MrAl you have failed utterly to prove anything, just because more complex math is required to define how ohms law relates to an 'active' circuit does NOT mean that ohms law fails in an active circuit. Again, I will say for ANY given moment in time a circuit has a V I R equivalent that will work out. To say ohms law is an ultimate ratio with no other dependancies is ignorant, because even the most commone resistors are non linear with both voltage (very slightly) and with current and temperature (symbiotically) as well.
Hi again,
First off, if more complex math is required then we dont have Ohm's Law
anymore. Ohm's Law is a very simple and elegant rule that allows us to
calculate a third quantity knowing two others. If you have to complicate
that rule then it can not be Ohm's Law.
For example, i think it is very clever to think of adding a series resistor to
a diode and then calculating the diodes dynamic resistance based partly
on the drop across the resistor, but it is not Ohm's Law because Ohm's
Law does not say that we have to connect other elements in series with
anything else, and it deals with one element not two, and furthermore
we dont have to subtract anything (as we do with a series resistor and
ANY other circuit element, including another resistor).
Second, if we say that resistors dont obey Ohm's Law because they
vary slightly in resistance (hence non constant resistance) with any
other variable such as current (self heating) then we have just reduced
Ohm's Law to nothing at all (again). In other words, we would eliminate
Ohm's Law from reality and gee i just wonder maybe we should tell
everybody that we no longer need Ohm's Law because we found out that
no element on earth truely obeys it 'perfectly'. Good idea? I dont think
so. I dont think so because Ohm's Law is still a very useful tool when
used appropriately.
Third, if you can say that a diode obeys Ohm's Law because for one
value of current I we can say that we know what R is by measuring V
and because V=I*R holds for that one value, then i can say an ordinary
resistor follows the ideal diode law:
v=N*Vt*ln(i/IS+1)
just by finding a new N for every new current i that I measure.
So what do we say now, that every law for different elements is really
the same? Gee, i think from now on i will only use the law of gravity
for everything including circuits
How about this...
We have a rotating resistor rotating laterally, so that it's leads come into
contact with two brushes mounted on opposite sides of rotation. When
the leads are rotated such that they come into contact with the brushes
we measure the resistors value, very quickly, and note the result.
We rotate it very fast...30,000 rpm. Now at some point in time it comes
into contact with the brushes and we measure its resistance, so i guess
we know V=I*R at that one instant in time (or over a short interval).
I guess that means we can say that this apparatus between the two
brushes follows Ohm's Law, because for that one instant we can measure
its resistance? I dont think so
Also, what is its resistance when
there is an open circuit between the two brushes?
Note that when we use Ohm's Law properly we can calculate a third
quantity from knowing two others, and that we only have to calculate
the resistance once.
Again, Ohm's Law is expressed as a ratio because Ohm's Law is calculated
as a ratio, but a ratio is not the same as Ohm's Law.
Elements that obey Ohm's Law have certain properties, and elements that
do not obey do not possess these properties...that's how we can tell the
difference between the two types of elements.
It's not really about semantics either, because the two types of elements
are very clearly different in nature.
What some of you guys need is some lab time. Take a few resistors and
do a few experiments using Ohm's Law. Measure the voltage, calculate
the current...see how easy it is to do. Then, take a few diodes and
try to calculate the current...see how much harder it is to do, and not
once can you use Ohm's Law to do so.
If i hand you a 100 ohm resistor to do the experimenting with, you can
quickly calculate the current knowing the voltage across the device,
but if i hand you a 1N4148 diode, it's not going to happen because
the diode is a very very different kind of device, and it is often
characterized by saying that it does not follow Ohm's Law while
the resistor does.
Another challenge for you is to find one notable reference that shows
Ohm's Law being used for a diode (not a piecewise linearization however
because we already know we can use Ohm's Law to approximate a
diodes behavior over a very short range of current).
We have presented several references that clearly show that a diode
does not follow Ohm's Law, so it's up to you now to find one that shows
different.