Anyone care to demystify ampere for the layperson?

1. Jan 6, 2006

hkhil

Hello

This is a section from a textbook...

"an ampere is defined as that current in each of 2 infintely long parallel wires 1 m apart that causes an electromagnetic force of 2 x 10^-7 per meter of its length to act on each wire"

Hmmm... was that in English ?

q1. So say we start at 0. After 1 meter, wire 1 will cause a force of 2 x 10^-7 N (Is it in Newtons?) on wire 2.
That would mean that after 2 meters, wire 1 will cause a force of 2( 2 x 10^-7) N of force on wire 2... and so on.
Is that what that passage is trying to say?

If so... what does that have to do with anything? Considering we learnt that an amp is basically the number of electrons flowing past a given point per second.

2. Jan 6, 2006

Staff: Mentor

Imagine two very long parallel straight wires, one meter apart, carrying equal currents. Ideally, you need infinitely long wires, but in practice it works well if you have very long wires (several meters?) and just look at the middle part of the wires. The reason for this is that the magnetic field produced by the wires is distorted at the ends because of "end effects", and we need to make those effects small enough to ignore.
Now take a one-meter section of one of the wires. If the magnetic force acting on that one-meter section is $2 \times 10^{-7}$ N, then the current through each wire is by definition 1 ampere. On a two-meter section of the wire, the force would be $4 \times 10^{-7}$ N, etc.
This seems like a strange way to define current. After all, electric current is a flow of charge, so it might seem more intuitive to define our unit of current in terms of some number of electrons per second. The reason we don't do this is that it's easier to measure forces and distances precisely, than it is to measure the precise amount of charge passing through a wire. Definitions of units have to be easily reproducible in a laboratory in a precise way.

3. Jan 6, 2006

chroot

Staff Emeritus
Since this is not quantum physics, I am moving it.

- Warren

4. Jan 6, 2006

hkhil

Hi JT

Thanks I got it now.

Last edited: Jan 6, 2006
5. Jan 10, 2006

rcgldr

There's no reason not to define an Amp in terms of electrons per second:

$$1 \ amp \approx \ 6.25 \ \times \ 10^{18} \ electrons \ / \ second$$

Last edited: Jan 10, 2006
6. Jan 10, 2006

ZapperZ

Staff Emeritus
The only problem with doing this is that "electron per second" has units of "number/s". One can get into quite a bit of a dimensional analysis mess if one doesn't actually realize that your "electrons" is meant as as "charge of electrons".

I certainly would not recommend doing this to anyone just starting out learning physics. There's nothing wrong with the standard definition, so why introduce an unnecessary ambiguity?

Zz.

7. Jan 10, 2006

rcgldr

Ok, electron charges per second. Last I remember 1 amp was defined as 1 Coulomb per second, and a Coulomb is about 6.25x10^18 electron charges. In an ideal metal conductor, where the only flow is electrons moving from molecule to molecule, then electrons per second should be close enough for a lay person to understand.

Last edited: Jan 10, 2006
8. Jan 10, 2006

ZapperZ

Staff Emeritus
Sure, but call it something else, such as "flux", which has a very flexible definition in various parts of physics. But you were attempting to change a very fixed definition that can cause a "lay person" to use it wrongly, which lay persons are apt to do.

Zz.

9. Jan 10, 2006

rcgldr

The web site below defines 1 Amp as 1 Coulomb per second and defines an electron charge at 1.6^10-19 Coulombs, making a Coulomb 6.25x10^18 electron charges. It also goes on to state that you have electron flow in wires and ion flow in fluids.

When did the defnition of 1 Amp change from 1 Coulomb per second?

http://www.gcse.com/glos.htm

10. Jan 10, 2006

ZapperZ

Staff Emeritus
When was it ever defined differently?

Zz.

11. Jan 10, 2006

rcgldr

From the original post in this thread:

"an ampere is defined as that current in each of 2 infintely long parallel wires 1 m apart that causes an electromagnetic force of 2 x 10^-7 per meter of its length to act on each wire"

Now where can I actually measure the force between 2 infinitely long parallel wires? I prefer the mks standard of 1 coulomb / second.

12. Jan 10, 2006

Galileo

You then still have the question of defining how much a Coulomb is. The point is that the Coulomb is defined in terms of the ampere.
The standard definition (force of the two wires) uses no new quantities, only ye old force which can all measure.

13. Jan 10, 2006

Staff: Mentor

If all you want is a conceptual definition, then this will work fine, provided of course that you specify the number of electrons precisely, and you specify the amount of charge on an electron.

However, the official definitions that people actually use in the laboratory must be practical. How can one count electrons precisely, in practice?

For the same reason, we still define the kilogram as the mass of a specific lump of platinium-iridium alloy in a basement in Paris. People are working on ways to count atoms precisely, so that someday we can define a kilogram as a specific number of atoms of some isotope, but we're not there yet.

14. Jan 10, 2006

ZapperZ

Staff Emeritus
No, look again. They're not that different. Use Biot-Savart Law and use dimensional analysis. They're identical. You can do the same thing with the electric field. You can define it in units of V/m, or N/C. Break each of them down to the fundamental units and you'll get the same dimensions. They are not different beasts.

But saying an Ampere is # of electrons/second is.

Zz.

15. Jan 10, 2006

rcgldr

Isn't a Newton just as conceptual? After all it is the force required to accelerate that same lump of alloy in Paris 1 meter / sec^2. At least meter has been defined as so many wavelengths of a certain frequency of light from a laser. Is any any easier to calibrate a Newton meter than an Ammeter?

16. Jan 10, 2006

rcgldr

Note I was trying to define what an amp means, not provide a method to measure current. Defining an amp in terms of electrons or Coulombs seems a lot simpler than decribing a method to measure current via the electrical field generated between two very long wires.

I'm also under the impression that if I have 1 amp of currrent in a typical wire, that at any cross section of that wire there will be net flow of 6.25 x 10^18 electrons / second across that section, unless there are magical electrons that don't have a standard amount of charge in typical wire.

17. Jan 10, 2006

ZapperZ

Staff Emeritus
There's a difference between "equivalent" and "equal". 1 amp is equivalent to having x many electrons flowing through a cross-sectional area in 1 sec. But 1 amp is NOT equal to x many electrons per second. That is just not right dimensionally since x electrons/s is number/second, where as an ampere is coulombs/second. You were trying to indicate that those two are identical. They are not, and cause quite a confusion to anyone just learning the subject matter.

Zz.

18. Jan 10, 2006

rcgldr

I understand your point now. You want it to be clear that 1 Amp is related to the charge, and not the mass of the electrons that I mentioned in my definition.

19. Jan 10, 2006

rbj

but, Z, they could define 1 amp to be equivalent to having x many elementary charges flowing through a cross-sectional area in 1 sec. and that would be dimensionally the same. but it would change the definition.

an amp is not quite equivalent to having some x number of elementary charges pass some cross-sectional boundary in 1 second since we do not know (vis-a-vis the present definition of the unit of charge) exactly what the elementary charge is. now the big guys at NIST or whatever international standards body could redefine the unit charge to be the sum of a certain exact number of elementary charges, but then, as a result, $\mu_0$ would no longer be exactly $4 \pi \times 10^{-7}$ as it is in SI units.

another way of wording what an ampere is (in the present definition) is whatever current it has to be to define $\mu_0 \equiv 4 \pi \times 10^{-7}$ H/m. sorta like the meter is presently defined to be whatever length it has to be to define c = 299792458 m/s.

it's like redefining Advogadro's Number, NA to be exactly 6.023 x 1023, but that would not be the same definition as the number of Carbon-12 atoms needed to weigh precisely 12 grams.

so which universal constant do you want to define exactly by use of the definition of the unit charge (or unit current)? $\mu_0$ or e?

Last edited: Jan 10, 2006
20. Jan 11, 2006

rbj

i didn't check the web site, but fundamentally, the Ampere was never defined as a Coulomb per second. The Ampere was defined first as whatever current it had to be to result in $\mu_0$ being exactly $4 \pi \times 10^{-7}$, or whatever current it had to be to result in 2 x 10-7 N/m force in two infinitely long parallel wires spaced apart by 1 meter. Then the Coulomb was defined to be whatever charge passes a cross-section boundary of a wire carrying 1 ampere in the time of one second. that is, the Coulomb is an Ampere-second, where the Ampere has a primary definition.

of course, it would be circular to define the Coulomb to be an Ampere-second at the same time as defining the Ampere to be the current of a rate of 1 Coulomb of charge per second.

Last edited: Jan 11, 2006