Re: R = V/I ?

[Mark Sylvester <msylvest@SPIN.IT>]

At 10.52 07/05/00 -0700, Leigh Palmer wrote:
> >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.
>
> I have no problem with resistivity as a physical quantity. The
> resistivity of tungsten at 2800 K has a meaning which I identify
> as a physical quantity. The resistivity of a light bulb, however,
> has no evident physical meaning.

Obviously not, but in deriving the resistivity from theories of materials
we are doing the same for resistance.

To quote Leigh from a parallel post:

>Well, if one wishes to call the ratio V/I the resistance of a light
>bulb at a specific operating point one may certainly do so. It is a
>matter of some concern to me that this resistance can't be used to
>calculate anything else. It does not condense the information in a
>way that will simplify a later calculation - operating voltage or
>current must still be specified along with this resistance, so how
>is that an improvement over specifying two other numbers, say
>voltage and power?

We do a lab where students measure I and V for a range of values, using a
12V bulb. They calculate R at each point in order to find the temperature
of the filament (and then go on to plot power vs temperature in pursuit of
the 4th power radiation law). I don't think this is a counter-example to
your comment, but it does show that the concept (of resistance) is useful
even when limited in the sense that you point out.

>The value of knowing the resistance of a real
>resistor is that only one parameter is needed to characterize the
>VI curve. I prefer to reserve the term "resistance" for devices
>that obey Ohm's law, except in applications where it is clear that
>both parties to a conversation are aware that what is being
>discussed does not obey Ohm's law.

Mark




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