From: ard(a)p850ug1.demon.co.uk (Tony Duell)
Subject: Re: The Unit (was: One-upsmanship (was: Secret Mac))
To: classiccmp(a)classiccmp.org
Date sent: Sat, 20 Apr 2002 00:38:02 +0100 (BST)
Send reply to: classiccmp(a)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/