# Homework Help: Why ampare is fundamental unt in S.I rather than charge

1. Sep 3, 2008

### 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

2. Sep 3, 2008

### LowlyPion

http://physics.nist.gov/cuu/Units/ampere.html

3. Sep 5, 2008

### 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}

4. Sep 5, 2008

### mgb_phys

No, as lowlypion described the ampere is derived from fundemental 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?

Last edited: Sep 5, 2008
5. Sep 5, 2008

### 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.

6. Sep 5, 2008

7. Sep 5, 2008

### LowlyPion

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

8. Sep 5, 2008

### 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 dont 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 couldnt find anything on google about it so dont ask me to prove it. I have no idea how it was derived.

9. Sep 5, 2008

### LowlyPion

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.

10. Sep 7, 2008

### Staff: Mentor

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

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.

Last edited: Sep 7, 2008
11. Sep 7, 2008

### Redbelly98

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

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