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International System of Units overhauled in historic vote

npl.co.uk

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Re: International System of Units overhauled in historic vote

#161

Post by yesenadam »

majewsky wrote: post #90
TylerE wrote: post #67

Earlier quoted context omitted.

Odd choices... since 1000 seconds is ~17 minutes and 1M seconds is 4.6 hours.

$ units --verbose Currency exchange rates from FloatRates (USD base) on 2018-10-25 3070 units, 109 prefixes, 109 nonlinear units You have: 1 megasecond You want: 1 day 1 megasecond = 11.574074 * 1 day 1 megasecond = (1 / 0.0864) * 1 day

Gee, I never knew about units, thank you. Very cool. It can convert meters/s to furlongs/fortnight, cm^3 to gallons.. Write fractions like '1|2'. And you can add your own units to the units file.

Re: International System of Units overhauled in historic vote

#162

Post by bcoates »

turdnagel wrote: post #145

So there was a definition of a kilogram based on an actual physical thing with an arbitrary amount of mass. Does this new definition essentially try to approximate that mass as close as possible? What is the margin of error there? I would love to know this for a second as well, although I don't know what we used to measure a second before we locked it to the vibrations of a cesium atom.

The Earth?

Re: International System of Units overhauled in historic vote

#163

Post by nas »

jackpirate wrote: post #153
jcranmer wrote: post #114

Earlier quoted context omitted.

Understanding mass in molar terms is necessary to do a lot of chemistry correctly. The mole is effectively a count of the number of molecules kicking about, although the count is large enough that terms like "quadrillion" don't cut it. (One mole is about 600 sextillion molecules). Knowing how many molecules are around lets you actually compute how much stuff can react in a given chemical environment, and other aspect…

I think the GP is making the claim that Avogadro's constant should be 1 (effectively eliminating it), not that there's no need to count molecules.

Yeah, I think that's his idea. So, instead of tables of molar mass in g/mol, you would have tables in terms of "amount of substance"/g. E.g. number of atoms or number of molecules.

Re: International System of Units overhauled in historic vote

#164

Post by cco »

shittyadmin wrote: post #130
Digit-Al wrote: post #119

Can anyone tell this poor ignoramus whether the kg is getting heavier or lighter - and how much by (approximately, I know it's not going to be an exact figure given the reason they are changing it)?

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, and that standard now will be unchanging?

Re: International System of Units overhauled in historic vote

#165

Post by laurentl »

piano wrote: post #21
throwaway2048 wrote: post #10

Earlier quoted context omitted.

It does actually, US measurements are legally defined in terms of metric units. https://en.wikipedia.org/wiki/United_States_customary_units

They really should cut the middle man. What surprised me reading that wiki page is that apparently the metric system was opposed in the US based on religious reasons . That is mind-boggling.

Well, you can’t suddenly replace the quarterpounder with cheese with a royale with cheese.

Re: International System of Units overhauled in historic vote

#166

Post by aylmao »

XorNot wrote: post #36
3pt14159 wrote: post #12

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?

As others have noted, knowing the count of entities (note: not atoms, entities - i.e. it can also be molecules) in relation to actual mass is very useful for the physical sciences - at a molecular level the quantities of molecules and atoms interacting matter, not their masses - but mass is the unit I do experiments with. EDIT: A simple example would be if you were trying to make water - H2O, from hydrogen (H2) and o…

I think what the commenter is referring to is why use Avogadro's number instead of say, just Sextillions or Septillions.

Re: International System of Units overhauled in historic vote

#167

Post by madhadron »

3pt14159 wrote: post #12

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?

Historically, the mole predates the acceptance of atomic theory. Stoichiometry of various reactions let you work out that there was some mass of oxygen that would entirely react with some mass of carbon to form carbon monoxide. They didn't have the same masses, but the relative amounts for each element were constant (or small integer multiples, such as twice as much oxygen to carbon for CO_2).

So Dalton took the lightest one, hydrogen, and defined a mole as the stoichiometric amount equivalent to that in 1g of hydrogen. Looked at that way, it's a pretty solid choice.

Re: International System of Units overhauled in historic vote

#169

Post by kibwen »

njarboe wrote: post #107
MichaelDickens wrote: post #73

Earlier quoted context omitted.

You could say the same thing about most Imperial units, but most people still prefer the metric system.

I would say most people prefer the one they learned first. For doing science metric is usually better. Easier unit conversions. I think inches/feet are better for construction as you have more options at what level of accuracy you want to work with and splitting things up unless you need to have a fifth of something.

And to think, we could unify Imperial and metric if we just shift the whole world to a duodecimal number system! :P

Re: International System of Units overhauled in historic vote

#170

Post by 24gttghh »

conbandit wrote: post #131
24gttghh wrote: post #118

Earlier quoted context omitted.

It is still not apparent to me. Is a kilogram of something now it's joules x seconds or something? Does the kilogram now vary depending on what the kilogram is of? Like, is a kilogram of Iron a different mass from a kilogram of Aluminum? >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, wh…

I believe it means something like: Given the definition of metre and the definition of second, the kilogram is whatever value that makes the Planck constant h precisely 6.62607015×10−34 kg m^2 s^(-1).

But isn't that a bit of a recursive definition? What might one use as the value of kg in that example?
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