Dave,

This characteristic is embodied in the equation:

I = V/R

Now it is correct to say not all devices obey this law for their V-I characteristic, diode being an obvious example where the I-V characteristic (note the switch from V-I to I-V) is defined by the non-linear Ebers Moll relationship.

I disagree. All devices obey the resistance formula V = IR at all times. Not all devices do it in a linear fashion to comply with Ohm's law. What in the world does the Ebers Moll transistor model have to do with this?

There is an important point to note - even non-linear devices display Ohmic behaviour - consider the diode which has an exponential I-V characteristic, that is as the voltage across the diode changes (ΔV) so the current varies non-linearly in an exponential manner. If we take the condition that ΔV tends to 0, i.e. we are look at the I-V characteristic locally we see that device behaves Ohmically - that is the resistance at that point, R = V/I. This is the same as saying the resistance is the reciprocal of the derivative of Ebers Moll defined at a voltage V where ΔV is zero. Therefore locally, diodes like all electronic devices, obey Ohms Law.

I can't quite figure out what you are saying. The derivative of a exponent is also an exponent. There might be a small region close to zero where a diode might be approximately linear. Within that range, it could be said that the diode is ohmic. But stray very far from zero and the diodes's nonlinearity becomes readily apparent. Anyway, the resistivity/conductivity of any semiconductor is well known to be nonlinear and nonohmic.

Ratch

 

beenthere,

Given these examples, how does this affect our understanding of Ohm's law? It really does not seem to have been invalidated by the above, nor must we pay careful attention to the exceptions when calculating voltage drops in a circuit.

Those examples don't affect my understanding of Ohm's law. And it doesn't matter whether a material is linear or not when using the resistance formula. Just be sure not to call the resistance formula Ohm's law. That would be a misnomer.

What is the point of scrupulously pointing out that Ohm's law is not absolutely universal? As I have said before, this is not a physics - I think I said 'condensed matter' previously

I never did any such thing. Ohm's law, the linearity or nonlinearity of a material's resistance/conductivity is universal like all material characteristics are. Whoever said otherwise?

Ratch

 

I disagree. All devices obey the resistance formula V = IR at all times. Not all devices do it in a linear fashion to comply with Ohm's law. What in the world does the Ebers Moll transistor model have to do with this?

Ohms Law is a linear relationship that defines resistance at a specific V and I value for any electrical/electronics device (as per Georg Ohm's 1827 definition). The I-V characteristic (across a range of voltages) for a diode follows the Ebers Moll model (i.e. is non-linear); however if you localise the characteristic at a particular voltage the diode is Ohmic - something you do say you agree with (even if you don't realise it).

I can't quite figure out what you are saying. The derivative of a exponent is also an exponent. There might be a small region close to zero where a diode might be approximately linear. Within that range, it could be said that the diode is ohmic. But stray very far from zero and the diodes's nonlinearity becomes readily apparent. Anyway, the resistivity/conductivity of any semiconductor is well known to be nonlinear and nonohmic.

You need to differentiate (not mathematically) between the V-I characteristic and the determination of the value at particular voltage/current. Yes, the derivative of a exponent is also an exponent, therefore the resistive characteristic of a diode is non-linear, however what I am saying is localise the characteristic at a particular applied voltage and Ohms law applies to the diode - it becomes Ohmic at that localised point.

Dave

 

Dave,

Ohms Law is a linear relationship that defines resistance at a specific V and I value for any electrical/electronics device (as per Georg Ohm's 1827 definition). The I-V characteristic (across a range of voltages) for a diode follows the Ebers Moll model (i.e. is non-linear); however if you localise the characteristic at a particular voltage the diode is Ohmic - something you do say you agree with (even if you don't realise it).

No, Ohm's law specifies linearity of a particular material charactistic. The resistance formula defines the resistance of the material. The textbook I quoted specified that many materials have limited linear resistivity/conductivity, and therefore are ohmic within a particular range of values.

You need to differentiate (not mathematically) between the V-I characteristic and the determination of the value at particular voltage/current. Yes, the derivative of a exponent is also an exponent, therefore the resistive characteristic of a diode is non-linear, however what I am saying is localise the characteristic at a particular applied voltage and Ohms law applies to the diode - it becomes Ohmic at that localised point.

I guess you can say that about any material if you want to specify the range so small that the curvature of its I-V or V-I is not apparent. But its derivative of a V-I or I-V curve of a diode is not constant, so its curvature will become apparent later if not sooner.

Ratch

 

No, Ohm's law specifies linearity of a particular material charactistic. The resistance formula defines the resistance of the material. The textbook I quoted specified that many materials have limited linear resistivity/conductivity, and therefore are ohmic within a particular range of values.

I suggest you brush up on your German and read Georg Ohms 1827 paper (ref. earlier) which defines Ohms Law. It does not specify the need for a material characteristic, but that the notion that "the current through a conductor between two points is directly proportional to the potential difference across the two points, and inversely proportional to the resistance between them" - at the point of localisation this even applies in non-linear V-I materials.

I guess you can say that about any material if you want to specify the range so small that the curvature of its I-V or V-I is not apparent. But its derivative of a V-I or I-V curve of a diode is not constant, so its curvature will become apparent later if not sooner.

The derivative of the V-I characteristic is not constant, but at the localised point the relationship proposed by Ohm applies.

Dave

 

Dave,

I suggest you brush up on your German and read Georg Ohms 1827 paper (ref. earlier) which defines Ohms Law. It does not specify the need for a material characteristic, but that the notion that "the current through a conductor between two points is directly proportional to the potential difference across the two points, and inversely proportional to the resistance between them" - at the point of localisation this even applies in non-linear V-I materials.

Brushing up on my German would be a lot of work, especially since I have had little encounter with it since high school. Spanish appears to be a more useful language to learn in the US. I won't deny that he articulated the resistance formula, but who said that was Ohm's law? How does your insistence that the resistance formula is Ohm's law square with the two web sites, two physics textbooks, and four physics professors? Surely they would never say such a thing unless they thought it was true.

The derivative of the V-I characteristic is not constant, but at the localised point the relationship proposed by Ohm applies.

Yes, proposed and articulated by Ohm. But is that really Ohm's law, or the resistance formula?

Ratch

 

Let me see if I'm following this...

Some dusty old tome by some mistaken author says Ohm's Law is not "Ohm's Law," so the rest of us should shift gears in mid-trip and toe a new line?

Just because a second tome defines the continuum form of Ohm's Law instead of the common form of Ohm's Law we should all begin speaking a different language?

I've got a better idea. Let's have the mistaken author (and anyone else who got lost by reading the obscure old tome) get on board with the other 99% of us. Or at least let them learn to accept the rest of us as happy with the status quo.

Unless, of course, you are posting on the "All About Physics" forum and I simply misread the top of my screen...

 

Brushing up on my German would be a lot of work, especially since I have had little encounter with it since high school. Spanish appears to be a more useful language to learn in the US.

Yes, I appreciate that.

I won't deny that he articulated the resistance formula, but who said that was Ohm's law? How does your insistance that the resistance formula is Ohm's law square with the two web sites, two physics textbooks, and four physics professors? Surely they would never say such a thing unless they thought it was true.

The "resistance formula" is borne out of Georg Ohms 1827 paper, which has duly given the name to it "Ohms Law". The two web sites, two physics textbooks, and four physics professors are merely interpretations of Ohms original work.

Yes, proposed and articulated by Ohm. But is that really Ohm's law, or the resistance formula?

See above.

Dave

 

Can we begin the new debate on the observed packing density of angels in pinhead-sized circles anytime soon? Unlike this exersise in gum-beating, it might lead to a useful and applicable insight.

 

Can we begin the new debate on the observed packing density of angels in pinhead-sized circles anytime soon?

Dancing or stationary? With vacuum or without vacuum? You know there will be an argumentum ad vericundiam dismissal of the status quo regardless, yes?

 

thingmaker3,

Let me see if I'm following this...

OK, go ahead.

Some dusty old tome by some mistaken author says Ohm's Law is not "Ohm's Law," so the rest of us should shift gears in mid-trip and toe a new line?

Cleanliness is irrelevant. You have not proven the authors mistaken. You have not criticized Dave for referencing a document 140 years older than my reference. Besides, these books are still in print. http://search.barnesandnoble.com/bo...for-Scientists-and-Engineers/Raymond-A-Serway and http://bcs.wiley.com/he-bcs/Books?action=index&itemId=0471320579&itemTypeId=BKS&bcsId=1458

Just because a second tome defines the continuum form of Ohm's Law instead of the common form of Ohm's Law we should all begin speaking a different language?

You mean instead of the misnomer. No, not speak a different language, correct the misnomer.

I've got a better idea. Let's have the mistaken author (and anyone else who got lost by reading the obscure old tome) get on board with the other 99% of us. Or at least let them learn to accept the rest of us as happy with the status quo.

You have not proven the authors mistaken. Now which old tomb are you refering to, mine or Dave's. You mean live with a mistake? Will your conscience let you do that.

Unless, of course, you are posting on the "All About Physics" forum and I simply misread the top of my screen...

Physics is a foundation of electronics. One can't ignore mistakes in either science.

Ratch

 

Dave,

The "resistance formula" is borne out of Georg Ohms 1827 paper, which has duly given the name to it "Ohms Law". The two web sites, two physics textbooks, and four physics professors are merely interpretations of Ohms original work.

Well, I have seen no proof that the resistance formula was named Ohm's law. I have seen other references that say Ohm's law references the linearity of the resistance, but they do not articulate it quite as well as the references I gave. I will keep looking for other references, especially textbooks that provide more information.

Ratch

 

Cleanliness is irrelevant.

Perhaps it is and perhaps it is not. It has been said to be "next to Godliness."

You have not proven the authors mistaken.

You have not proven them correct.http://bcs.wiley.com/he-bcs/Books?action=index&itemId=0471320579&itemTypeId=BKS&bcsId=1458

You mean instead of the misnomer. No, not speak a different language, correct the misnomer.

Very well, I shall correct the misnomer! HEY, RATCH! "OHM'S LAW SAYS: I=E/R!!" There. Corrected.

You have not proven the authors mistaken.

You have not proven them correct. Is there an echo in here? (Is there an echo in here?)


Dang. This out-of context quoting and blithe response stuff is kind of fun! (If completely unproductive.) I begin to see why you do it, Ratch!

 

Well, I have seen no proof that the resistance formula was named Ohm's law. I have seen other references that say Ohm's law references the linearity of the resistance, but they do not articulate it quite as well as the references I gave. I will keep looking for other references, especially textbooks that provide more information.

Georg Ohms work was the first published work to draw the relationship you refer to as the "resistance formula". Any work that references either Ohms Law or the "resistance formula" stems back to Ohms 1827 conceptual frameworks.

Dave

 

thingmaker3,

Perhaps it is and perhaps it is not. It has been said to be "next to Godliness."

It is not. Dirty or clean, its contents are the same. This is not a religious point.

You have not proven them correct.

Because of their academic credentials, status and position, it is not improper to assume they know what they are talking about. In other words they are more likely to be right than wrong.

Very well, I shall correct the misnomer! HEY, RATCH! "OHM'S LAW SAYS: I=E/R!!" There. Corrected.

You did not correct it, you propagated the mistake.

You have not proven them correct. Is there an echo in here? (Is there an echo in here?)

Same answer as above. Same answer as above.

Dang. This out-of context quoting and blithe response stuff is kind of fun! (If completely unproductive.) I begin to see why you do it, Ratch!

I am giving you the straight insight.

Ratch

 

Dave,

Georg Ohms work was the first published work to draw the relationship you refer to as the "resistance formula". Any work that references either Ohms Law or the "resistance formula" stems back to Ohms 1827 conceptual frameworks.

I will agree with that, but it still begs the question of what Ohm's law specifically means, linearity or resistance calculations.

Ratch

 

It is not. Dirty or clean, its contents are the same. This is not a religious point.

You have yet to prove that cleanliness is not next to Godliness.

Because of their academic credentials, status and position, it is not improper to assume they know what they are talking about. In other words they are more likely to be right than wrong.

I could say the same thing of George W. Bush. He has academic credentials, status, and position. By your logic, it is not improper to assume he knows what he is talking about, and that he is more likely to be right than wrong.(1)

You did not correct it, you propagated the mistake.

Actually, I propagated the correction.

Same answer as above. Same answer as above.

There IS an echo in here! There IS an echo in here!(2)


(1)Argumentum ad verecundiam
(2)Argumentum ad nauseum

 

I will agree with that, but it still begs the question of what Ohm's law specifically means, linearity or resistance calculations.

Ohm stated without material or application condition that "the current through a conductor between two points is directly proportional to the potential difference across the two points, and inversely proportional to the resistance between them". Linearity occurs at a characteristic tangent (which may occur across the whole characteristic in the case of a resistor), and the relationship he described is also applicable to all resistance calculations within appropriate conditions (for example in a diode, R at I for a conditional value of V).

Dave

 

thingmaker3,

You have yet to prove that cleanliness is not next to Godliness.

Doesn't matter, its the contents that count.

I could say the same thing of George W. Bush. He has academic credentials, status, and position. By your logic, it is not improper to assume he knows what he is talking about, and that he is more likely to be right than wrong.(

Knowing all the resources and databases he has available, it would be right to assume he knows more about things in his purview than you do. Being right with respect to what? Making good decisions, or knowing things?

Actually, I propagated the correction.

A false correction.

There IS an echo in here! There IS an echo in here!(2)

Where is it coming from?

Ratch

 

Dave,

Ohm stated without material or application condition that "the current through a conductor between two points is directly proportional to the potential difference across the two points, and inversely proportional to the resistance between them". Linearity occurs at a characteristic tangent (which may occur across the whole characteristic in the case of a resistor), and the relationship he described is also applicable to all resistance calculations within appropriate conditions (for example in a diode, R at I for a conditional value of V).

I already agreed with the first sentence. I don't understand what you are saying in the rest of the paragraph, especially the part about the "characteristic tangent". Again I say, semiconductors are well known to be nonohmic. That means they do not follow Ohm's law.

Ratch