On Apr 14, 9:57*pm, David Nebenzahl wrote:
On 4/14/2011 5:48 AM spake thus:





On Apr 13, 9:07 pm, "
wrote:

On Tue, 12 Apr 2011 21:23:03 -0700, David Nebenzahl
wrote:

On 4/12/2011 9:06 PM spake thus:

The current will only flow if there is a difference in
voltage.

Correct. *Ohms Law.

That is *not* Ohm's Law. Where do you get that? Sheesh--you're
trying to lecture *me* on this stuff???

I think he gets it directly from Ohms Law. * V=IR.

Or, I = V/R

If V, the voltage is zero, then I, the current must be zero. *Or, in
other words, current will only flow if there is a difference in
voltage.

But that's not Ohm's Law (the statement "current will flow only if there
is a difference in voltage"). Actually, that is a *tautology* (look it
up). In other words, that's the very definition of current, which
requires a potential difference (voltage 0) to flow. Ohm's law didn't
establish that, because it was already established by the time he came
along.

I never said Ohm established it. Only that from Ohm's law for the
circuit
under discussion you can directly verify that with zero potential you
get
zero current.




You've correctly stated Ohm's Law, but that's not what it says. Strictly
speaking, what Ohm determined was that the current flowing in a circuit
is proportional to the voltage and inversely proportional to the
resistance--but only for certain resistors. Specifically, his carefully
calibrated metal resistances, at a certain temperature. So "Ohm's
law"--what he determined experimentally and published--is only this:

* * I = E / R


And if E is 0, what does this say I will be? Zero. Yes, Ohm isn;t
the
first guy to discover that current only flows from a potential
difference.
But his law clearly reflects it and shows it to be true.




and that only at fixed temperature. Turns out "Ohm's law" does *not*
hold for a lot of things that look like resistances in the real world
(for example, any humble tungsten filament fails to observe it).

Total nonsense. Just because a filament changes resistance with
temperature
does not mean Ohm's Law doesn't apply. Ohms law applies at every
discrete
temperature/resistance point the filament has. If we followed your
logic
almost nothing would behave according to Ohm's Law. Even the
simplest resistor changes resistance slightly as current flows
through it and it's temperature rises slightly. That means the
resistance
has changed, not that Ohm's Law no longer applies.




But that's going waaaaay deeper into it than we need to here ...

At least one step deeper than you should have gone.