Russell,

I still think that it is misleading to say "Ohm's law always works" It
isn't an arbitrary definition. Current and voltage were each defined
and discovered independent of the concept of resistance. It was then
discovered that, for most materials, the ratio between voltage and
current was a constant for that material (at a particular temperature,
with a particular geometry, etc.) It wouldn't have to be this way - it
could have been that most materials had a highly nonlinear V-I
relationship, in which case, simply defining V=3DIR would not have made
much sense.

This is more than just semantics or pure theory - it has very
practical implications. If one thinks of V=3DIR as a fundamental
definition of resistance, then he is likely to try to compute the
"resistance" of a battery, for example. It has a certain V. It has a
certain I, so it should have an R=3DV/I. But of course, in this case, it
makes much more sense to talk about R=3DdV/dI, which is often called the
internal resistance of the battery, which is a much more useful
concept for a battery than the simple ratio of V/I.

Sean


On Mon, Jul 6, 2009 at 9:21 AM, SME<shoemakerself@gmail.com> wrote:
> PS: Ohm's law always works, by definition. ie I =3D V/R. R =3D V/I. V =3D=
 IR
> ie it's essentially a tautology.
> BUT as noted - for some substances the ratio of V/I (=3DR) changes with
> V and or I. R can also changes with temperature, pressure, exposure to
> EM field etc. For most purposes I =3D V/R is valid enough to be useful.
>
> For semiconductor devices the ratio of V/I can change inmmensely with
> V. Which is exactly what happens in the LED case being discussed on
> this thread. Aty say 20 mA an LED may have a Vf of 3.0V, so R =3D V/I =3D
> 150R. However, at 30 mA Vf may be 3.1V. And at 100 mA Vf may be 3.3V.
> Giving R's of 103 R and 33 R. Clearly R is VERY dependant on V. We
> don't usually think about this example in these erms, but doing so
> demonstrates that Ohms law, as it is usually understood, can be easily
> 'broken' in everyday life.
>
>
>
>
> =A0 =A0 =A0 =A0 =A0 Russell.
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