Why is the Ampere the fundamental SI unit concerning electric phenomenon, and not the Coulomb?
I've done my own research over the years to find the answer to this question, with little result.
It seems to me that the Coulomb is more basic than the Ampere afterall, the Ampere is a rate of charge -- Coulombs per second -- while Coulombs are fundamental attributes of charged particles (electrons, protons).
Sure, Amperes can be measured independantly as a Force (kg*m/s^2, etc.) in 2 wires, but Coulombs can also be measured independantly (Milikan's experiment, etc.). So, is it because we must rely on the mass of subatomic particles to be an absolute constant, that the Coulomb is not used as a fundamental SI unit?
One other thought... If the Coulomb would be considered to replace the Ampere, then I would also propose a new measure of current: A mole of fundamental charge per second. This would be approximately 96,500 Amps which is a very large current in practice. Look at the unit, "Farad", it is also a very large unit, in practice. I don't have a name for the new unit.
Sturm Vogel2008-04-24T11:30:33Z
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You have a very great thought. One answer I can think of is that scientists care a lot about transfer of energy in terms of electricity. Current reflects off of that based upon transfer of charge (and energy too) over a period of time. A Coulomb is nothing but a scalar quantity. It has magnitude, but no direction.
However, I'm also looking at what you said about units such as Farads being very large. That's very true. Coulombs are large, Teslas are large, Farads are large, and Amperes are large. Ususally quantiities in electromagnetism are small, yet I do not know why large units are SI units.
Overall, I honestly don't know why scientists choose certain units to be the SI units. Its possible that other units have been rejected for whatever reason, even if they made more sense. Anyways, I do hope that you find an answer to your question one day.
While I agree that the Coulomb is more fundamental, the fact is that the Ampere is easier to measure. You can measure current in a wire quite easily based on the magnetic field it produces (e.g. with a galvanometer), but it is difficult in practice to aggregate a bunch of charged particles together to measure the total charge. I see the point you are making with Millikan, but the truth is that it is a LOT easier to measure the current in a wire than it is to set up and perform a Millikan experiment.
So I suspect that the Ampere came to be established historically as the fundamental unit because scientists could quanitify it much more easily.
That's a reasonable question, and I think it's more an artifact of history and custom than anything else. It's a good choice because it's defined in its relationship to its effect, and therefore the other base units. We will be refining the accuracy of our measurements of elementary charge for a long time. Also, e is no longer considered the fundamental unit of charge. Look at the charge values of quarks. Even the definition of the second is quite arbitrary, and historically based.
When you are working with electrical devices you are working with devices that have moving electrical charges. Electric motors won't turn and computers won't compute if the electric charges didn't move. So we use amperes to describe the moving electrical charges.
By the way, I think the real fundamental classical measuring units are distance, mass, and time. As you said, the units of force really boil down to kilograms (kg, which is mass), meters (m, which is distance), seconds (s, which is time.) This breaks down at the quantum level.