Re: R = V/I ?

[brian whatcott <inet@INTELLISYS.NET>]

Leigh's position on light bulb resistance is not untypical of the
usage of electrical engineers ( you know - the people who have
contributed the device that has transformed world culture and lifted
national productivity in bulk over the last few years).

 They think of the resistor as a discrete device that can be specified
in terms of mounting method, size, resistance value & tolerance with
temperature and time, power dissipation.

A person with this mindset sees at least four values
for an incandescent filament - comprising its hot and cold values
of course.

 It is difficult for him to enter this in a circuit simulator, unless
he selects a thermistor style (non-ohmic) device which for him it
effectively is.

 It is in this way that he specifies (for instance) the incandescent bulb
crucial to the Wien Bridge oscillator.
It would be unnecessary to remind such a person that the linear devices
he specifies as resistors, may be carbon composition (but probably not),
metal oxide, or metal.

He can recall that the accumulators that powered
 his grandfather's tube radio may well have been recharged with the carbon
filament stabilized battery charger in the radio store. He knows that
 the carbon filament was used there as a non linear element. He knows
the metal filament provides a non linear stabilization in the oscillator.

 Yet these same materials are the linear chess pieces with which he plays.

Perhaps it is something of the same philosophy as the physician's,
   who knows his illegible scrawl is specifying a deadly poison quite
likely, but in such proportion as to be therapeutic.

Brian


At 23:26 5/6/00 +0200, Mark Sylvester wrote:
> This is OK if indeed you want to *define* an electrical parameter in terms
> of temperature and geometry (I wouldn't in this instance), but notice what
> Leigh is saying:
>
> At 11.38 05/05/00 -0700, Leigh Palmer wrote:
> > I've never considered R = V/I to be anything so general as the
> > definition of a physical quantity. It is the definition (with
> > Ohm's law implicit) of a parameter which we call the resistance.
> > This parameter may apply to many two-terminal devices, notably
> > to resistors, and it is inappropriate to apply it to devices
> > which do not obey Ohm's law.
>
> He has just recently made a strong case for considering a lightbulb to be
> "a device which does not obey Ohm's law". It seems to follow that it is
> inappropriate to speak of the electrical resistance of a lightbulb. This
> seems to be an idiosyncratic position to take, and I'm interested to see
> how he will extricate himself from this corner.
>
> Mark

> > At 15.09 06/05/00 -0500, Rick Tarara wrote:
> > If [] resistance is a
> > temperature and geometry dependent property of the material, then
> > [] the light bulb has a well-defined resistance for every
> > temperature encountered within a given range of currents.  The I-V graph is
> > then a convenient way to MEASURE this resistance.  ;-)

> > From: "Mark Sylvester" <msylvest@SPIN.IT>
> > > But Leigh, would you not say that the (by now overworked) lightbulb has a
> > > well-defined resistance given by R = V/I at every point on the I-V graph?
> > >
> > > Mark

brian whatcott <inet@intellisys.net>
Altus OK