From: ard_at_p850ug1.demon.co.uk (Tony Duell)
Subject: Re: The Unit (was: One-upsmanship (was: Secret Mac))
To: classiccmp_at_classiccmp.org
Date sent: Sat, 20 Apr 2002 00:38:02 +0100 (BST)
Send reply to: classiccmp_at_classiccmp.org
> > > It is a derivated unit, as Coulomb, but you'r right, the
> > > name is Degree Celsius (=B0C), and youre right, C is Coulomb.
> > Coulombs aren't derived.
> Unforutnately they are :-(. The SI base unit for electical quanitities is
> the Ampere (A) for electric current. All other electrical units are
> derived from that (together with the other base units). The Coulomb is
> defined as an A*s
> (and FWIW, the Ohm is the kg * m^2 / (A^2 * s^3),
> the volt is the kg * m^2 / (A * s^3), erc)
> IIRC, the ampere is defined by a totally impractical experiment
> concerning the force between 2 infintely long, zero diameter parallel wires
> carrying a current in a vacuum. Obviously the only way to do this
> experiment is to calculate the force between 2 coils of real under the
> suitable conditions as a function of the current, and then to actually
> measure that force when the current-under-test is flowing).
No, the definition is quite theoretical, but it includes the calculation
of one of the magnetic constants (dont ask me which, school is a quite
long time ago) which again can be measured - and when you know this constant
all the rest is stable without building the gedankenexperiment in reality.
> IMHO, the coulomb should have been taken as the base unit (say define 1 C
> as the charge on 6.24...*10^18 electons. Charge (which is a property of
> a particle) seems to be much more fundamental than current (which is a rate
> of flow of charge).
Well, the problem is still how you count the electrons ...
A definition without a way to measure it is quite wothless.
Gruss
H.
--
VCF Europa 3.0 am 27./28. April 2002 in Muenchen
http://www.vcfe.org/
Received on Sat Apr 20 2002 - 07:10:39 BST