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11210205 No.11210205 [Reply] [Original] [archived.moe]

Why does current have a SI base unit and not charge?

>> No.11210227

>>11210205
It's about what was a measurable quantity in the late 19th century. Counting ~1019 electrons would take a long time, but it's "straight-forward" to measure the force two wires exert on each other.
Also, consider that electric current was well-known and widely-studied for many years before the existence of electrons was known and their charge was measured. I don't know a date for the first observation of electric current, but Ohm's law was published in 1827, while the electron charge wasn't measured until 1908.
Since they were first established, we've changed our choice of fundamental units very little, and only as improved measurement technology has come along. At the moment it's still considered easier to measure the force on parallel wires than to count quintillions of electrons, so we still consider the ampere a fundamental unit and the coulomb a derived unit, defined as an ampere-second.

>> No.11210251

>>11210205
mol is the stupidest unit. it literally is a unitless unit. friggin retarded

>> No.11210261

>>11210251
Why is it unitless? Because it's based on a constant and describes the number of atoms?

>> No.11210273

>>11210261
i guess the word i'm looking for is "dimensionless". it just means "an avogadro's number of". it has no dimension, it's just a number. kilograms have the dimension of mass, seconds the dimension of time, meters the dimension of length. mol is dimensionless, it is just a number of something. you could for example have an avogadro's number of meters which would be equivalent to a mol of meters. or a mol of picoseconds would be 19,000 years. real units don't work like that. you can't have a kilogram of seconds

>> No.11210316
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11210316

>>11210273
>>11210251
Yeah it's kind of an un-unit, but it is far more useful than most other units, at least for chemistry. I mean, the other units?
[math]\hbar=m_e=e=4\pi\varepsilon_0=1[/math], done.
However, the macroscopic dynamics of a process do not depend on the mass of the substance but on the amount.

>> No.11210339

>>11210205
SI is for industrial practicality primarily, and used in science incidentally. That's why you have pants-on-head retarded units like candela as a base unit, which is based on human psychosensory perception.

>>11210227 gives a good perspective on historical roots.

>>11210261
it's like making "dozen" a fundamental unit. We already have a unit for counting; it's called 1. btw, the new definition of mol breaks the traditional interpretation of the number of atoms in 1g of C12, as in this is no longer true with the new units

IMHO, base units should describe length, mass, time, and charge (in all its varieties like electric and color charge. mass is a type of gravitational charge, but since it is also inertial mass, it probably should be given it's own unit). Candela, mol, and kelvin are fucking imbecilic units.

>> No.11210465

>>11210339
>kelvin
>imbeciliic unit
bullshit. Of course the unit's scale is arbitrary as it is cancelled out by [math]k_b[/math], but similar things are true for other units as well. For example, energy and time can be chosen unitless, because they form fourier pairs. Does that mean they should be unitless? Similarly, it makes no sense for length and time to have different units, because they are converted into one another freely using lorenz boosts.

>> No.11210679

>>11210465
time and space are not on equal footing, even when considering relativity. there is something very different about our one time dimension and three spatial dimensions, even if you need to consider combined spacetime to define invariants

>> No.11210708

>>11210273
>you can't have a kilogram of seconds
A kilogram-second is a perfectly valid unit.

>> No.11210757

>>11210679
Of course you have funkiness of the metric tensor and all which essentially means the local "otherness" of time is globally preserved, but transition from one frame to another still computationally requires matrix-vector multiplication in the simplest case, mixing the constituents. However, the point is that physics can be formulated using dimensionless quantites only, so there is no real distinction between the "arbitrariness" of giving lengths a unit and temperature a unit

>> No.11210861

>>11210708
not the same thing. a kilogram-second has dimensions mass times time. however a mol of seconds has dimensions of time. a kilogram of seconds is nonsense just like a kilogram of electric charge or a kilogram of degrees kelvin

>> No.11210937

>>11210757
>dimensionless quantites only
a dimensionless ratio might be unitless, but that doesn't make the interpretation of the numerator and denominator arbitrary. when you do shit like describe speed as a ratio wrt the speed of light, the ratio is unitless but you are still describing speed.

>> No.11210948

>>11210205
That is outdated. They switched the definition of SI this year to be closer to natural units

>2019 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

>> No.11210984

>>11210861
kilograms of seconds = kg * s

>> No.11211001

>>11210984
Not him, but the problem is that a mole of seconds gives you a quantity of seconds while a kilogram-second produces an entirely new unit, so a kilogram of seconds is meaningless. Kilogram has no associated quantity with it whereas mole has the ~6.02*10^23.

>> No.11211026

>>11211001
>>11210861
>a mol of seconds has dimensions of time
no it doesn't. it has units of mol (of seconds). also, mol is used to count discrete objects, so using it to count a continuous quantity is stupid.

>> No.11211049

>>11211026
>it has units of mol (of seconds)
>hurr durr, a dozen seconds has units of dozen (of seconds)

>also, mol is used to count discrete objects, so using it to count a continuous quantity is stupid.
whatever, then we can change the example. electric charge is discrete (even taking into account quarks) and a mol of elementary charges has dimensions of charge. spin is quantized in quantum mechanics so if you had a mol of spin then it would be units of angular momentum (or magnetic moment, depending on how your convention). either way, mols are a stupid unit because they are dimensionless. as anon said and i said before, it's like calling a "dozen" a fundamental unit. that's retarded and stop defending chemistry retardation

>> No.11211060

>>11211049
a mol not dimensionless, moran. it has units of mol

>> No.11211070

>>11210316
would you really want to express the action of earth’s orbit in units of h_bar? Seems more convenient to convert into something macroscopic, like Js.
But wait... then the conversion is just a freaking number!!?.! This is so fundamental and much more intricate!

>> No.11211095

>>11211060
no, idiot. you can't say "a kilogram has dimensions of kilogram" because that is nonsense. a kilogram has dimensions of mass. similarly, a mol has dimensions of just a number. it's just a number. like a "dozen" has units of number. not any actual unit like mass or time or length or charge. it's dimensionless

i know you probably are basing your argument on what your chemistry education has taught you, but i assure you, it is retarded.

>> No.11211098

>>11210205
You fucking disgusting lizard creature, who the FUCK do you think you are?

>> No.11211149

>>11211095
dozen and mol are units of counting, like how kilogram is a unit of mass. none of these are dimensionless. you need to take a ratio if you want a dimensionless quantity, like (x mol) / (y mol). if you treat counting units as a dimensionless quantities, then 1 mol + 1 dozen = 2 would be dimensionally consistent, which it isn't. of course, it seems like what I'm trying to say is 6.02x10^23 + 12, but that's not what the previous equation says. it seems absurd to make this distinction, but make it you must (else you are secretly doing a unit conversion in your head and not admitting to it)

>> No.11211311

>>11211149
>units of counting
what the fuck anon. units of counting are called numbers, and numbers are dimensionless. the difference between say, 12 (a dozen) or Avogadro's number (6.blah * 10^23) and 1 kilogram is that 1 kilogram corresponds to a measurable quantity of some thing. 12 doesn't. 1 kilogram does. units correspond to things in reality that we need to measure and aren't just mathematical indices. things like charge, mass, length, time, etc. a "number" of anything is just a mathematical thing that isn't a real thing we measure using rulers and clocks and scales and etc. you don't measure pure numbers using instruments.

yet a mol is just a pure number multiplicative factor of 6*10^23. just like a dozen is a multiplicative factor of 12 and a "giga" is a multiplicative factor of 10^9. is a "giga" a fundamental unit? no.

just stop bro. i'm beginning to think you are a troll.

>> No.11211437

>>11210316
Interesting unit choice. The speed of light is approximately 137?

>> No.11212789
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11212789

>>11211437
exactly!
Also, the unit of energy is simple. Consider the energies of a hydrogen like atom
[eqn]E_n=-\frac{e^2}{4\pi\varepsilon_0}\cdot\frac{Z^2}{2a_0n^2} =-\frac{1}{2}\cdot\frac{Z^2}{n^2}\text{, since}\\
a_0 =\frac{4\pi\varepsilon_0\hbar^2}{m_ee^2}=1[/eqn]
For this reason this unit system is called "atomic units".
>>11211070
That choice is specifically used in molecular physics as it produces natural scales for atomic and molecular processes (see above). The general Hamiltonian for a molecule becomes trivial, as all precators become one or an integer (charges of nuclei and electrons).

>> No.11212925

>>11212789
Yeah it's pretty good. For high energy, as you probably know, we take c = 1 instead of e, and usually we leave one remaining scale instead of taking m_e=1 (we would take the Planck mass = 1 if anything)

>> No.11212976

>>11212925
Naturally, often you eventually switch to "natural units" of system anyway, often induced by some property of your lagragian/hamiltonian/action. The Harmonic oscillator is a great example for this, where you can use the rotational symmetry in phase space (which translates in QM to the hamiltonian being invariant under fourier transformation, given the right units). Condensed matter people have it easy, they can just use their coupling constants [math]J[/math] or what have you as energy scales, the analytical solution for spherically symmetric systems in GR also essentially utilizes some conversion to natural units iirc. Atomic units are only special in fixing so many units so nicely. Though even in chemistry you usually convert the energy scale depending on what you need it for, as molecules have a notorious threefold scale separation (rotation, vibration, electonic excitation) in their spectral profile.

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