and a derived quantity is speed (metres per second)Cifrocco said:1. Time [seconds]
2. Length [metres]
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).
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.
Dean Huster said:...the definition of the ampere relates to what can be measured with the best precision
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 :?:
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.
Well, that shouldn't be too difficult to measure with primitive equipment ... ?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.
"The Earth is flat."checkmate said:universally recognized.
Is this supposed to be funny or are you trying to bring across any meaning? Because I don't quite get it.mechie said:"The Earth is flat."checkmate said:universally recognized.
mechie said:Well, that shouldn't be too difficult to measure with primitive equipment ... ?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.
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 ???
Russlk said:It is now determined by wavelengths of light from krypton 86.
Sorry for being a bit too terse there,checkmate said:Is this supposed to be funny or are you trying to bring across any meaning? Because I don't quite get it.mechie said:"The Earth is flat."checkmate said:universally recognized.
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.
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.
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?
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