram the base unit instead of gram?
Acceleration due to gravity is a nice number between 0 and 10 if measured in "kilogram-meters per second squared".
Earlier quoted context omitted.
> For example, you can't define entropy as an absolute number; it needs to be assigned a unit, You can define entropy directly without reference to other units, although it's a bit awkward. Entropy is the log of the number of microstates that correspond to a system's macrostate. Concretely, if you put n mols of ideal gas molecules in a box of volume V at a pressure P and temperature T, there is some large number of m…
While you are right that this is the statistical definition of entropy, there are two issues: first, entropy is a unitless quantity, so defining it exactly doesn't actually move the ball forward in terms of defining units of measure. Second, outside of its theoretical underpinnings, physicists and chemists hardly ever talk about absolute values of entropy, they almost always use differences in entropy -- which factor…
The parent post asked about defining temperature in terms of more fundamental units. Entropy is typically written in J/K. If you treat it as dimensionless, then you get a definition of temperature in terms of energy for free. My point is that you can, in principle, actually do an experiment to make this useful.
More exciting is that the mol and Avogadro's constant will be pushed off to the side on their own. Hopefully an even more future update will remove them entirely. They're really quite redundant and pretty much only exist to facilitate the needless presence of the non-SI mass unit, the unified atomic mass unit, which thankfully will now be defined in terms of the kg instead of having two independent mass units like we…
More exciting is that the mol and Avogadro's constant will be pushed off to the side on their own. Hopefully an even more future update will remove them entirely. They're really quite redundant and pretty much only exist to facilitate the needless presence of the non-SI mass unit, the unified atomic mass unit, which thankfully will now be defined in terms of the kg instead of having two independent mass units like we…
The mole and amu's (similar concepts) will never go away. It's a fundamental concept in chemistry. Reactions happen between individual particles but counting out individual atoms or molecules to supply for a reactions is impossible or ridiculously impractical at best. So chemicals have to be measured by mass (which we can measure) and then converted into number of particles. Until a molecular counting device exists,…
With the 2018 SI change, Avogadro's constant will be defined as an arbitrary number without any physical basis, and the amu will be a constant multiple of the kg. No more 1/12 the mass of carbon 12.
Of course we'll still need a way to represent large numbers, but there's no fundamental reason it has to be such a complicated number. It could be exactly 10^24, for instance. Again, I agree this isn't going to happen because of legacy inertia.