I am sure they had good reasons for their decision but I gotta say the Kibble Balance (aka Watt Balance) method to measure Planck’s constant seems hugely complex.
Just watching that video there is so much going on (e.g. measuring the length of wire in the contraption, measuring the gravity differentials every few feet in the chamber where the experiment is done, etc.). With so many bits and pieces it seems it would be impossible for different labs to ever get the same result. Sure they would be close but could they really replicate it all perfectly?
The sphere approach seems simpler. A sphere made of silicon of exactly a given diameter is a kilogram. Another attempt won’t get exactly the same diameter but they’d get pretty close and be able to calculate the number of atoms difference pretty easily to get the right answer.
I guess the thing here is that one experiment literally becomes THE definition of the kilogram and anyone else who replicates it will not be the kilogram and will have to note their offset from that one experiment.
F = ma is the same equation as m = F/a, and no matter how you write that equation, it makes no sense to call a unit of force by the same name as a unit of mass.
And a mole is most certainly absolutely not “the amount of matter in an object”. A mole is a count, like a dozen. A count of what? It could be anything. You could have a mole of atoms, or a mole of ions, or a mole of molecules, or a mole of small burrowing mammals. Well, OK, you couldn’t have that last one, since it would be a sizable fraction of the mass of the Earth, but in principle, you could refer to a mole of anything.
The mole is the SI base unit for the amount of a substance, and yes it is a bridge between the atom and the macroscopic quantities we use. In the case of SI mass is a measure of a bodies inertial property or the property of matter that measures its resistance to acceleration. We could get into the whole complexity about invariant mass, rest mass, intrinsic mass, proper mass, and relativistic mass but there is no need to.
For common use, when you care about mass more than forces it makes sense to simplify. If I need to go on a diet or buy some coffee why complicate with densities etc…when the mass works out within my needs. I don’t care the force I am exerting on the ground when I am on a diet…nor do I care about how many atoms or particles I gain or loose. The needs just don’t require the accuracy or precision in this context, and I never plan to travel to another planet.
But if you are arguing that the SI Kilogram is not a measure of mass, which in this context is a measure of resistance to acceleration, please provide a cite. In this context mass is a quantity that only depends on inertia. While it is a useful approximation of the amount of “stuff” it is not a measure of the amount of stuff.
But to answer your question, I would take the sample mass divided by the molar mass of the substance. Water is ≈ 18.015 g/mol as an example for molar mass.
How do you define “stuff”, such that the amount of it is not mass?
And a mole isn’t a measure of substance, or of anything else. You can’t say “What’s the mole of that box?”. You can’t even ask “How many moles are in that box?”. You could ask something like “How many moles of molecules are in that box?”.
The entire point of the mole is that it is a number. Chemical compounds are ratios of the count of constituent element atoms, and in order to provide a bridge between the mass of reagents and products, and to provide a quantitative approach to stoichiometric chemistry a number was needed of a suitable size.
The BIPM definition cited above uses the word “amount” to mean something very specific…
The mole isn’t a unit of measure of stuff, it is a count of constituent elementary components of stuff. If your stuff isn’t made up of elementary entities that can be counted, you can’t define its constituents in moles. If you describe a lump of stuff in moles, you must know what the elementary components it is made of are. Otherwise you can’t define the measure. Stuff that doesn’t have an easy discrete definition of structure gets hard. You can’t say you have a lump of a mole of steel. A lump of pure iron yes.
This is a good description of how standards are used and maintained in a working lab, but not the best example.
For a couple hundred thousand dollars (not a big number if you’re manufacturing systems worth hundred’s of millions of dollars) you can own a primary voltage standard. It will be as accurate as any other primary voltage standard and if your lab participates in the periodic comparisons of national standards, it will actually be a traceable source standard.
The voltage standard is not only an example of a standard that depends only on physical constants (e, h) and universally measurable quantities (f), it is also an example of reducing the lab setup to an off-the-shelf turnkey product.
I had to pick the nit on this one, even though its a great description otherwise…
This is just a side discussion anyway, but not all things that have mass are massive and so matter simply doesn’t count here.
I didn’t choose the SI base quantities, nor did I choose their definitions. The problem with visualizing a mole of something because Avogadro’s constant is extremely large, not because it isn’t the unit of quantity which my cite provided it is under SI.
To solve the same problem knowing the mass requires knowing the Molar mass, or similar to the following.
g * (1/(mol/g) * (atoms/mol)) = Atoms (or molecules)
While the SI system fails in both quantum and relativistic domains, explain to me how a hypothetical weightless box of mirrors full of photons will have “mass” or other situations when the term “matter” doesn’t quite work when trying to use kilograms as a measure of “stuff”
The kilogram and mass in SI is purely dependent on inertia, and this is true despite issues with usability of the chosen base unit for for quantity.
Irrespective of your concerns about practical visualization of a mol
I cannot defend or justify the SI design choices, but if this isn’t true you should be able to provide a cite.
“Mass is the amount of matter in an object” simply doesn’t work post John Dalton’s atomic model. Mass being a quantity of matter and an inertial property works in Newton’s Philosophiæ Naturalis Principia Mathematica because he didn’t know about atoms etc…
If you are making the claim that SI uses the density and volume form from Newton:
It should be pretty easy to show how the Kilogram uses density and volume, but BIPM and modern classical mechanics tends to define it according to Newton’s second law, where a body has a mass m if, at any instant of time, it obeys the equation of motion (F =ma).
You are trying to re-debate operationalism, Mach did enough of that at the time to impact how SI was set up. To avoid recreating Mach’s arguments, which seem to have gained favor by the BIPM, here’s a link to Mach’s book and page.
Please do point me to a source that shows that the BIPM uses the primitive form of mass as a “quantity of matter” as I obviously don’t know the Metric system very well if I need to consider anything else except inertia for the kilogram.
Am I the only one who finds it odd ampere is a fundamental unit, but coulomb is not?
I mean, ampere is coulombs per second, right? The second is a fundamental unit. One would think the coulomb would also be a fundamental unit, and therefore ampere would simply be a combined or derived unit (C/s).
But really it is probably because they could figure out how to derive the ampere and because moving from the old CGS system to the MKS system that SI adopted in 1960 was due to problems with Electric and Magnetic Units.
rat avatar, what you’re talking about has nothing to do with relativity or quantum mechanics, and dragging them in just confuses matters for no good purpose. And no issues arise from Avogadro’s number being large; in fact, it would be useless if it were not large. But even if we consider a smaller unit of the same sort, the problem with your argument still become clear: One would never, for instance, ask “What is the dozen of the eggs in my refrigerator?”, which is how you’re claiming moles would be used.
And what on Earth could you possibly mean when you say that “not all things that have mass are massive”? That’s literally the exact definition of “massive”.
I EXCLUDED relativity or quantum mechanics to avoid that.
But as the newton and the MKS system was added because of the needs of electromagnetic energy it cannot be ignored in this case. While it is a common claim, I will note you still haven’t shown any cites that “mass” in the context of SI was intended to be a unit of the amount of substance.
The SI unit of force is the newton, but that is a technical unit of measure introduced in 1960 which replaced the dyn from CGS and the kilopond from the Metric system.
It is absurd to claim that the committee restricted their ideas to 1687 and Newton’s Principia. The base units of candela and mole were added on purpose and the form of “mass” they use is related to F=ma.
Resolution of the 3rd CGPM (1901)
Matter being any substance that has mass and takes up space by having volume does not include electromagnetic radiation in non-quantum/relativistic contexts. But as you keep arguing for 1687 I did error to saying that “massive” was related to “matter” and thus light etc… wouldn’t take up space.
But as I have provided cites and you have not provided a single one, but lets see about a coherent system of measure.
Explain to me how you account for changes in chemical energy changing inertia with your model of mass being a sum of all parts and a measure of quantity. Then show me where density is defined in Newton Principia.
I get that you find this convenient, and it may be acceptably precise for your needs but it is not precise enough for a coherent system of measurement.
Density is the mass per unit of volume and the SI derived unit for volume is cubic meters, explain how you can match the Newtonian “amount of stuff” definition of “The quantity of any matter is the measure of it by its density and volume conjointly” while still having mass be a base unit?
There is a good reason that they got rid of the volume of water version of the kilogram from the Metric system and created the IPK in the first place. As this is GQ I am not going to abandon the evidence that says that kilogram is only tied to the inertial properties without citations.
OK, I can’t keep up with all of this, but where did I ever say that mass was “a sum of all parts and a measure of quantity”? A mole is a quantity. Mass is an amount. You’re the one who is saying otherwise. And who’s talking about density?
I suppose Le Grande K is made out of Platinum-iridium and kept under 3 bell jars-worth of inert gas to prevent oxidation…adsorbing atmospheric oxygen would cause a microscopic addition of mass.
You didn’t read that page, did you? You are conflating the use of SI vs US customary with the difference between mass and weight.
This NIST page talks about it. Essentially, they started trying to define the gram in terms of the cm, because they thought that size would be convenient, but decided it wasn’t convenient after all. So they reverted to defining the kg.
I came across this in college once, but SI doesn’t use the kilopond or kilogram-force. The SI unit of force is the Newton.
The root of this problem is the history of how the terms originated. The pound was originally a measure of force and mass - they were considered the same thing. With Newton came the understanding of the difference, and the need to separate the units because the relationship is offset by the acceleration of gravity (standard 32.2 ft/sec[sup]2[/sup]). Slugs aren’t the problem, the problem is the pound mass. Whenever you use that, you have to use a conversion factor, 32.2 ft-lbm/lbf-sec[sup]2[/sup].
A mole is a count of matter, which is a form of amount. Mass is a different measure of amount - the resistance to acceleration.
If you are trying to determine the amount of rocket fuel, you definitely need mass. You have to account for the mass of the fuel that is being used, and the momentum transfer.
No. The whole point is to move away from having to measure a specific item each time you want to compare. The standard is being based on unchanging fundamental constants. The experiments are being used to refine the most precise values for those constants given the existing base standards. It is a cumulative result from numerous experiments.
Because 