Why ampare is fundamental unt in S.I rather than charge

[physical2]

hii

i have a problem that why ampare{unit of current} is fundamental unit, ampare is

I=Q/t

and it is drived.while electric charge is not a drived unit and it also measured by millican mattod.and if we place coulomb{unit of electric charge} as fundamental unit in place of ampare{unit of current} definition of charge and its unit "coulomb" change OR not.please tell
me as soon as possible because i want to complete my assignment.

in short i seems that charge is a fundamental quantity in S.I than current,so why coulomb is not a fundamental unit rather than ampare.PLEASE DO NOT IGNORE.HELP ME

 

[LowlyPion]

hii

i have a problem that why ampare{unit of current} is fundamental unit, ampare is

I=Q/t

and it is drived.while electric charge is not a drived unit and it also measured by millican mattod.and if we place coulomb{unit of electric charge} as fundamental unit in place of ampare{unit of current} definition of charge and its unit "coulomb" change OR not.please tell
me as soon as possible because i want to complete my assignment.

in short i seems that charge is a fundamental quantity in S.I than current,so why coulomb is not a fundamental unit rather than ampare.PLEASE DO NOT IGNORE.HELP ME

Perhaps this link will help you?
http://physics.nist.gov/cuu/Units/ampere.html

 

[physical2]

hii

i think i could not explain my question .my original question is why coulamb is not a fundamental unt.and why ampare is a fundamental unit.while i seem that ampare{unit of current} is drived from coulamb{unit of charge}

 

[mgb_phys]

No, as lowlypion described the ampere is derived from fundamental units (mass,length,time).
The coloumb is derived from the ampere - a current of 1A flowing for 1 second.
If the coloumb was the base unit, how would it be defined?

 

[atyy]

It's easier to measure the force between two wires, than the force between two charges. From the point of view of theory, it makes no difference if the base unit is current or charge.

 

[mal4mac]

For a *quick and easy* run through, with some references, check out:

http://simple.wikipedia.org/wiki/Ampere

 

[LowlyPion]

There is also this observation about the fallout of Coulombs becoming a standard:

In principle, the coulomb could be defined in terms of the charge of an electron or elementary charge. ... Combined with the present definition of the ampere, this proposed definition would make the kilogram a derived unit.

 

[granpa]

a better question is why is either one considered fundamental? both CAN be defined in terms of more fundamental dimensions but you need fractional exponents. physicists don't like fractional exponents so they added another 'fundamental' dimension to get rid of it.

thats what I read a long time ago and I fully believe it but I couldn't find anything on google about it so don't ask me to prove it. I have no idea how it was derived.

 

[LowlyPion]

a better question is why is either one considered fundamental? both CAN be defined in terms of more fundamental dimensions but you need fractional exponents. physicists don't like fractional exponents so they added another 'fundamental' dimension to get rid of it.

thats what I read a long time ago and I fully believe it but I couldn't find anything on google about it so don't ask me to prove it. I have no idea how it was derived.

The point I think is to have a practically manageable set of basic units which unify the physical sciences and their application in society, not muck it up through minimization using complicated calculations derived from as few units as possible.

Current is electron flux and relates fundamentally to Magnetism through the Maxwell relationships and is the more easily measured. Since a Coulomb of electrons doesn't fit conveniently in a beaker or sit easily on a scale, but a coulomb in flux does yield more easily measurable effects, it seems to make better sense to then just ... stay current.

 

[Borek]

Current is electron flux and relates fundamentally to Magnetism through the Maxwell relationships and is the more easily measured. Since a Coulomb of electrons doesn't fit conveniently in a beaker or sit easily on a scale, but a coulomb in flux does yield more easily measurable effects, it seems to make better sense to then just ... stay current.

No no no, you may conveniently catch a Coulomb of electrons in a beaker, using electrochemical methods

Honestly, while I have no doubts that there were logical and practical reasons to make ampere fundamental unit, I don't get the 'whys'. Coulomb - defined as 1 ampere times 1 second - is only as accurate as our measurements of time and current are. Coulomb - defined as number of electrons - would be exact. That's what is proposed to be done to Avogadro constant, I suppose it will work for Coulomb as well. To some extent that's what has been already done to time and length.

 

[Redbelly98]

Measuring current can be done more accurately than counting electrons, hence we use current rather than charge as the fundamental unit.

No no no, you may conveniently catch a Coulomb of electrons in a beaker, using electrochemical methods

That's impossible. However, one could catch -1 Coulombs of electrons.