Re: Is zero even or odd?
From: Michael Mendelsohn (invalid_at_msgid.michael.mendelsohn.de)
Date: 12/24/04
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Date: Fri, 24 Dec 2004 00:17:14 +0100
John Fields schrieb:
> On Thu, 23 Dec 2004 22:45:26 +0100, Michael Mendelsohn
> >> "Michael Mendelsohn" <invalid@msgid.michael.mendelsohn.de> wrote
> >> > If in measuring a resistor, we find 0.0A at 0.0V, is the resistance 1
> >> > Ohm, then?
> >When checking it turned out that some thief had actually stolen the
> >resistor where 0V,0A was measured. The circuit was broken, but noone
> >noticed because the voltage was zero.
> In truth, the E in
>
> E
> R = ---
> I
>
> refers to the voltage _across_ the resistor, (a shunt, was it?) which
> you didn't measure. What you measured was the voltage from the low
> side of where the resistor was supposed to be to ground, which gave
> you zero volts which corresponded, also, to zero amps. Had you
> measured the voltage _across_ where the resistor was supposed to be
Well, I did - do you think I'm stupid? ;)
The problem is, the measurement was automatic, and since there was
short-circuit somewhere (presumably in parallel to the supposed
resistor), the voltage was zero across the measurement points even
before the resistor was stolen. The current is of course measured the
proper way.
The software computed resistance by Nick's rules and hence never noticed
anything unusual.
> you would have measured the entire supply voltage minus what was being
> dropped across the load by the current flowing through the meter and
> you would have concluded that by subtracting the meter current that
> you would have had:
>
> E E
> R = --- = --- = oo
> I 0
>
> Which would have been right!
Unless E=0 too, in which case the result is 1 (says Nick).
On a short circuit you can detect no voltage, but you can measure a
current.
E 0
R = --- = --- = 0
I I
This leads to a contradiction when E=I=0.
Cheers
Michael
--
Still an attentive ear he lent Her speech hath caused this pain
But could not fathom what she meant Easier I count it to explain
She was not deep, nor eloquent. The jargon of the howling main
-- from Lewis Carroll: The Three Usenet Trolls
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