I would like to call your attention to a misnomer in electrical science of the formula V = I*R, which is wrongly called Ohm's law.

I will first quote from a college textbook called Physics, by Halliday & Resnick, 1967, page 780. It was written by David Halliday, Professor of Physics, University of Pittsburgh and Robert Resnick, Professor of Physics, Rensselaer Polytechnic Institute

-----------------------------------------------------------------------
"We stress the relationship V=I*R is NOT a statement of Ohm's law. A
conductor obeys Ohm's law only if its V vs. I curve is linear, that is, if R
is independent of V and I. The relationship R=V/I remains as the general
definition of the resistance of a conductor whether or not the conductor
obeys Ohm's law. . . . . . . . . Ohm's law is a specific property of
certain materials and is NOT a general law of electromagnetism, for example,
like Gauss's law."
-----------------------------------------------------------------------

Next a quote from another college textbook called Physics for Scientists & Engineers, by
Raymond Serway, Third Edition, 1990, page 745. It was written by Raymond A. Serway of James Madison University

-----------------------------------------------------------------------
"A current density J and an electric field E are established in a
conductor when a potential difference is maintained across the conductor.
If the potential difference is constant, the current will also be constant.
Very often, the current density in a conductor is proportional to the
electric field in the conductor. that is J=sigma*E where sigma is called the
conductivity of the conductor. Materials that obey the above equation are
said to follow Ohm's law, named after Georg Simon Ohm (1787-1854). More
specifically,

Ohm's law states that for many materials (including most metals) the
ratio of the current density and electric field is a constant, sigma, which is
independent of the electric field producing the current.

Materials that obey Ohm's law, and hence demonstrate this linear
behavior between E and J, are said to be ohmic. The electrical behavior of
most materials is quite linear for very small changes in the current.
Experimentally, one finds that not all materials have this property.
Materials that do not obey Ohm's law are said to be nonohmic. Ohm's law is
not a fundamental law of nature, but an empirical relationship valid only
for certain materials."
-----------------------------------------------------------------------

OK, what could be clearer? Ohm's law refers to the linearity between voltage and current, not relationship between voltage, current, and resistance. Yet the formula, V = I*R has been misnamed across countless classrooms, books and discussions. What does your textbook or school teach?

To summarize, certain materials like conductive metals follow Ohm's law, in that their V vs.I curve is linear. Ohm's law is a property of a material, not a general law of nature. Other conductive entities like diode junctions or gas discharge bulbs do not have the Ohm's law property, because their conductivity changes depending on what voltage or current is applied, causing their V vs. I curve to be nonlinear. In all cases, V = I*R is always correct, but it should not be called Ohm's law. I suggest it should perhaps be called Ohm's formula.

No matter what you call V = I*R, circuits will still get designed and analyzed, and science will still progress. In any case, be aware that V = I*R is not Ohm's law, but the V vs. I linearity, if present, is the law.

I await the fallout from those who may think this post is outrageous. I hope they can shed more light than heat on this matter. Ratch
Posted on 2003-05-27 00:16:19 by Ratch
30 years of circuit design down the drain.:mad:
Posted on 2003-05-27 09:03:33 by drhowarddrfine
drhowarddrfinedrhoward,

30 years of circuit design down the drain.


Not at all. Read the second to the last paragraph of my post. You were only joking, were you not? Ratch
Posted on 2003-05-27 09:24:35 by Ratch

. Other conductive entities like diode junctions or gas discharge bulbs do not have the Ohm's law property, because their conductivity changes depending on what voltage or current is applied, causing their V vs. I curve to be nonlinear. In all cases, V = I*R is always correct, but it should not be called Ohm's law. I suggest it should perhaps be called Ohm's formula.


My instruction of Ohm's law was: The current in a circuit is DIRECTLY proportional to the applied voltage and INVERSELY proportional to the circuit resistance.




Above is a PN junction diode. Ohm's law tells me at point A with a forward bias of 1v and 5ma that the junction is 200 ohms. At point B with 3v and 50ma the junction measures 60 ohms. And finally, reverse biasing the diode at point C with 80v and 100?a there exist 800k ohms of resistance.

It's not linear but Ohm's law yields the resistance of the junction. Maybe I don't understand what you are stating.
Posted on 2003-05-27 22:21:56 by alpha
alpha,

Don't worry, you got it. 100% got it.

Some people have their particular take on life that doesn't match what the rest of us do.

You'll meet more people like this in life. Best thing is to listen politely (as long as they are quick about it), smile, say something non-commital but positive sounding, then leave.

After all, they just talk to hear themselves.
Posted on 2003-05-27 23:20:15 by Ernie
alpha,
It's not linear but Ohm's law yields the resistance of the junction. Maybe I don't understand what you are stating.

The last sentence of your above quote is true. The point I am trying to make is that Ohm's law does not give the resistance of a particular operating point of a PN junction, or the resistance of anything else. The formula R = V/I gives you those values corrrectly, but that formula should not be called Ohm's law. Ohm's law is a property of a material and not a method of calculating resistance, voltage, or current when two of those quantities are known. That material property is the linear relationship of V and I as described in the material from the college texts that I quoted. Since PN junction diodes do not have this linear property, they do not follow Ohm's law and are nonohmic. The formula R = V/I is always correct whether the material or device follows Ohm's law or not. I like to call R = V/I Ohm's formula, to differentiate it from what Ohm's law really is, the linearity of V vs. I.

I like your graph by the way. Ratch
Posted on 2003-05-27 23:29:56 by Ratch
Ernie,

I think you are trying to say that I am wrong. Fine, but how do you explain or dismiss what is said in two preeminent college textbooks. I did not make up those sources; they really exist. Did I misintrepret what was said by those physics professors? Please don't just smile and leave. Put forth your own arguments about the validity of my proposition. Ratch
Posted on 2003-05-27 23:41:48 by Ratch
What he's trying to say, I think, is that while the subject of this topic is probably true, for 99% of us Ohm's law works fine as it is. As you know, there are many things in electronics and other professions that don't work exactly as most people assume they do but "rules of thumb" do work in most or all cases. While you're post is very important to those working on that critical level, most designers don't give it a thought even if they're aware of it.
Posted on 2003-05-28 09:03:00 by drhowarddrfine
drhowarddrfinedrhoward,

What he's trying to say, I think ....

I think that you are referring to "he" as Ernie. I understand and appreciate what you are saying. Lotsa things are described and labeled wrong. I just wanted to point out that most people's preception of Ohm's law is one of them. Politicians are the worse offenders, but sometimes scientific organizations don't do too well either. For instance, NASA contantly refers to its astronauts as walking in space, when everyone who observes them can easily see that they are floating. If their tethers become unfastened, they would not walk away, would they? It was more descriptive when they used to described it as extravehicular activity (EVA). Ratch
Posted on 2003-05-28 10:12:32 by Ratch
When George Simon Ohm formulated his theory he was working with Alessandro Volta's invention of the battery, he didn't have access to the exotic materials we have today. AC is another issue he couldn't explore. In AC circuits you can only use Ohm's law to solve purely resistive circuit problems. I was taught that Ohm's law cannot be applied to alternating-current circuits since it does not consider the reactance which is always present in such circuits. However, by a modification of Ohm's law which does take into consideration the effect of reactance we obtain a general law which is applicable to ac circuits. Impedance or Z, which represents the combined opposition of all the reataances and resistances in an AC circuit can replace resistance to give a general law: V=I*Z. This general modification applies to alternating current flowing in any circuit, and any one of the values may be found from the equation if the others are known.

I understand that Ohm was ridiculed during his time for his findings which disagreed with the status quo. As technology marches forward and goes quantum towards nanotechnology the rules will certainly change as we know it today. An analogy of this is how Newton's laws of gravity evolved with Einstein's work.
Posted on 2003-05-28 11:32:17 by alpha
The only thing Ratch is saying is that the formula which most of us call Ohm's Law, is mislabelled. I checked an old (1977) textbook on basic electronics, and it had this mislabelling -- it has been going on for a long time. That's why drhoward laments (jokingly) that he just lost 30 years of circuit design experience.

Most of us will continue to refer to the DC formula as "Ohm's Law", in spite of Ratch's (and physicists') objections.
Posted on 2003-05-28 15:54:41 by tenkey
alpha,

From what you wrote, I think you still do not understand what I am saying. So, let's look at some links.

First a biography, http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Ohm.html . Here is a quote from the ninth paragraph. "In fact he had already convinced himself of the truth of what we call today 'Ohm's law' namely the relationship that the current through most materials is directly proportional to the potential difference applied across the material. " Of course, some materials do not have a direct proportional relationship between V and I, so those are not ohmic.

Next look at http://maxwell.byu.edu/~spencerr/websumm122/node50.html#SECTION00066000000000000000 . This tells you what resistance is and what is not Ohm's law.

Then look at http://www.launc.tased.edu.au/online/sciences/PhysSci/done/electric/resistnc/Resistance.htm . This should convince you that what you think of as a formula to calculate resistance or impedance is NOT Ohm's law, even though you and almost everyone else thinks it is. Ohm's law is a material property involving linearity between V and I, and is NOT a means of calculating the relationship between V,I, and R. The formula V = I*R or V = I*Z does that, and is not Ohm's law.

Finally, review what is said in the textbook quotes of my first post. You will see that it backs up what is on the links. Ratch
Posted on 2003-05-28 16:49:20 by Ratch
While your with in your right to do so, i simply wish you would stop confusing what needs not be confused. :rolleyes:
Posted on 2003-05-28 16:52:12 by NaN
tenkey,

The only thing Ratch is saying is that the formula which most of us call Ohm's Law, is mislabelled. I checked an old (1977) textbook on basic electronics, and it had this mislabelling -- it has been going on for a long time.
You are spot on the money there, tenkey.

Most of us will continue to refer to the DC formula as "Ohm's Law", in spite of Ratch's (and physicists') objections.
Sad but true. Ratch
Posted on 2003-05-28 16:53:53 by Ratch
Nan,

While your with in your right to do so, i simply wish you would stop confusing what needs not be confused.

Au contraire. I am pointing out what should be clarified. The truth should not confuse anyone. Ratch
Posted on 2003-05-28 17:02:53 by Ratch
Truth confuses everyone.
Posted on 2003-06-02 03:16:27 by inFinie
If we demote Ohm's law to Ohm's formula, we should also consider all the other formulas we currently call laws. Perhaps if Ohm had used wording more similar to Newton, you might be satisfied. Perhaps you would accept revising Ohm's Law to state:

An instantaneous electric current tends to continue to the immediately following moment in time in an unchanged state until acted upon by an instantaneous unbalanced vector force. This force is described by the vector sum of any instantaneous electromotive force and any instantaneous electromagnetic force. Once acted upon by an instantaneously unbalanced vector force, the instantaneous elecric current will tend to change it's instantaneous state in an equal and opposite vector direction in the immediately following moment in time.

For you see, at any given moment in time, in the non-quantum world, Ohm's Law does hold true. Notice I said at any given moment in time. What happens in the next moment is determined by outside forces acting upon the electric current in the previous moment.

At any given moment in time, the amount of pressure being applied by any locomotive force from any vector on any mass (including a stream of electrons through a medium like carbon or copper) tends to be exactly equal in magnitude and exactly opposite in vector direction to the amount of opposition being offered back to the force by the mass. If the exact direction (phase) and velocity (instantaneous current) of a mass (electron stream) is known in one moment, and we also know the sum of all vector forces that are present to instantaneously act on that mass (voltage, reactance, capacitance, resistance), then the exact direction (phase) and velocity (instantaneous current) of the object in the immediately following moment in time tends to be exactly predicatable using Ohm's Law. At that very moment, Ohm's Law also tends to predict exactly how much force (Voltage) the object is feeling applied to it, and from which direction (phase). If this does not hold true, then neither do the nine laws of conservation hold true, including the theory of conservation of energy, and the theory of conservation of momentum.

The non-Ohmic nature of certain substances, and even certain situations all exist as static values inside a single moment and can be accounted for in the calculation of the unbalanced vector force that acts upon the mass or electron stream in the given moment.

I hold that Ohm's Law in it's orignal form is still a law, but must be applied using vectors instead of just real numbers, and can only be applied at single moments in time. I hold that it is true for analyzing instantaneous current flow through non-Ohmic substances as well as Ohmic ones, as long as all instantaneous vector forces that are present to act upon the instantaneous electric current are known. To say that Ohm's Law does not hold true for all instantaneous examinations would be like saying that when I push against a tree, it does not necessarily push back with the same amount of force, depending on what kind of wood it is composed of.

The only change that we make to Ohm's Law in working electronics, is that we accept the use of imaginary numbers and that we can only apply Ohm's Law at a single moment in time to get what we refer to as instantaneous values. Imaginary or Vector/Polar numbers tend to work in any expression that was developed before Imaginary numbers were defined and accepted. The expansion of an expression to include imaginary numbers does not in my opinion invalidate the original expression. In the case of Ohm's Law, imaginary numbers re-inforces it's integrity because the formula still works in what tends to be an entirely predictable fashion.

(please excuse my habitual use of the redundant phrase 'current flow' if present)
Posted on 2005-01-28 20:39:14 by gluespill
gluespill,
I've seldom come across so much obfuscating blather. Is your post a joke?

This force is described by the vector sum of any instantaneous electromotive force and any instantaneous electromagnetic force....


Electromotice force (volts=Joules/columb) is not a vector quantity.


For you see, at any given moment in time, in the non-quantum world, Ohm's Law does hold true.


Ohm's law is a property of a material. It has nothing to do with time.



At any given moment in time, the amount of pressure being applied by any locomotive force from.....


Pressure is force per unit area. That is irrelevant to the discussion except as a descriptive analogy.


the sum of all vector forces that are present to instantaneously act on that mass (voltage, reactance, capacitance, resistance),...


Voltage, reactance, capacitance, resistance are neither mass or force. Neither are they vector quantities.


.At that very moment, Ohm's Law also tends to predict exactly how much force (Voltage) the object is feeling applied to it, and from which direction (phase).


Ohm's law is a property of a material. It is not the resistance formula V=RI. Voltage is not force and phase is not a direction. Force is not used to describe what the resistance formula is. It might be used to explain its working at the quantum level.


If this does not hold true, then neither do the nine laws of conservation hold true...


Well, your statement does not hold true. What are the nine laws of conservation?


The non-Ohmic nature of certain substances, and even certain situations all exist as static values inside a single moment and can be accounted for in the calculation of the unbalanced vector force that acts upon the mass or electron stream in the given moment

The above is jabberwocky, in my humble opinion.


I hold that Ohm's Law in it's orignal form is still a law, but must be applied using vectors instead of just real numbers, and can only be applied at single moments in time.


What do vectors have to do with a property of a material?




In the case of Ohm's Law, imaginary numbers re-inforces it's integrity because the formula still works in what tends to be an entirely predictable fashion.


What do imaginary (duplex) numbers have to do with Ohm's law, the property of a material.


(please excuse my habitual use of the redundant phrase 'current flow' if present)


You should say charge flow.

Here is the way it works. The voltage difference between any two points forms a electrostatic field at the speed of light. The electrostatic field (EF), which is a vector, exerts a force on a charged particles like a electrons or protons. The EF only affects charges, not mass. For instance, a EF will not affect a stream of neutrons. The force on the charged particles can cause them to move, resulting in a current. The current will change according to the EF (formed by the voltage difference) at the speed of light. Ratch
Posted on 2005-01-31 11:03:16 by Ratch
There?s a second side to this debate which has been overlooked. Yes, being overly picky about the details is really just being anal (or should that be anal retentive or perhaps it should be anal-retentive ;) ) But on the other side being overly lax with the specifics, particularly misrepresenting something named after someone, shows a lack of respect for that person.

Perhaps just to give an example, take the Turing Test, and I?d like to quote from The Turing Test Page which gives the original formulation of the test in italics:


I propose to consider the question "Can machines think?" This should begin with definitions of the meaning of the terms "machine" and "think."

Turing Test is meant to determine if a computer program has intelligence. Quoting Turing, the original imitation game can be described as follows:

The new form of the problem can be described in terms of a game which we call the "imitation game." It is played with three people, a man (A), a woman (B), and an interrogator (C) who may be of either sex. The interrogator stays in a room apart from the other two. The object of the game for the interrogator is to determine which of the other two is the man and which is the woman. He knows them by labels X and Y, and at the end of the game he says either "X is A and Y is B" or "X is B and Y is A." The interrogator is allowed to put questions to A and B.

When talking about the Turing Test today what is generally understood is the following: The interrogator is connected to one person and one machine via a terminal, therefore can't see her counterparts. Her task is to find out which of the two candidates is the machine, and which is the human only by asking them questions. If the machine can "fool" the interrogator, it is intelligent.


The last paragraph above is the test as most of us know it, in fact an even simpler form of it (often presented in the media) just has one human talking to one unknown via a terminal or whatever.

Crucially the quoted bits of the original test neglects an important paragraph from Turing paper, though a link is provided.

We now ask the question, "What will happen when a machine takes the part of A in this game?" Will the interrogator decide wrongly as often when the game is played like this as he does when the game is played between a man and a woman? These questions replace our original, "Can machines think?"


And therein lies the huge difference between Turing?s Imitation Game and the test most of us know as the Turing Test. The game isn?t about a machine trying to trick someone into believing they?re talking to a human, its about pitting a man against a machine in a game to see who is best at pretending to be a woman.

For an example of why I see this distinction is so important I?ll point ye to a page by my own AI lecturer, Mark Humphrys, the page is very critical of the Turing Test and includes a link to a second page where he tells of a program he wrote in 1989 which ?passed the Turing test?

Did it? Well in the loosest interoperation of the third formulation (the media formulation) of the test I suppose it did pass. But under no circumstances would it have passed the actual test. Personally I see the test as the ultimate, final test an AI would have to pass to quash any questions of its intelligence, but its certainly not the first test.

I think it?s a credit to Turing that he formulated quite probably the hardest test in AI and I see it as an huge insult to him that he is almost universally misrepresented.

But well I?ve digressed ( clearly I?ve been waiting to vent that particular misgiving for a while and this has probably become my biggest post ever in the process :) ) but I do hope that perhaps my point has been made a bit more clear-

As someone who would think of himself as a helpful person I do not like having minor mistakes (or even big ones) pointed out particularly while I am giving of my time to help people. But there are some things which just simply should be correct, if even just out of a respect for the great minds that?ve come before us.
Posted on 2005-01-31 19:30:00 by Eóin
E?in,
But there are some things which just simply should be correct, if even just out of a respect for the great minds that?ve come before us.


What you say if fine, but where is the disrespect? And what does it have to do with the Ohm's law misnomer? I am trying to see the link between your posting and mine. Am I right about Ohm's law? Ratch
Posted on 2005-01-31 20:13:29 by Ratch