221–230 of 589 posts
This is probably the dumbest thing I'll type on HN. In university I just gave up trying to understand why we even needed the Avogadro constant / mole as a fundamental constant. It still confuses me. Why have a difference between molar mass and mass? Why couldn't it just be "1" and everything else change around it?
The candela is a real abomination of a unit.
Earlier quoted context omitted.
An Avogadro's constant number of molecules is one mole. One mole of hydrogen has much less mass than one mole of iron. You have to choose an arbitrary mass of a single element as the base quantity. Hydrogen might be the best theoretically, it's just a proton and an electron, but it is tricky to work with because it's a gas at room temperature. So instead the mole has been defined as the number of molecules in a speci…
I think this is exactly the rationale. But, from a more practical perspective... If instead of g/mol, we referred to molecules/g -- we would end up populating tables and charts with really big numbers. This would make lookup tables hard to read, difficult to publish, and hard to work with. Imagine if you had to do math with a bunch of 10^23 exponents all of the time. Instead, it was agreed to effectively pull out a c…
What if it was just set to 10^24 then? Much easier to remember and serves the same purpose.
If it was 1 it would work too, since the SI system already has a way to deal with large numbers: prefixes. So we might wrote Ymol for yotta mole = 10^24 mol.
This is probably the dumbest thing I'll type on HN. In university I just gave up trying to understand why we even needed the Avogadro constant / mole as a fundamental constant. It still confuses me. Why have a difference between molar mass and mass? Why couldn't it just be "1" and everything else change around it?
I don't believe the Avogadro constant needs to be an SI unit. It's like having 12 as an SI unit, or a million, or 1. Sure, it's a useful constant scaling factor, but it doesn't need to be canonized at the heart of the SI system. It doesn't actually express anything fundamental about how we measure our universe. And if nothing else, it can be derived: a mole is the number of atoms in a kilogram of carbon-12. Done.
No longer is. Not quite.
Has a "metric" version of time ever been proposed? Seems odd to me that we're "ok" with divisions of 12 for time but not length. I guess for science seconds are basically the only measure of time that usually matters?
And really, you can use kilo seconds or terra seconds just as easily as micro seconds. Its just harder to relate to as human time-scales.
Earlier quoted context omitted.
It just doesn't have the same ring to it. "He won't move an inch," or "He won't move a centimeter." "She won't quit until she's six feet under," or "She won't quit until she's two meters under." "I'll go the whole nine yards," or "I'll go the whole nine meters." "You can see for miles and miles," or "You can see for kilometers and kilometers."
In case you are not joking... in metric countries we still say the same sentences you say, with miles and inches. That doesn't mean we use the olde units to actually measure something...
I get that this is a significant milestone but this seems extremely overstated: "the new changes will have wide-reaching impact in science, technology, trade, health and the environment, among many other sectors." Am I missing something?
With the old definitions, the values of the known constants were just approximations, but now that the unit is defined based on the constants, we can consider the constant as being the exact value, no matter how imprecise it was when it was measured using old definitions of the units.
Earlier quoted context omitted.
This doesn't impact the actual weight of a kg. What this impacts is how the kg is defined, no longer does it rely on a physical kg. It used to literally be defined as the mass of "IPK" - the international prototype kilogram. https://en.wikipedia.org/wiki/Kilogram#International_prototy... This switch now defines it in terms of the force equivalent of the energy exerted by a single photon - if my understanding is corre…
That doesn't make sense to me, the definition has changed. The IPK was 1 kg and the copies of it were all slightly heavier or lighter, for argument let's say IPK1 was 1.1 kg and IPK2 was 0.9 kg. Now that we have a constant for the mass of 1 kg, all of those IPK's will now have a new mass relative to the new definition. Unless the new definition was based on the current IPK and set its mass as of today as the standard…
Correct. The current IPK will still be 1kg to within margin of error at the time the new definition comes into effect, but any further changes to it will make it, for the first time, drift away from that value.
I know several bipm participant nations had clones but they were always understood to be daughter weights.
Level surfaces (for instance) used to be made as triplets and never pairs: pairs can form lenses.
Not that a kibble balance definition isn't better: I just don't understand why the kg definition was a Singleton and not statistically satisfied.
Earlier quoted context omitted.
An Avogadro's constant number of molecules is one mole. One mole of hydrogen has much less mass than one mole of iron. You have to choose an arbitrary mass of a single element as the base quantity. Hydrogen might be the best theoretically, it's just a proton and an electron, but it is tricky to work with because it's a gas at room temperature. So instead the mole has been defined as the number of molecules in a speci…
I think this is exactly the rationale. But, from a more practical perspective... If instead of g/mol, we referred to molecules/g -- we would end up populating tables and charts with really big numbers. This would make lookup tables hard to read, difficult to publish, and hard to work with. Imagine if you had to do math with a bunch of 10^23 exponents all of the time. Instead, it was agreed to effectively pull out a c…
I came to complain about the article calling the mole a "base unit of the SI", and this seems like an appropriate thread.
Why is the mole a defined unit at all? As far as I understand things, "one mole" is the same thing as Avogadro's number -- neither can be a unit, because they're both dimensionless constants (well, they're both one and the same dimensionless constant). Applying actual units, "one mole of water molecules" is the same thing as "Avogadro's number of water molecules". Avogadro's number, and therefore the mole, is the conversion factor between atomic mass units and grams. Similarly, 3 is the conversion factor between feet and yards, but nobody thinks 3 is a fundamental base unit of the imperial system. The foot is a base unit of the imperial system, measuring length, the yard is a non-base unit also measuring length, and 3 is a number with no special relationship to the system at all. It would be total nonsense to say that yards are defined by reference to 3. How is Avogadro's number different?
Wouldn't "fixing the glitch" be abandoning the idea of calling the mole a unit in the first place?
Earlier quoted context omitted.
I like to commend my American friends for their undying loyalty to the memory of the Empire. U.S. customary units are derived from units that helped the British conquer the world, but even the British themselves have shamelessly abandoned them for the units of a filthy, monarch-less republic. It's good to see that Americans still subconsciously yearn for the firm ruling hand of their rightful Queen. Note: Americans h…
It just doesn't have the same ring to it. "He won't move an inch," or "He won't move a centimeter." "She won't quit until she's six feet under," or "She won't quit until she's two meters under." "I'll go the whole nine yards," or "I'll go the whole nine meters." "You can see for miles and miles," or "You can see for kilometers and kilometers."