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.
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.
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"
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.That equation is a definition that came before Ohm's law.
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.
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
... set of equations like "Ohm's law" has.
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
Could you please define that with historical reference or at least some factual reference of any kind?Ratchit said: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?
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.Ratchit said:I remember when an SI unit conductance was Ohm spelled backwards. That was really funny.
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.DerStrom said: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.
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!
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!MrAl said: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!
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)
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.
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.
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.
Could you please define that with historical reference or at least some factual reference of any kind?
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.
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.
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.
A "definition" of something is a law. The definition of resistance is a law, which we today call "ohm's law" in this context.
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.
"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.
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