Date: Sun, 5 Apr 2009 13:23:24 -0300
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Learning Electricity Part Two
In-Reply-To: <20090405114121.D6EB61165C1@hamburg.alientech.net>
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I was hoping nobody would bring this up -- but
Mike's correct. Two points, however:
At 08:28 AM 4/5/2009, Mike S wrote:
>The coulomb is a derived unit ( C = s·A ).
First, I learned electricity in the '50s. Yes, I
know I was only eight...<shrug>. The Systeme
Internationale did not exist prior to 1960 and
wasn't taught to the trade until long after; and
previous to that the coulomb was indeed the
fundamental unit, and the ampere was defined as
the flow of one coulomb per second. So I'm
teaching 1950s practical electricity which worked
fine then and still does even though it isn't
"legal." And I'm doing it for one reason: It's a
lot easier to understand. This also means that
we'll be talking "conventional" current, which
flows out of the positive terminal and into the
negative one. Ben Franklin flipped a coin and he
got it wrong; but it works fine in practice, and
it was confusing as heck when the young'uns
started getting taught 't'other way round -- it
meant that every conversation had to start by
defining terms. So call me a grumpy old fart,
and consider if necessary that you're in an electrical time machine.
>The SI base unit of electricity is the ampere,
>which is that constant current which, 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 x 10-7 newton per meter of length.
Second, just for amusement -- according to
wikipedia, the SI ampere is due to be redefined
within the next couple years in terms of a
certain number of charge units passing a point in
a given time. Which in practical terms means
that the new definition will be awfully close to
"one coulomb per second." Not exactly, but awfully close.
Finally (our *three* main weapons) I got the
numerical value of the coulomb confused with
Avogadro's number. As a kid I mentally set them
equal. The coulomb is actually a bit larger, 6.24 and change.
Cheers,
David
--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'89 Po' White Star "Scamp"
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