Actually, I’ve never heard of colomb before, I just ran to google and looked at what it had to say.
I do know that 1 dimentional measurements (miles, for example) Don’t measure the same thing as 2 dimentional measurements (miles per hour)
So how do you get coulomb from amps? I mean, knowing there are 5 amps running through a circuit isn’t gonna help you figure out how much charge that is, is it? Unless you know the 1.602x10^-19 figure to start with, which would defeat the point…
Seems to me that it’d just be simpler to use coulombs 
Hey, let’s use one billionth of the distance that light travels in one second! We could call our system the Fundamental Orthometry Of Time system.
Umm, aren’t all units of measurement arbitrary by nature? The flaw in the OP is that while a metric system based on the circumference of the Earth and the density of water is inconvenient when measuring the speed of light, the converse would also be true. Considering that engineering problems deal more often with water than with any other single substance, a system that is convenient for dealing with masses and volumes of water is inherently more useful than one that is not.
I see what you’re saying. If you want units that aren’t based at all on other units, there’s the kilogram (based soley on the mass of the protoype), the second (based solely on counting cesium oscillations), the mole (great show), and the kelvin (based solely on a fraction of water’s triple point). Originally, the meter was an independent unit (based solely a fraction of the earth’s circumference) and so was the candela (based solely on the luminous intensity of carbon filament lamps, IIRC). So 6 out of 7 ain’t bad. If we lose some “independence” for the sake of precision, I have no problem with that.
But I based my definition of independent on the definitions provided by the BIPM and I don’t think that’s what the BIPM mean by independent. Sure, in order to know the length of a meter you need to know the length of a second. But think of it this way. Every derived unit is a combination of the base units. Force is measured in newtons, a derived unit, and can be expressed in a combination of base units, namely: m•kg•s[sup]-1[/sup]. The meter can’t be expressed as simply a combination of base units. Neither can the candela, it’s not “is just energy divided by time”. Correct me if i’m wrong, but energy divided by time is an merely a generic expression of power, not of luminous intensity.
I am perfectly correct to say that the mole is a unit, simply because the International Bureau of Weights has defined it as such. You disagree with the Bureau’s definition? Write them a letter AFAIK, they consider it a basic unit because you need it to express derived units, like the katal (mol•s[sup]-1[/sup]).
The coulomb (measure of charge) is a more fundamental quantity. However, it is more practical to define the coulomb in term of the ampere, as opposed to defining it in in terms of the actual charge of an electron or proton.
An ampere is the amount of electric current in each of two long, parallel wires that give rise to a magnetic force per unit length of 2 x 10[sup]-7[/sup] N/m on each wire when the wires are separated by one meter. The coulomb is thus defined as the quantity of electric charge that passes a given point in one second when the current is one ampere (or one coulomb per second).
If you now want the charge of a single electron, you must conduct an experiment (such as Robert Millikan’s oil drop experiments of 1909-13).
Thus the coulomb, in and of itself, has nothing to do with the charge of an electron (or proton).
Quoth enolancooper:
Not quite, on the Planck distance: The size of the electron has nothing to do with this. The Planck units are chosen in such a way that c (the speed of light), G (Newton’s gravitational constant), and hbar (the quantum constant) are all equal to one. If you like, you can extend the Planck system by also requiring that k[sub]e[/sub] (Coulomb’s electrostatic constant) and k[sub]b** (Boltzman’s constant, relating energy and temperature) also equal one, so now you’ve got fundamental units for charge and temperature, too. It should be noted that the fundamental charge thus derived is not the same as the charge of the electron: In this system, the electron has a charge of approximately 0.085 Planck charges. It’s also worth noting that this means that theoretical physicists looking for a Theory of Everything can get away with using no units at all for their calculations: All quantities are considered to be in the appropriate combination of Planck units. Hence statements like “The temperature of a black hole is equal to one fourth its surface gravity”, even though surface gravity is usually meters/second[sup]2[/sup] and temperature is usually Kelvins.
Quoth jbird3000:
You can’t have it both ways. If you want to use the Planck units, then the density of water is some silly-looking number that nobody would ever remember (don’t have my references handy, so I can’t get the exact number).
I know that. What I meant was the Planck system would make sense, instead of doing something silly like saying 1 “distance-Planck” is the distance something can travel at the speed of light in 2 “time-Plancks”.
Who has said that coulombs measure current?
tv snake
No. The coulumb is an absurdly large number of electrons (or rarther, their charge). The mole is just an absurdly large number. It’s not an absurdly large number of atoms, or an absurdly large number of molecules, or of anything else. It’s just an absurdly large number. It does not represent anything physical.
MattTheCroc
If you have 5 amps for 10 seconds, then there are 5*10=50 coulumbs that passed through the wire. You don’t need to know the 1.602x10^-19 number unless you want to know how many individual electrons passed through, which is different from the charge.
Alphagene
Can you explain to me what the difference is? It seems to me that this is like saying that “meter” is just a generic measure of distance, not of the distance between Paris and London. Do we need a new unit for every application?
Well then, I guess “dozen” and “pi” are units, too.
Either you’re wrong or they are, because the katal has the same dimension as the hertz. Anything that can be measured in katals can be measured in hertz (although you need much larger numbers).
OK, I missed this one the first few times.
You seem to have gone the wrong way in converting kilograms to grams in presenting the Planck mass. In grams, a Planck mass would be around 10^-5 gram. Actually, pretty hefty, no?
Hopefully not!
A meter is a one-dimensional unit, as Mikahw pointed out above. We use it to measure distance. Period.
What Alphagene’s discussing is the interpretation of a two-dimensional unit (energy per time unit). The difference is, therefore, that they’re two different things altogether.
Which brings me to another quote of yours in this thread:
I’m not sure if I understand you. But I’d say that distance has nothing to do with time. London - Paris (to use your example) will be 500 kilometers, no matter how long it will take you. How is distance “imaginary time”?
**
Eh, you’re probably right. I probably assumed too much.
I must respectfully disagree. The mole measures an amount–not a mass, mind you, but an amount. One Avogadro’s number of something is one mole. But that doesn’t mean that’s the same as the mole just being a large number.
I work with a unit called molarity rather often, being a biochemist. A one molar solution of X is defined as having one mole of X per liter of solution. By what you’ve been saying, this could be simply defined as N[sub]A[/sub] L[sup]-1[/sup] of X. Great. N[sub]A[/sub] what of X per liter? Grams? Kelvins? Submarine sandwiches?
The mole very simply defines this. In one mole, there are N[sub]A[/sub] formula masses of X. If one wanted, one could define the gram as: the mass of one-twelfth of a mole of Carbon-12 atoms.
From the NIST page on this: “When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.” (Bolding mine)
I will freely admit that the mole is a convenience unit–it gives us the ability to say “One mole of carbon” rather than “6.022045×10[sup]23[/sup] atoms of carbon.” But that doesn’t make twelve a unit–because you can’t say “One twelve of carbon.” You can say “Twelve atoms of carbon,” which is something different.
We don’t need a new unit for every application, but light has its own special properties that make measuring it rather important–see the derivation of the length of a meter from the speed of light.
Energy divided by time gives power. That’s simple enough. But it’s completely different from luminous intensity, for this basic reason: The wavelength of the light is a factor in determining the power.
In other words, if you have a red light and a blue light of equal luminous intensity, the blue light will have about 1.6 times the power as the red light, by virtue of having more energy per photon. I suppose if you wanted to be picky, you could say that a candela was a certain number of photons, but we’ve yet to be able to count those, so for now I’ll stick with a candela.
LL
The difference is that they’re different? That sounds rather circular. Why are they different?
Time squared minus space squared is invarient. So since their squares differ by a factor negative one, space and time differ by a factor of imaginary one. For instance, if you measure one event in London as happening one millisecond before one in Paris, then the total distance is sqr((5/3)^2-(1)^2)=4/3 (500km=~ 5/3 light milliseconds). If I’m traveling at half of light speed, I will measure the same distance, even though time and space dilations will cause each individual meaurement to be different. Relativity says that not only is time another dimension, but it’s essentially the same dimension.
LazarusLong42
Yes it does. If one NA of carbon = a large number of carbon, then one NA is a large number.
If there is one mole of submarine sandwiches for every liter of your solution, then you have one molarity of submarine sandwiches. Of course you have to specify what you’re talking about. Try coming into work tomorrow and asking for one molarity. If someone asks “One molarity of what?” just say “One molarity. Give me one of whatever a molarity measures.”
No, in one mole of X, there are NA of X. One mole, in itself, with no substance specified, represents absolutely nothing. It’s just a number.
I never said that twelve is a unit. I said that a dozen is a unit. And one can indeed ask for one dozen carbon molecules.
If that is so, is it not of the same nature as the mole? Could we not as easily talk of one mole of photons?
And wouldn’t it be much easier to define ‘room temperature’ as 300K instead of the current 298K (25 degrees)?