ohm's law clarification

[meowth08]

Hi,

I have been browsing previous threads and was intrigued by the very long thread on ohm's law. I have read most of the posts. I got confused on three things.

1. Ohm's law applies only with conductors or also with semiconductors?

2. The relationship V=IR or V=IZ is NOT a statement of ohm's law.
It is a resistance or impedance formula??

3. Ohm's law is a property of a material???

The formula V=IR is the first formula I learned in electronics. I believe it is basic. And I know that without full understanding of the basics, I can never learn fully the complicated things that are yet to come.

 

[Nigel Goodwin]

It couldn't really be much simpler, V=IR is pretty explicit and tells you all you need to know.

It works where you have those three values - and while it 'works' for semiconductors it only works 'if' you can reduce it to those values, which you very often can't as they aren't fixed.

 

[MrAl]

Hi meow,

Ohm's Law is a statement of proportionality, not a statement of units. Just because we can calculate the units in ohms doesnt mean the material obeys Ohm's Law. A material that obeys Ohm's Law follows the familiar equation V=IR, or more succinctly, R=V/I, so often the formula is confused with the law itself.

A good example of where Ohm's Law applies is for a pure resistance R. Whatever current we apply we will get a voltage V=I*R because V is proportional to I.

A good example of where Ohm's Law does NOT apply is for a real diode. When we apply a certain current we get a certain voltage and so therefore we get a certain resistance where we use the same formula V=I*R, but the difference is that when we apply a different current we dont get voltages that are proportional to the first and second currents so the diode does not obey Ohm's Law. We do find certain current ratios where it almost does, but to be more accurate we have to turn to a new law, a law for diodes.

So Ohm's Law is a statement of proportionality, and while we can use V=I*R (more aptly called "Ohm's Formula") that formula can be used for simply calculating resistance at a certain operating point too in a device even if that device does not follow Ohm's Law.

Unfortunately you'll find Ohm's Law stated as V=I*R and R=V/I as if that is the law itself. That's probably because it is used for calculating the units of "Ohms". In other words, the following statement is NOT true:
"We calculate the units in Ohms therefore we are using Ohm's Law"

It's unfortunate that this confusion has worked it's way into the field so that many people say and even teach that Ohm's Law actually *is* the formula V=I*R (or similar), but that's actually only true when also V is proportional to I.

In order to determine if a device or material obeys Ohm's Law you have to make *at least* TWO tests with TWO different test currents and use R=V/I TWICE. By contrast, to measure resistance you only have to make ONE test with ONE test current and use R=V/I ONCE.

 

[Ratchit]

meowth08,

I have been browsing previous threads and was intrigued by the very long thread on ohm's law. I have read most of the posts. I got confused on three things.

Yes, I participated in that thread

1. Ohm's law applies only with conductors or also with semiconductors?

Since Ohm's is a property of a material, and conductors including semiconductors are material, then it applies.

2. The relationship V=IR or V=IZ is NOT a statement of ohm's law.
It is a resistance or impedance formula??

Yes, it is the resistance or impedance formula. That formula defines what resistance or impedance is quantitatively.

3. Ohm's law is a property of a material???

As stated above, yes it is.

The formula V=IR is the first formula I learned in electronics. I believe it is basic. And I know that without full understanding of the basics, I can never learn fully the complicated things that are yet to come.

As stated above, that formula is the definition of resistance. It can never be wrong unless the definition of resistance changes. It just should not be called "Ohm's law".

MrAl in the above post has pretty much explained it. I will expound on it a bit. First I will post what I wrote in that thread you read.

There seems to be some confusion about what Ohm's law is. The formula V=IR or V=IZ is NOT Ohm's law. It is the resistance or impedance formula. Ohm's law is a property of a material, not a method of calculating current,impedance, or voltage. Read what the physics books say about this.

"We stress that the relationship V=IR is not a statement of Ohm's law. A conductor obeys Ohm's law only if its V--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."
The above snippet is from Physics, by Prof David Halliday, University of Pittsburgh & Prof Robert Resnick,Rensselaer Polytechnic Institute, 1967 , page 780.

And the following.
"Ohm's law states that for many materials (including most metals), the ratio of the current density and electric field is a constant, which is independent of the electric field producing the current.
Materials that obey Ohm's law, and hence demonstrate this linear behavior 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 emperical relationship valid only for certain materials."
The above is from Physics for Scientists and Engineers, Raymond A Serway, James Madison University, Third edition, 1990, page 745.

An interest site is here. **broken link removed**

Now an observation. Folks back in the 18th century England knew that the trip from London to Cambridge took less time if they traveled in a faster coach. Smart men like Newton accepted the definition of speed as the distance divided by time. Now, you have never heard of Newton's speed law, have you? Of course not. It is a definition, not a discovery. Same with Ohm's law. Ohm discovered different materials either had or did not have resistive linearity. No doubt he was familiar with the definition of resistance, but he did not "discover" it.

So, in conclusion. V=IR or V=IZ is an always correct definition formula, and not Ohm's law.

Ratch

 

[meowth08]

Hi,

Thanks for the replies.

If only my teacher had explained it the way MrAl did, I'm sure I would not have that thinking that V=IR is ohm's law. With those examples, it's very easy to comprehend.

The second statement of Nigel is very short but is already substantial for me to grasp his point.

Ratchit, that's exactly the thread I read. The site you posted is really interesting. It really helps to clarify things when there are supporting documents.

meowth08

 

[Sceadwian]

I think I was the primary person that disagreed with the general usage of the term "Ohm's Law" my point of contention was simple; Ohm's law came about due to Ohm's very detailed study of the voltage and current in relation to a metal conductors resistance, however the resistance was a metallic rod the voltage and currents were very limited and thermal constraints had to be observed to get the data values that approximated the equations he came up with.

Ohm's law as it is used currently is applied to any relation of V=IR or it's derivation(s) to any number of non linear systems (Ohm only studied the most linear experimental system he could devise) The linearity of Ohm's law breaks down even if you look closely at the true V=IR in a real metal rod, especially with temperature.

I applaud Ohm's experimental results, keep in mind however that the paper he published was originally a complete flop in academia until the reach of Ohm's law was found to have equivalents outside of the linear systems he studied.

To this day, non linear systems are considered non-ohmic even if the equations V=IR can be applied to solving their interactions.

Again it was my intention that although this is the first time I've said anything about it in a considerable amount of time that the bulk majority of the use of the term "Ohm's law" would best be written as "the equations derived from Ohm's law"

Everything currently known about physics can have V=IR equivocation but they are derived from more complex systems.

I don't mean to say these things to slight Ohm, someone obviously had to be given the credit for the equation, but Ohm's only other major study was "Ohm's Acoustical Law" which much like "Ohm's Law" proved to be not quiet true =) The history and semantics are quiet interesting to me.

I recommend you look into the details yourself.

 

[Ratchit]

Sceadwian,

Again it was my intention that although this is the first time I've said anything about it in a considerable amount of time that the bulk majority of the use of the term "Ohm's law" would best be written as "the equations derived from Ohm's law"

What equations are those? Surely not V=IR. That equation is a definition that came before Ohm's law.

Ratch

 

[Sceadwian]

That equation is a definition that came before Ohm's law.

Ohm's law is according to Wikipedia I=V/R So V=IR would in fact be a derivation of Ohm's law accord to Wikipedia's declaration, which I find sparsely credible. Henry Cavendish noted the same thing in experiments with Layden jars almost 50 years before Ohm's experiments but never published anything, James Clerk Maxwell published Cavendish's work, but that was 50 years after Ohm published his work, so George got the credit. If there's anything you'd care to add to historical understanding of the equations commonly refereed to as Ohm's law (which is all three) sometimes four when power is factored in.

My favorite line in the Wikipedia entry is "Modern developments in electromagnetic theory and circuit theory do not contradict Ohm's law when they are evaluated within the appropriate limits."

Basically what they're saying is they've found out that Ohm's law is insufficient scientifically unless you cross your eyes to blur the details. Ohm himself had to have known under which experimental conditions the equation failed otherwise he would have used larger boundaries to the limits of his experiments. The narrow range of his experimental data strongly suggests that he chose experiments that proved something mathematically within it's given dataset, which is junk science. I don't like to slight someone that is noted in history for such a 'great acomplishment' but when you look at the details of what he actually did there's very little of anything to look at, and this is his single 'claim to fame' nothing else he did except the failure of Ohm's Acoustic law is ever noted in science that I can find, certainly not enough to justify the virtual saint hood that is applied to him for contribution to electrical understanding.

The one nice thing is that the standard unit for measuring conductance is the siemens named for Ernst Werner von Siemens who actually contributed a lot not necessarily to hard science directly but to engineering and practical understanding and application of technique. I do however dislike that the SI unit for both resistance impedance and reactance is the Ohm, which is a joke. Impedance and reactance weren't even described until the late 1800's early 1900's

It's my firm thought that George gets the credit cause you can't reassign it 50 years earlier once it's already gone as mainstream as a name for a very important set of equations like "Ohm's law" has.

It's used by rote now not for it's historical significance. What amazed me is how many people will attack you if you so much as hint that George Ohm wasn't some genius that rightly deserves the use of his name on such a wide reaching set of equations. History speaks for itself, if you doubt me look it up, if you disagree with me take it to private cause this should not turn into "Ohm's Law" thread.

I am however glad to see that the previous thread sparked the curiosity of a new user and I think all the posts before the history answered it quiet well and that my followup posts on the contention I have with the semantic uses of the phrase are now currently well addressed =>

 

[Ratchit]

Sceadwian,

My favorite line in the Wikipedia entry is "Modern developments in electromagnetic theory and circuit theory do not contradict Ohm's law when they are evaluated within the appropriate limits.

I think they are referring to the range of resistive linearity. No compound or element is completely linear for all current values.

I still don't know what equations Ohm discovered. The V=IR is a definition equation, and is not due to Ohm. He studied and published a work on electrical linearity, but what equation came from this work?

The one nice thing is that the standard unit for measuring conductance is the siemens named for Ernst Werner von Siemens who actually contributed a lot not necessarily to hard science directly but to engineering and practical understanding and application of technique.

I remember when an SI unit conductance was Ohm spelled backwards. That was really funny.

I do however dislike that the SI unit for both resistance impedance and reactance is the Ohm, which is a joke. Impedance and reactance weren't even described until the late 1800's early 1900's

Since resistance and reactance both lower current, albeit in different ways, it is probably best that the units are the same. It also makes sense that the phasor combination of resistance and reactance into impedance also has the same unit.

... set of equations like "Ohm's law" has.

What equations are those?

Ratch

 

[DerStrom8]

Just going to throw my two bits in here--

Georg Simon Ohm did not make the law. He simply put a name to it. Fighting over the name is completely unimportant. What is important is that it is a natural relationship that has existed since the beginning of time. V=IR, I=V/R, and R=V/I are all Ohm's law, not just one of them. Ohm gave his name to the relationship, which is the same, regardless of the order of the variables.

Regards

 

[DerStrom8]

I do however dislike that the SI unit for both resistance impedance and reactance is the Ohm, which is a joke. Impedance and reactance weren't even described until the late 1800's early 1900's

Sorry, but a unit of ohms for resistance, impedance, and reactance does, in fact, make complete sense. The only difference between impedance/reactance and resistance is that they are a resistance on the complex plane. They have their purposes, and react the same way in an AC circuit as resistance does in a DC circuit.

Regards

 

[Sceadwian]

I still don't know what equations Ohm discovered. The V=IR is a definition equation, and is not due to Ohm. He studied and published a work on electrical linearity, but what equation came from this work?

Could you please define that with historical reference or at least some factual reference of any kind?

I remember when an SI unit conductance was Ohm spelled backwards. That was really funny.

It never was, the SI unit of conductance has always been the siemens before then the mho was commonly used but no standards body validated it, it was just common practice.

Fighting over the name is completely unimportant

Um.. Why? George Ohm didn't create this law or equation yet it holds his name as a law that is externalized so far outside of his field of study and methods that it not longer should have the same word usage. If anyone can using Latex post the equation that Ohm actually published I'd like to see it.

Ohm did not give his name, history was given his name, and it was little more than the first one on record as having something to do with it. My issue is with the specifics, the science the understanding of what is attributed to him which has nothing to do with anything he actually did or discovered outside of blinders even GIVEN the limits of the instruments of the time.

 

[DerStrom8]

Um.. Why? George Ohm didn't create this law or equation yet it holds his name as a law that is externalized so far outside of his field of study and methods that it not longer should have the same word usage.

I agree with that, but the name of the law has nothing to do with its function, and is not what the OP was asking about.

Ohm did not give his name, history was given his name, and it was little more than the first one on record as having something to do with it. My issue is with the specifics, the science the understanding of what is attributed to him which has nothing to do with anything he actually did or discovered!

Yes, I agree 100%. I did not phrase that very well. I did not mean that Ohm named it himself--only that his name was given to it. Sorry for the confusion.

 

[MrAl]

Hi again,


Ohm came up with the law of proportionality in conductors, the fact that we can have a relationship R=E/I in a conductor. The interesting thing which everyone here noticed right away was that there is no material that obeys this 'law' perfectly. What it does do however is it gives us a simplification, a way to think about conductors without constantly referring to their non linearity. Non linear circuits are much more difficult to solve, if solvable at all, but thinking in terms of linearity helps figure these things out in many cases. Ohm used wires of various metals and sizes in his test setup, and deduced not only the law of proportionality, but also that science could be found not only by experiment but by pure reasoning...quite interesting i'd say for his time.

It's interesting that when we measure or calculate circuits with resistors we usually assume that the resistance in a constant unless there is good reason not to (LDR for example). But there is no material that obeys that law completely, so what gives us the desire to use that law anyway?
It's the simplification of thought that spurs this desire, and that gives us the ability to do many things much easier and faster, and also a means to measure things that dont follow that rule.

A comparison to magnetic design is in order. When designing an inductor from scratch we always consider the nonlinear character of the core metal, unlike the resistor. However, we still end up thinking of the inductor as a linear device with an anisotropic core. But why that when there is probably no metal that exists in the universe that has a true anisotropic core. We're looking for simplification of thought again. Thinking of the inductor as a linear device, we can come up with little approximate theories like, "the inductance is proportional to the square of the turns ratio".
The inductance is never going to be proportional to the square of the turns ratio because the core is made of metal and all metal (so far) has a limit on its flux density before it turns into a metal PLUS an air core inductor. But we still think about that good ol' anisotropic curve, but we also keep in mind that there are operating limitations to it.
So do we go around saying that there is no such thing as an anisotropic core, or do we just use that mental tool to make things simpler...the answer is apparent.

So we have 'pure' resistance (a constant) and we have the anisotropic core. They dont truly exist yet we use them both as mental tools in everyday practice. Does this mean that Ohm's Law isnt really a law? That's possible because Ohm's Law doesnt work unless the material obeys Ohm's Law. That's quite convenient isnt it. But we still use them as if they were both some sort of law of nature. Instead of calling them a "law" of nature i think we should call them an "idealism" of nature.

 

[Sceadwian]

Ohm used wires of various metals and sizes in his test setup, and deduced not only the law of proportionality, but also that science could be found not only by experiment but by pure reasoning...quite interesting i'd say for his time.

Could you post some substantiating documentation to support that supposition? His results were WRONG even for the data set that he came up with the statistical non-linearity (noise floor) was huge!) His only other major theory was also proven wrong, he exhibited no truly pure scientific reasoning... If you study even the same experiements he used then with modern instrumentation you'd SEE the non-linearity directly which disproves the core equations derived from the observations by renewed observation!

He discovered nothing, he experimented and documented what he chose to share carefully rather than the results outside of his experimental methods which cause the equations he came up with to be nonsensical (non-linear)

 

[DerStrom8]

Could you post some substantiating documentation to support that supposition? His results were WRONG even for the data set that he came up with the statistical non-linearity (noise floor) was huge!) His only other major theory was also proven wrong, he exhibited no truly pure scientific reasoning... If you study even the same experiements he used then with modern instrumentation you'd SEE the non-linearity directly which disproves the core equations derived from the observations by renewed observation!

It would help if you post links to YOUR sources as well (other than wikipedia, which isn't always correct). I don't know about anyone else, but I for one can't just take someone's word for that type of thing.

 

[MrAl]

Could you post some substantiating documentation to support that supposition? His results were WRONG even for the data set that he came up with the statistical non-linearity (noise floor) was huge!) His only other major theory was also proven wrong, he exhibited no truly pure scientific reasoning... If you study even the same experiements he used then with modern instrumentation you'd SEE the non-linearity directly which disproves the core equations derived from the observations by renewed observation!

He discovered nothing, he experimented and documented what he chose to share carefully rather than the results outside of his experimental methods which cause the equations he came up with to be nonsensical (non-linear)

Hi Scead,

Well, what you are saying there is that Ohm was a dang idiot, and that he had no knowledge of R=E/I. That's just too hard to believe without some proof. If it is true i think the burden of proof lies in your court because almost everyone else here believes Ohm contributed in at least some way. If what you say is true or partly true then you should have some proof or at least where you found this totally contradictory information.
I wont pretend to be a historian, but i've seen much more work that states he did contribute.

I have a book with his experiments in it but unfortunately when i moved i put many things in storage which is quite a distance from where i live now, and im not too inclined to go get it. It's been a very long time since i read up on his works too. A trip to the library might help.

 

[Ratchit]

DerStrom8,

Georg Simon Ohm did not make the law. He simply put a name to it. Fighting over the name is completely unimportant. What is important is that it is a natural relationship that has existed since the beginning of time. V=IR, I=V/R, and R=V/I are all Ohm's law, not just one of them. Ohm gave his name to the relationship, which is the same, regardless of the order of the variables.

I believe Ohm discovered the law of electrical linearity, which states that some materials have electrical current linearity and others don't. The equation V=IR and its variants are not Ohm's law, it is the definition of resistance. Ohm did not discover a definition.

Sorry, but a unit of ohms for resistance, impedance, and reactance does, in fact, make complete sense. The only difference between impedance/reactance and resistance is that they are a resistance on the complex plane. They have their purposes, and react the same way in an AC circuit as resistance does in a DC circuit.

Not really they don't. A resistor reduces current by dissipating electrical energy as heat. A reactance reduces current by setting up a back voltage and storing electrical energy, which is released later with no energy loss. Those are two entirely different mechanisms for lowering the current.

I agree with that, but the name of the law has nothing to do with its function, and is not what the OP was asking about.

That is true of any law isn't it? The OP was asking about whether "Ohm's law" meant V=IR. It doesn't. It means instead resistive linearity.

Sceadwian,

Could you please define that with historical reference or at least some factual reference of any kind?

Didn't I do that in post #4 of this thread where I quoted 2 repected physics books and a website? The physics books definitely state that Ohm's law is a property of a material, not a fundamental law of nature like Gauss's law is.

Ratch

 

[DerStrom8]

DerStrom8,
I believe Ohm discovered the law of electrical linearity, which states that some materials have electrical current linearity and others don't. The equation V=IR and its variants are not Ohm's law, it is the definition of resistance. Ohm did not discover a definition.

Oh boy, here we go again....
A "definition" of something is a law. The definition of resistance is a law, which we today call "ohm's law" in this context.

Not really they don't. A resistor reduces current by dissipating electrical energy as heat. A reactance reduces current by setting up a back voltage and storing electrical energy, which is released later with no energy loss. Those are two entirely different mechanisms for lowering the current.

I'm not talking about how they work. I'm talking about what they end up doing in the circuit. Impedance and reactance work in ohm's law in an AC circuit the same way "just plain resistance" does in a DC circuit. They also work in the voltage/current divider principles, and any other calculations having to do with "resistance" in an AC circuit.

That is true of any law isn't it? The OP was asking about whether "Ohm's law" meant V=IR. It doesn't. It means instead resistive linearity.

"Ohm's law" refers to that law of electricity, that V=IR, I=V/R, R=V/I, etc. It would help you to look at the big picture, rather than at each individual little piece of it, Ratch.

I don't want this to get into another big long argument like it did last time without the OP's permission. Before I reply again, I'm going to ask--meowth08, is this the type discussion you were looking for?

Kind regards,
Der Strom

 

[Ratchit]

DerStrom8,

A "definition" of something is a law. The definition of resistance is a law, which we today call "ohm's law" in this context.

Not so. For instance, speed = distance/time is a definition of speed. You have never heard of it called the "speed law", have you? A definition and a law are two entirely different things.

I'm not talking about how they work. I'm talking about what they end up doing in the circuit. Impedance and reactance work in ohm's law in an AC circuit the same way "just plain resistance" does in a DC circuit. They also work in the voltage/current divider principles, and any other calculations having to do with "resistance" in an AC circuit.

Not only do they work differently, the results are different. Resistance takes electrical energy out of a electrical circuit, while reactance stores energy, redirects it, and gives it back. Ohm's law is not used to calculate circuit values. The definition of impedance is used instead (Z=V/I).

"Ohm's law" refers to that law of electricity, that V=IR, I=V/R, R=V/I, etc. It would help you to look at the big picture, rather than at each individual little piece of it, Ratch.

That is the crux of the matter. You keep insisting that V=IR and its variants are Ohm's law. I and two good physics books I referenced in post #4 of this thread state otherwise. No matter how big the picture is, we have to get past that fact.

Ratch