MrAL, I think the point of contention here is that Ohm's experiments where this 'law' came from used conductors with a fixed R to get the results that made sense. You have to remember that these experiments were done over 150 years ago, the basic reasons for these forces were almost completely unknown, in order for his experiments to work he early on realized that he had to restrict the variables in his experiments to fixed conductors and fixed temperatures, because he knew that he couldn't explain flows in ionic fluids or complex materials. So the root of Ohms law is in fact based on these highly limited set of original test conditions, however the root applicability of the equation extends to non-ohmic materials as well. Ohm's law CAN be applied to non-ohmic materials you have as yet to disprove this. A PN junction or any of the junctions in a transistor will have an equivalent resistance and voltage across it and that current will balance out perfectly so that there is no imbalance in the circuit at any given point in time. If using the V=IR equation on non-ohmic materials isn't Ohm's Law then why do the numbers ALWAYS work out? R is in fact a complex thing, even in metallic conductors, Ohm just picked a set of experiments that kept R almost completely static to prove the general proportional response of the V and I which at the time were better understood than resistance.
Again, I'll repeat myself one more time, just because his experiments used fixed resistances does NOT mean that Ohm's Law does not apply to variable resistance. At any given moment in time an electric circuit will have a set V=IR values that will ALWAYS work out
One caveat, I'm a little iffy on saying always, simply because ohms law like most macro laws break (or at least become insanely complex) on a quantum level.
Then the onus is on you MrAl, shut the hell up or tell me what V=IR is if it isn't ohms law, because that's the 'law' that they described from his results, it does NOT NEED A CONSTANT R. EVERY SINGLE LAST CIRCUIT IN EXISTENCE WILL ALWAYS HAVE AN R equal to V\I. Ohm's own measurements confirm this, his R was however always static, this in no way shape or form states that R being dynamic doesn't meet the law! R is not a real number assigned to a substance, it is inferred ONLY from the results that are measured from real world materials, and not one single material on this planet meets Ohm's law requirements for linearity at every voltage and current state, period.
Again let me state for the record, if Ohm's law is what you say it is. R=V/I for every value of V and I for any given material this will NEVER result in a static R. Again let me repeat that. NOT ONE SINGLE KNOWN PHYSICAL MATERIAL IS OHMIC BY YOUR DESCRIPTION. Take temperature out of the equation and you HAVE to limit I. I asked this of Wayne Saslow when I tried to ask him what the slope variation was allowed to be to declare a material ohmic. Never got an answer, because if the slope is required to be 1 it fails instantly for ANY material outside of Ohm's very narrow limits. Nothing in the V=IR equation or it's derivatives infer a limit of any kind for any value or describe linearity in any way. Which is smart not because he was trying to be sly about it, but because at the time he knew R related under those conditions, but he KNEW they didn't under some other conditions and he had no theory for those other conditions.
Ohm's experiments ONLY showed that R was constant under the physical constraints HE placed on the systems he tested, NOT that they couldn't make sense in system where R wasn't constant. I don't have to prove anything the ratio that describes Ohm's law is used outside tuse of perfectly linear resistors in just about every field related to electrical discharge, and it's most of the time still called Ohm's law.
I'll state this one more time. Just because Ohm's inferences were based on a static R does NOT IN ANY WAY SHAPE OR FORM mean that it can't apply to a non static R. Ohm was just a smart scientist, he only tested things he knew would give him good results. Not to say he was trying to be sly, he KNEW the results become skewed with temperature and with various other materials, he wasn't trying to deal with that because at the time nothing was known about the why. If he included those 'facts' in his results he'd be laughed out of academia, because he couldn't explain them.
Assuming the voltmeter and ammeter are average responding instruments, a switch opening and closing with a 50% duty cycle would do it.
But I'm not going to argue semantics, that's futile, if you want to say that 'it's always possible to divide V by I (presumably I non zero) and get a quantity called R' then that's fine. No-one will argue with you. I'd just like you to post some examples to show where this is useful, or indeed examples of anyone else apart from yourself doing this. Surely if we are going to give this result the exalted classification of a 'Law' it should be useful or others must know/use it?
I know I am a relative newcomer and so am careful of forum etiquette, but I did see you criticise Colin for providing advice that didn't live up to your standards, so in the same spirit, if you don't know what you are talking about, don't give advice. Today's post was a fairly incoherent rant, for example:
- saying that a device didn't need to have a constant R to satisfy Ohms Law, when you yourself posted material describing Ohms law as the discovery that R was essentially constant
- saying that Ohm's Law only applies over very narrow limits, I assume you have a reference for this, I posted a link that suggested that in conductors it applies over 20 orders of magnitude (at constant temperature).
The followupDear Mr Saslow,
I appear to be at odds in a basic conversation about Ohm's law in a forum and one of the users paraphrased section 7.1.2 of your book "Electricity, Magnetism, and Light" the first two paragraphs which state that Ohm's law requires the slope to be 1 for Ohm's law to hold true. Where does this linearity requirement come from as it's not explicitly stated in Ohm's Law Equation itself. I understand using the term 'non-ohmic' to describe materials which don't have a linear curve but Ohm's law still applies to them as I see it.
My basic argument is that in a semi conductor material, even though it's resistance depends on the voltage/currents developed across it's material qualifies it as a non-ohmic device that for any given moment in time it still has a very real resistance that would calculate properly for Ohm's Law. I would appreciate any time you could set aside to respond.
Dear Mr Saslow,You really have the idea; just be a little more flexible.
-Ohm's discovery of the law named after him was made studying wires. His results included the linearity of the current vs voltage, which gives R to be independent of the voltage, as well as the proportionality of R to the length of the wire. I don't know if he studied the dependence of R on the cross-section of the wire.
-You can always write down I=V/R, but it's really only useful when R is pretty nearly independent of V.
-Of course it has a real resistance, but R=V/I is only a definition. The fact that for many materials it is independent of V is what makes it a law; Ohm's Law.
I downloaded a translation of "The Galvanic Circuit Investigated Mathematically" and thumbed through it for an hour and I understand exactly what you said now, however I am still confused about the linguistics behind the term 'Ohm's Law' as it is most frequently referenced. I can find on the Internet it's simple ratio not necessarily relating to his experiments. I think the real argument is coming from what do you call the V=IR equation itself and it's derivatives if not Ohm's law? As the equation itself or it's derivatives reaches into many other equivalent circuit descriptions, also as what Ohm did was experimental rather than mathematical what is the true 'cutoff' point of describing a "Non ohmic" device? How far does the slope have to deviate? If no deviation is allowed not one single material in the real world is truly 'Ohmic'
I don't recall ever seeing a statement of how linear the I vs V relation has to be in order for the system to be considered ohmic.
You are worrying too much; you know there is a problem if you take things too literally, so don't take them so literally.
Prof. Wayne Saslow
521 ENPH
Department of Physics
Texas A&M University
College Station, TX 77843-4242
ohmicHere's a quote from the University of Guelph, Department of Physics:
2. Material that obeys Ohm's Law is called "ohmic" or "linear" because
the potential difference across it varies linearly with the current.
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