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This is very much a navigational byway, but it's a matter I would like to
help clear up.

In thread Ohm's Law???????????, Doug Royer commented-

"George,
Where or how did you come up with "Ohm's Law only applies to those
conductors that Ohm's law applies to.Try a filament lamp, zinc oxide
resistor,a semiconductor junction etc. Ohm's Law applies to none of
these.Nor does it apply toa contact between 2 oxide corroded metal
surfaces".
Am I missing something?
Ohm's Law most assuredly still applies to everything you mention.
Any PN junction is nothing but a variable resistor.It doesn't operate in a
linear fashion but it is still subject to Ohm's Law.
You'd better guess again about your statement reguarding it doesn't apply to
contacts between 2 corroded metal surfaces either."

In a later message he added-

>After thinking about this for awhile I still don't know where you're coming
>from George.
>Here's what I'm thinking about this.
>Ex. # 1
>2 plates of iron are fastened together so there is a large surface area
>touching each plate.A + potential is connected to 1 and the negative
>potential is connected to the 2nd. X amount of current will flow because of
>the low resistance. After a time corrosion builds up between the plates.The
>resistance between the 2 surfaces is now greater so less current will be
>able to flow with the same potential.
>Ohm's Law is valid.
>Ex. # 2
>A  5 volt,1 watt zener diode is regulating the voltage in a circuit at its
>max knee current.The input voltage rises to the point that the wattage of
>the zener is exceeded thus destroying the junction.
>Ohm's Power Law is valid.
>Same with any other type PN junction.
>
>Will you please explain what you were getting at in your post reguarding
>this?Perhaps I misunderstand what you're trying to get across.
>This does have relavance to navigation because someone started the thread of
>lightning on the navigation list and there are plenty of responces along
>with George's statement.

=====================

Ohm's law simply stated that the current through a conductor is
proportional to the voltage you apply across it.

When Doug states- "Ohm's Law most assuredly still applies to everything you
mention.
Any PN junction is nothing but a variable resistor.It doesn't operate in a
linear fashion but it is still subject to Ohm's Law.", that's where he goes
wrong.

If it doesn't operate in a linear fashion then it doesn't obey Ohm's law,
because Ohm's law says nothing more than that "conductors operate in a
linear fashion", which is another way of saying that the current is
proportional to the voltage.

When Doug refers to the "knee voltage" of a zener diode, the mere fact that
it has a knee in its plot of voltage versus current implies that it doesn't
obey Ohm's law, and it's just that characteristic that makes it so useful.

Doug refers to "Ohm's Power Law". What is Ohm's Power Law? I do not
recognise it. Does Doug refer to the law that states that "power
dissipation = voltage x current", which does indeed apply (for d.c.) to any
device, whether it's linear or not. Was Ohm responsible for that one?  I
doubt it (but could be wrong).

Doug states-

>Ex. # 1
>2 plates of iron are fastened together so there is a large surface area
>touching each plate.A + potential is connected to 1 and the negative
>potential is connected to the 2nd. X amount of current will flow because of
>the low resistance. After a time corrosion builds up between the plates.The
>resistance between the 2 surfaces is now greater so less current will be
>able to flow with the same potential.
>Ohm's Law is valid.

Well, oxide can be a rather good insulator, so if you applied 1.5 volts
from a torch bulb across the rusty interface, you might not see any
measurable current flowing at all. But try increasing that to 100 volts, or
1000 even. At some point there will be a change, as the rust-film breaks
down, and the current will suddenly increase to a high value. It's likely
to be accompanied by some sparking and crackling. Depending on how much
current your power-source can deliver, you could end up with a spot-weld
between the plates. As Ken James has pointed out, sparking generates a
local plasma which can carry the current. The "instantaneous" local heating
can result in melting at the surfaces, as happens in arc-welding.

In the case of a lightning strike, the available voltage and current are
pretty well unlimited, so there's no rust-film (or between-links chain of
rust-films) that's ever good enough to provide a barrier.

That said, a heavy Copper conductor will no doubt do a better job. I
wouldn't claim that a chain is in any way superior. It's just that you use
what you have around.

George.







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