Re: R = V/I ?

[Mark Sylvester <msylvest@SPIN.IT>]

At 14.01 05/05/00 -0400, Ludwik Kowalski wrote:
> I used to say it the coefficient of proportionality between the
> DOP and current.
> From now on I will try to emphasize that
> R is a coefficient of proportionality between dV and dI, at any
> given I. Is this acceptable?

This dV/dI is, I think what electrical engineers call "dynamic resistance".
Consider the I - V graph for a lightbulb: the filament has a resistance at
some current I, which is (measured or defined?) as V/I. dV/dI is something
else - 1/gradient of the tangent to the curve at the point of interest.

Mark

> Mark Sylvester wrote:
>
> > At 12.29 05/05/00 -0500, Richard W. Tarara wrote:
> > >Another reason to avoid this usage is the confusion it causes the
> beginning
> > >student.  They may look at R = V/I and say OK, if we increase V then R
> > >increases.  While this might happen (to some extent) with a light bulb, it
> > >is definitely not the behavior we use Ohm's Law to describe.  Again,
> we have
> > >the problem of just how students interpret equations.  Many will look at
> > >them as definitions rather than functional relationships.  In my mind, the
> > >only time to use R = V/I is in a situation where you know both the voltage
> > >across and current through an object and want to know the resistance.  In
> > >this sense, the relationship gives you a MEASUREMENT of R but not a
> > >definition of R.
> >
> > Hmmm... what, then, is the quantitative definition of R?
> >
> > Mark
> >
> > _____________________________________
> > Mark Sylvester
> > United World College of the Adriatic,
> > 34013 Duino TS, Italy.
> > _____________________________________


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