Slashdot Mirror
Ampere Could Be Redefined After Experiments Track Single Electrons Crossing Chip
ananyo writes "Physicists have tracked electrons crossing a semiconductor chip one at a time — an experiment that should at last enable a rational definition of the ampere, the unit of electrical current. At present, an ampere is defined as the amount of charge flowing per second through two infinitely long wires one meter apart, such that the wires attract each other with a force of 2×10^-7 newtons per meter of length. That definition, adopted in 1948 and based on a thought experiment that can at best be approximated in the laboratory, is clumsy — almost as much of an embarrassment as the definition of the kilogram, which relies on the fluctuating mass of a 125-year-old platinum-and-iridium cylinder stored at the International Bureau of Weights and Measures in Paris. The new approach, described in a paper posted onto the arXiv server on 19 December, would redefine the amp on the basis of e, a physical constant representing the charge of an electron."
why would the weight of the platinum/iridium slug fluctuate? I could imagine the size fluctuating, but not the weight.
Unless we're willing to dump our current units, the new definition will be some arbitrary-seeming number chosen to be consistent with a fairly precise version of what we're already using.
"almost as much of an embarrassment"
You would have done better with the technologies at hand at the time how?
A kilogram is straightforwardly defined as 2.20462 pounds. Simple enough.
that's almost 214 newtons per metre of length!
It's hard to consistantly and accurately measure weight when the force of gravity constantly changes, add to the fact that there may be radioactive decay of trace elements, oxidation of metals, Dust/erosion, sublimation of trace components), it's easy to understand how using a physical object to consistantly measure a weight, would fluxuate. when your "constants" are actually "variables" it's really hard to nail down constants...
units are so much better!
This is a shovel-balancing approach to tweezer level measurements really.
a kilogram is a measure of mass, a pound is a measure of weight...
weight does not equal mass.
The Ampere was only chosen as an SI fundamental unit because it was easier to measure than a Coulomb. To me, an Ampere will always be 1 Coulomb per second.
And since the electric charge is 1.602E-19 Coulombs, we can just invert that number to find the number of electric charges (ie, electrons) in a Coulomb.
It may look like I'm doing nothing, but I'm actively waiting for my problems to go away.
--Scott Adams
The fine article is incorrect. How an Ampere is defined does not change.
What may change is how you can measure current in the lab using other known standards because it's really hard to count electrons. Or perhaps the way a Coulomb is defined may change but the Ampere will not change.
One Ampere will remain defined as One Coulomb per second.
"File to fit, pound to insert, paint to match" - Aircraft Maintenance 101
And ohms law still works out. ... hang on, that's already a nice easy calculation to define and Ampre. I = E/R
Almost as much of an embarrassment as the definition of the kilogram? I'm kind of sick of this complaint... The weight of a kilogram is precise enough for nearly all applications. Those rare applications where its imprecision makes a difference should use a DIFFERENT unit of measurement. Invent a new one, like the mass of a hydrogen atom or something. "This thing here is 458 H masses" There, problem solved. The same with the Amp... it works fine for installing my celing fan. Don't go screwing with it just to satisfy some particle physicists. If Particle physicists don't think the amp is precise enough, USE A DIFFERENT UNIT.
Are they next going to complain that the AU is too imprecise for them to do proper measurements at the atomic level so we should therefore spend millions refining what an AU is? No... because that would be stupid. Measuring things that way thousands of grams with a unit that's precise to within a few hundred atoms is good enough. Stop using measurements designed for Newtonian physics at the atomic level and there's no longer a problem.
> the fluctuating mass of a 125-year-old platinum-and-iridium cylinder
Why is nobody looking into this?
Once we understand and control that mass fluctuation effect, we have our stardrive.
Make the part we want to keep on board (payload, living space, drive, fuel) go to the lowest possible mass, and make the exhaust flame have the highest possible mass.
Speed of light -- almost attainable.
Work on it, people!
How about we change "At present, an ampere is defined as" to "Currently, an ampere is defined as"?
To pick the cotton of course
Weight applies only within a gravity field. Gravity varies over the surface of the earth.
Weight is most often measured within air which varies in density and so the amount of air displaced varies as does the upward force of that displaced air.
Mass however is more constant.
The material (platinum-iridium alloy) was chosen for its hardness and corrosion resistance, but air pollution deposits a thin film on the surface.
This has long been known, and there is a standard washing procedure which removes most of it, but it turns out there are contaminants that this does not remove. Hydrocarbon contaminants can be removed by a recently developed technique using ultraviolet light and ozone, but mercury appears unremovable. (Without damaging the underlying Pt-Ir metal.)
The accumulation of additional mercury can be prevented in future by storing some gold leaf next to the weights (gold attracts mercury strongly), but the hope is to replace the physical weights with something else.
a banana (for scale)
redefine the amp on the basis of e, a physical constant representing the charge of an electron.
Until some smarty pants physicist comes along and determines that e or the charge of an electron changes depending on [pick something, this is physics after all]
Huh? You don't have two infinite lengths of wire lying around? They're fairly easy to get; you just take one infinitely long wire and cut it in half.
More seriously though, what's wrong with just reversing the definition (1 A := 1C/s rather than 1C := 1As), and then redefining 1 Coulomb as a precise number of electrons?
I am shocked.
My puny brain doesn't have the power to Coulomb through this information.
However, I am sure my fellow slashdotters can tell me Watt is going on.
Maybe a car analogy using the Chevrolet Volt?
If Joule help me, I can make it worth your while.
Silence is a state of mime.
My country's Black Hawk helicopter does one to zero terrorists (binladen) in roughly 9 years and 8 months.
http://www.youtube.com/watch?v=ZMByI4s-D-Y
This youtube video answered most of my questions about the kilogram.
Bearded Dragon
Exactly how ancient is the poster of this blurb? Their definitions are all *OLD*! And some of them are bullshit!
1 Coulomb is 6.241×10^18 electrons. (its a definition)
1 Coulomb of electrons passing a single point in 1 second has a current of 1 Amp. (definition) (Nothing about infinitely long wires or anything else).
1 kilogram is 1 cubic decimeter of pure water. (definition)
1 metre is 1,650,763.73 times the distance travelled by ultra voilet radiations emitted by Krypton-86. (definition)
Note that the last definition holds up under general and special relativity.
Any system of measurement needs some base units - that will be arbitrary.
In the metric system, they tried to have few base units and derive the rest. So, area and volume is simply the length unit squared or cubed. This makes some calculations simple - "if one liter of rain falls on a square meter, how deep will the water be? Exactly one millimeter"
The English system has more base units - which creates more conversion constants. "If one gallon of rain falls on a square foot - how many inches deep will the water be?"
What I'd like to know is why are units named after people? It is just crazy. If you want to honor a physicist, erect a statue to him/her. Why name a unit after them and obfuscate the meaning of the unit? What is a Newton? What is an Ampere? What is a Volt? How can you use such a unit in dimensional analysis? Naming a unit after someone guarantees extra effort and adds a layer of obstruction for anyone to learn and understand more physics. It hides the physical meaning of the process/properties that underlie the unit. If I were a physicist and they asked me if I wanted some constant/complex unit named after me, I'd say hell no. Physics is tough enough without pointlessly naming units after people.
And what is up with the Ohm and the Siemen? One of those guys got a free pass...
It was only a matter of when.
So can you tell me what Africans have given to the world?
More than you ever will even if you end up making Methuselah look like a dayfly.
How was the general form of Newton's 2nd Law defined in the prequels?
There earliest definition of Ampere was accepted in 1893 by the International Electrical Congress as: "current which, when flowing through a water solution of AgNO3, deposited silver at a rate of 1.118 mg per second." The IEC endorsed a new definition based on force between two wires as part of the MKSA system in the 30s, which because of WW2 wasn't "officially" implemented until 1950, and got taken over by SI in 1960 retaining the same definition. At no point did those systems take a Coulomb as an exact definition. The closest is the conventional electrical units developed in the 80s, which defines the Josephson constant and the von Klitzing constant, from which you can than derive the elementary charge (but it won't be a perfectly round number).
In 1795, the definition of a gram was the mass of a cubic centimeter of water at 0 C. That only lasted four years before being replaced a kilogram prototype, especially after some problems with determining exactly what water conditions would be needed for consistent results. As you tried to say, the definition is "*OLD*!"
The definition of the meter was the length 1,650,763.73 wavelengths of a transition by Kr86, but it was no a UV emission, but a hyperfine transition that puts it solidly in the red part of the visible spectrum. That definition was replaced in 1983 though, so it is not the definition now, which is instead the based on how light travels in one second now that the second is better defined.
The English system of measurements has the exact same problem with the pound because...Here it comes a pound is defined as 0.45359237 kg. And we are back to the bar sitting just outside of Paris.
No sigs in BETA. Beta SUCKS.
*Metre
Dear slashdot, the beta is broken again, as in no visible comments.
One coulomb is 6.241 X 10^18 electrons
One amp is one Coulomb per second.
Or one mole divided by one Faraday.
Those of us who do electrochemistry don't care about Newtons/meter.
Goofy mathematicians are trying to mess up a perfectly good system of units.
So ... my electric bill is going up again??
Comment removed based on user account deletion
Now we need differentiate the right or left rotation of electrons in the valance ring. Then determine which rotation of electron to allow to pass, jump to a higher energy level or freeze both rotations from passing. Just a thought...................