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Re: R = V/I ?



At 16.27 06/05/00 -0700, Leigh Palmer wrote:
>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.

Please explain the corner I'm in. I believe I implied it is not
appropriate to describe light bulbs by their resistance as they
do not behave like resistors. A VI characteristic is necessary
to describe their behaviour in circuits.


I begin (finally) to see where you're coming from. I use the term
"resistance" of a device as I would "pressure" of a gas - a physical
parameter with a clearly defined meaning, whose value depends on contingent
conditions. "Ohm's Law" I teach as the statement that for some materials
(notably metallic conductors) the resistance is constant (wrt applied pd in
particular), at constant temperature.

I think this usage has some justification. After all, in solid state
physics we (attempt to) derive the dependence of conductivity (and hence
resistance) on concentration and mobility of charge carriers, giving the
parameter a more general physical significance.

Mark






_____________________________________
Mark Sylvester
United World College of the Adriatic,
34013 Duino TS, Italy.
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