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As far as I know, R (incandescent lamp) = E/Isub0 x G(E.I)
Isub0 current at ambient temp (low power)
G is a power function of temperature.
This is a non linearity not unlike the diodes!
All very true.
But in the demo of different wattage bulbs in series and parallel,
and if you need to know the maximum transient current at startup,
Ohm's law doesn't seem to work very well, at least to an unsophisticated
student, which is why I threw in the comment.
It is perilous to do so, but in the interest of removing what
I think may be a nascent misconception, I will have to disagree
with David Bowman here. Tungsten may be an "ohmic substance"
(though I am unable to define what that means). I'll say more
about it later later. Light bulbs, even though their filaments
are made of tungsten, are *not* ohmic devices.
Ohm's law is not
a law of nature any more than Hooke's law is. Both are simply
useful approximations of actual behaviour, and the degree to
which a particular resistor (or spring) is ohmic (or Hookeish)
has reasonable tolerances.
Light bulbs operate in a range of
currents for which their behaviour would most certainly be
considered non-ohmic by any engineer,
and by this physicist as
well.
A measure of "ohmicity" might well be in order here. I'm at
home today nursing a cold, and I don't have access to the
references (and laboratory) I would like, but a good measure
might be the quantity
V dI
r' = --- ----
I dV
. . . . Light bulbs in the vicinity of their operating
points deviate greatly from the ideal, though without a
laboratory I can't estimate how greatly.
David's idea of immersing a tungsten filament in a constant
temperature environment is invincibly problematic. It would be
possible to do such a thing if Joule heating could be ignored,
but it can't. I appreciate the theoretical approach to many
problems, but a dissipationless flow of current in an ohmic
substance is, I'm afraid, out there with the spherical cow.
Provided, of course, that you operate your "ohmic material" in its ohmic
range of behavior, ie, where R=V/I is the *linear* fit of the plot of V vs
I. Light bulbs are ohmic as long as they are not operated as light bulbs!
Once you start getting near incandescent, the behavior is extremely
non-linear. Karl