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Base physical quantity: Current?

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Cifrocco

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The study of electronics must begin with an understanding of physical quantities beginning with the most fundamental, the base quantities, which are physical quantities that are not derived from other quantities. Seven of the most common base quantities are:

1. Time
2. Length
3. Mass
4. Electric current
5. Thermodynamic temperature
6. Luminous intensity
7. Amount of substance

Of particular interest for the sake of this thread is Electric Current. The definition is no less than the following: An ampere is the constant current that, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 E-07 newton per meter of length.

Wouldn't it be easier to define a base quantity as Electric Charge, measured in coulombs, a coulomb being equal to the charge of 6.25 E18 electrons? From there, the ampere is then easily defined as 1 coulomb per second. I just find it strange that the ampere is considered basic when it is easily derived from what I believe are truly basic quantities: time (seconds) and charge (coulombs or electrons).

Your toughts?
 
I think you have a good point here...

If basic quantities include
Cifrocco said:
1. Time [seconds]
2. Length [metres]
and a derived quantity is speed (metres per second)

then logically it would follow that amps (coulombs per second) would be a derived quantity.
Never mind the "infinite length, negligible cross section" definition of an amp being a nightmare to test :?

Anybody else ?
 
Wouldn't it be easier to define a base quantity as Electric Charge, measured in coulombs, a coulomb being equal to the charge of 6.25 E18 electrons? From there, the ampere is then easily defined as 1 coulomb per second. I just find it strange that the ampere is considered basic when it is easily derived from what I believe are truly basic quantities: time (seconds) and charge (coulombs or electrons).

First of all, 6.25 E18 is a rounded number. Second, every try to count out a few electrons? Ever try to measure ANYTHING about an electron? Stuff like that is difficult to do, and the results are often sketchy. Magnetic fields, gravity and other goofy stuff like that affects the measurement.

Your idea of resolving the definition of the ampere to basic definitions is good, but those definitions must be measureable to good precision and accuracy. One of those you mentioned is an excellent choice, for TIME is what we can measure best. As a matter of fact, we can resolve time to a precision that is probably at least six orders of magnitude better than anything else we can measure.

I would suppose that the definition of the ampere relates to what can be measured with the best precision without having to go through several other defining units to get to the measurement.

My understanding is that research is moving along to try to relate as many units of measure as we can to time or at least to atomic attributes such as resonance or wavelength, to not only provide greater precision and repeatability, but a more universal standard.

In the end, who cares? We measure our circuits with a 4-1/2 digit DMM and are perfectly happy without knowing worrying about the actual definition upon which our units of measure are derived. I don't care whether my bread is made from wheat from Kansas, Oklahoma or North Dakota, as long as it's fresh, soft, tastes good and isn't moldy!

Dean
 
Cifrocco said:
...two straight parallel conductors of infinite length, of negligible circular cross section,...a force equal to 2 E-07 newton per meter of length.

So an infinite length that can be measured :?

Dean Huster said:
...the definition of the ampere relates to what can be measured with the best precision

Is this tending towards "do you want GM bread or GM bread ?"

I use accelerometers quite a lot at work, their output is measured in coulombs, feeding a charge amplifier to give mV out - coulombs doesn't seem to be that difficult a unit in industry, surely a lab could do something :?:
 
mechie said:
Cifrocco said:
...two straight parallel conductors of infinite length, of negligible circular cross section,...a force equal to 2 E-07 newton per meter of length.

So an infinite length that can be measured :?

Dean Huster said:
...the definition of the ampere relates to what can be measured with the best precision

Is this tending towards "do you want GM bread or GM bread ?"

I use accelerometers quite a lot at work, their output is measured in coulombs, feeding a charge amplifier to give mV out - coulombs doesn't seem to be that difficult a unit in industry, surely a lab could do something :?:

Problem is my ammeter can't count electrons, and accelerometers came AFTER an Amphere was defined.

You have to remember, the people that came up with the Amp were simply looking for a standard universal way of measuring something... so all labs across the world would know what '1 amp' meant. They had to do this with primitive equipment.

I'm sure whoever invented "1 pound" as a measurement of weight, just picked up a rock and said "ok, this is what we're gonna reference everything in terms of weight from from here on out"... not taking into consideration gravity, force, etc.
 
plot said:
Problem is my ammeter can't count electrons, and accelerometers came AFTER an Amphere was defined.

You have to remember, the people that came up with the Amp were simply looking for a standard universal way of measuring something... so all labs across the world would know what '1 amp' meant. They had to do this with primitive equipment.

I'm sure whoever invented "1 pound" as a measurement of weight, just picked up a rock and said "ok, this is what we're gonna reference everything in terms of weight from from here on out"... not taking into consideration gravity, force, etc.
http://www.unc.edu/~rowlett/units/cgsmks.html said:
Unit of length meter The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
Well, that shouldn't be too difficult to measure with primitive equipment ... ?

Don't forget thata the "SI system" was only adopted in 1960 and is still being 'adjusted', The "degree Kelvin" became the kelvin in 1967.

I realise we don't stand much chance of redefining the whole system in this thread but Cifrocco has a good point as far as I can see ...

... or has he just kicked up a sandstorm and disappeared ???
 
I agree with Dean. The base units are chosen with respect to how reproducible the measurements can be made up to the highest precision. The S.I. system may not have achieved that aim perfectly, but it's the best attempt so far, and universally recognized.
 
mechie said:
http://www.unc.edu/~rowlett/units/cgsmks.html said:
Unit of length meter The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
Well, that shouldn't be too difficult to measure with primitive equipment ... ?

Don't forget thata the "SI system" was only adopted in 1960 and is still being 'adjusted', The "degree Kelvin" became the kelvin in 1967.

I realise we don't stand much chance of redefining the whole system in this thread but Cifrocco has a good point as far as I can see ...

... or has he just kicked up a sandstorm and disappeared ???

I have a hard time beleiving that that's how the meter was originally defined. I think someone just came up with that defination after the fact... because who really would have picked 1/299 792 458 of a second?
 
Napoleon Bonaparte started the metric system by decreeing that the meter should be one tenmillionth of the distance from the equator to one pole of the earth. The distance could not be measured accurately at the time. It is now determined by wavelengths of light from krypton 86.
 
Re: Scientific Assumptions

checkmate said:
mechie said:
checkmate said:
universally recognized.
"The Earth is flat."
Is this supposed to be funny or are you trying to bring across any meaning? Because I don't quite get it.
Sorry for being a bit too terse there,
My point was that it used to be universally accepted that the earth was flat, carried on the back of a giant turtle, then it was universally accepted that the Sun, Moon, planets and stars revolved around the Earth (the cenre of the Universe ?).

Modern 'scientific thinking' is sometimes at odds with old 'knowlege'

I fully accept that the amp is a base unit in our current SI system but that doesn't make it the best, most consistent unit for any future system.

Cifrocco has not posted here since starting this thread, WHERE ARE YOU ? I could do with an ally here.
 
I was very curious to see how this thread would develop.

I still haven't heard a valid reason why the base quantity cannot be Electric Charge from where we would derive Electric Current and other quantities.

Some here say that such quantities were developed in such a way as to be easily measured. Well then chew on these, the complete list of the basic physical quantities, universally-accepted and officially defined:

Meter. The length equal to 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton-86 atom.

Kilogram. The mass of the international protoype of the kilogram, which is a platinum-iridium cylinder maintained at the International Bureau of Weights and Measures near Paris. The kilogram is approximately equal to the mass of 1,000 cubic centimeters of water at its temperature of maximum density.

Second. The duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

Ampere. The constant current that, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 E-07 newton per meter of length.

Kelvin. The unit of thermodynamic temperature is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

Mole. That amount of substance of a system that containes as many elementary entities as there are atoms in 0.012 kilogram of carbon-12.

Candela. The luminous intensity in a given direction, of a source that emits monochromatic radiation of frequency 540 E+12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.



So which of these quantities would you say is easily measured?

The beauty of the coulomb is that it is an exact number of electrons or protons, an integer value. In fact, there are exactly 6,241,450,383,000,000,000 electrons or protons in a coulomb of charge. This is what physicists call graininess, I believe. You cannot have 0.43 of an electron, for example. I'm not saying an electron or proton is easier to measure or to count in a lab, but the definition is the most precise of all, in my opinion. So why can't we say that Electric Charge is the true base quantity? There must be some reason why and that's what I'm curious to find out! Is it something about the behavior of charged particles like electrons that don't make them a good, reproducible standard?
 
i still like my theory Cifrocco, they were created in times where we couldn't measure so precisely, so they simply created a universal way to measure everything.

as technology progressed, we were able to look back and specifically define our different means of measuring with natural occurances.

as technology and science continually progress, i'm sure they will get more precise, down to the charge.

An example was already given in this thread, the meter. Napolean "invented" the meter.... you think he decided it's length by using this definition?
Meter. The length equal to 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton-86 atom.

No, he guestimated, cut off a peice of metal or something, said "this is a meter", and everyone copied the exact length of it... eventually some scientist figured out that hey, a meter is equal to 1,650,763.73 wavelengths blah blah blah... so let's use that to define the meter instead.
 
I think when trying to establish a definition for something as basic, fundamental, and important as one of the seven physical quantities listed previously one has to place emphasis on immutability. In other words, can the standard be reproduced exactly every time, and for the rest of time?

The definition of the ampere seems to be almost haphazard in its specification, while the coulomb is as timeless and precise as can be. Has the charged particle, whether electron or proton, ever changed physically in the past millenia and will it ever in the distant future? I would tend to think not.

I don't think the quantity needs to be able to be measured in a lab on a routine basis. I think that the definition simply needs to be anchored to something irrefutable and immutable, although certainly measurable when necessary to calibrate dependent systems.

When I look at the definition of the ampere I tend to think that if you were to ask, say, five different world-class labs to measure it the way it is defined, that the physical setup of their equipment and apparatus would differ and that you would not obtain exactly the same value at each.

But counting electrons, if you can count one, then you can count two, and so on, until you have counted the number required. Not only is Electric Charge a truly basic and fundamental physical quantity in my opinion, but its definition based on the electron (or proton) is more solid (not to mention more easily understood!) than that of all the other six physical quantities.

Of particular note is that of the seven base physical quantities, only the ampere is dependent on other base physical quantities, three of them, as a matter of fact: the meter, kilogram, and second (by way of the newton) therefore it is really a derived quantity, not a fundamental one! Add to that the fact that in its definition you have expressions such as infinite length, and negligible cross-section and it is easy to see why I prefer the coulomb over the ampere as a fundamental quantity worthy to be included with the other six.

Either way, they're both French!
 
One can always measure one amp of current based on definition, at least to a value close to that. But doubt any lab can count that many electrons to that precision.
Anyway, you can't claim that ampere is a derived quantity simply based on I=Q/t. I can say that Q=It and coulombs are a derived quantity. What is important is that they cannot be expressed only in terms of the other base units.
 
checkmate said:
One can always measure one amp of current based on definition, at least to a value close to that. But doubt any lab can count that many electrons to that precision.

If you can count one electron, then you can count as many as needed. The precision is established as soon as you can show that you've counted one.

checkmate said:
Anyway, you can't claim that ampere is a derived quantity simply based on I=Q/t. I can say that Q=It and coulombs are a derived quantity. What is important is that they cannot be expressed only in terms of the other base units.

I did not claim that ampere is derived due to it being equal to coulombs per second, it is derived because the definition is dependent on three other, truly fundamental quantities: meter, kilogram, second. The coulomb, although can be derived from ampere-seconds, is first and foremost equivalent to a finite and precise number of indivisible charged particles. In 1909, Robert Millikan confirmed that electric charge always occurs as some integral multiple of some fundamental unit of charge, e. In modern terms, the charge Q is said to be quantized, where Q is the standard symbol used for charge. That is, electric charge exists as discrete “packets”. Thus, we can write

Q=Ne

where N is some integer. I believe this makes the coulomb a physical quantity that is as precise as the concept of an integer itself.

The rebuttals offered here to my original proposition are not addressing the point I'm trying to make, that is, that the coulomb, not the ampere, is better suited to play the role of fundamental physical quantity, alongside the meter, kilogram, second, etc.

The reason it hasn't been chosen by the science world, I'm led to speculate, is that the electron is not easy to measure, perhaps because it doesn't sit still. Any attempt to measure a charged body will cause the number of electrons in it to change. On the other hand, the ampere is measured by allowing a current to flow in two conductors and then measuring the force between them, where the act of measuring does not alter the flow of current.
 
Cifrocco, it isn't always what's easiest to measure, but what can be measured most accurately, most precisely with the highest degree of repeatability. That's why the second is the unit of measure with which we have the best control. In the SI system, we can resolve the second with the relative combined standard uncertainty of our primary standards to 2 parts in 10^15. You won't find anything in this world tighter than that at this time. Not even close.

Dean
 
Cifrocco said:
<snip>
Of particular interest for the sake of this thread is Electric Current. The definition is no less than the following: An ampere is the constant current that, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 E-07 newton per meter of length.

Wouldn't it be easier to define a base quantity as Electric Charge, measured in coulombs, a coulomb being equal to the charge of 6.25 E18 electrons? From there, the ampere is then easily defined as 1 coulomb per second. I just find it strange that the ampere is considered basic when it is easily derived from what I believe are truly basic quantities: time (seconds) and charge (coulombs or electrons).

Your toughts?

What happens to these two measurements as the conductors approach the speed of light? It's been 15 years since I opened a quantum physics book - but it would seem that an Ampere would still be an Ampere - even as the number of electrons flowing past any point per second approached zero (because their energy has increased). But the energy represented by 1 coulomb per second in conductors approaching the speed of light would approach infinity.

Of course, this all assumes that you could find a way to make conductors of infinite length (and therefore infinite mass) move at all, much less make them move near the speed of light.

So what is it that you want to measure? The Ampere, by the definition posted in the opening post, is a measurement of Force. The second, proposed definition, amounts to a measurement of charge carried by a volume of electrons. Force is based upon mass, which varies as relative speed varies. A volume of electrons will not vary, however, as the conductor approaches the speed of light.

(I am admittedly out of my element here. Maybe the fact that electrons are already moving near the speed of light makes all of the above irrelevant. Anyway, please be polite when you call me a moron)
 
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