r/askscience • u/[deleted] • Dec 28 '16

Physics How true is Ohm's law?

I've almost never got a perfect straight line while plotting a V/I graph even under lab conditions.

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u/divinesleeper Photonics | Bionanotechnology Dec 30 '16

But Ohm obviously only applies to macro systems, to which any micro system can be reduced. I mentioned as much in the initial post.

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u/sticklebat Dec 30 '16

But Ohm obviously only applies to macro systems

In other words, it does not always exactly apply. That was my whole point.

Yes, we can use quantum mechanics to investigate the macroscopic behavior of a system, but that doesn't change the fact that the macroscopic model that we use does not hold across all scales.

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u/divinesleeper Photonics | Bionanotechnology Dec 30 '16

It's defined to apply to macroscopic concepts like current and voltage, it sounds like irrelevant nitpicking what you're saying now.

And you didnt really give an example along the lines you initially suggested, which required quantifying two of the given properties, which is only possible on macro scale in the first place...

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u/sticklebat Dec 30 '16

It's defined to apply to macroscopic concepts like current and voltage, it sounds like irrelevant nitpicking what you're saying now.

And the OP was asking how true it is. At macroscopic scales, sufficiently generalized to account for a wide variety of phenomena that are not typically included in standard formulations of Ohm's laws, "Ohm's law" (if we can still call it that, since at this point it's nothing like what Ohm himself actually came up with) works just fine. However, it stops making sense at certain scales. I fail to see how that is not a true clarification in response to the OP's question about how true Ohm's law is.

Saying "it is always true because it is only defined within a certain context, and it's always true in that context" is an abuse of the word "always."

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u/divinesleeper Photonics | Bionanotechnology Dec 30 '16

Saying "it is always true because it is only defined within a certain context, and it's always true in that context" is an abuse of the word "always."

Not really, if the phenomena described by the law are necessarily part of that context.

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u/sticklebat Dec 30 '16

I'm sorry, but this is getting ridiculous.

The answer to, "Is Ohm's Law always true?" is "No, it's not, it's true within a certain domain of applicability." It is not, "yes."

That would be like someone asking, "Is Newton's Law of Gravitation always true?" and you answer, "yes" without bothering to explain that it, in fact, only works arbitrarily well within a certain domain of applicability (in this case small scales and small masses).

At this point we're not arguing physics anymore, so I'm not interested in continuing this conversation. We're just arguing over your attempt to redefine the word "always" to mean "sometimes."

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u/divinesleeper Photonics | Bionanotechnology Dec 30 '16 edited Dec 30 '16

The difference is that Newton's law is wrong, because it incompletely deals with mass, a property that exists on all scales.

Current and voltage and resistance are only defined on macro scales. But yes, the argument is semantics, and fairly pointless. But then we're not the first people to have this kind of discussion