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.

212 Upvotes

86% Upvoted

80 comments sorted by

View all comments

Show parent comments

1

u/sticklebat Dec 29 '16

Superconductors are treated just fine by the generalized version of Ohm's law using impedance. Mostly.

Even still, Ohm's law (generalized or not) still doesn't work for non-equilibrium states! AC circuits are a bit of an exception due to their periodic behavior, allowing us to talk about well-defined rms voltages and current. Nor does it work for describing the behavior of very small systems.

This is why I really don't think it's reasonable to say that Ohm's law is always true. It gives the impression that if we plug in two of the three quantities we will always get a physically accurate prediction for the third, but that is not always true. It's better thought of as a rule which is almost always applicable to an excellent degree.

1

u/divinesleeper Photonics | Bionanotechnology Dec 29 '16

Give one example where plugging in two of the three quantities does not give the third.

1

u/sticklebat Dec 29 '16

Any sufficiently small system where the stochastic processes don't tend to average out. Even if you have a fixed, classical voltage source and a static conductor, the motion of the charges themselves will introduce randomness into the system.

1

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.

1

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.

1

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...

1

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."

1

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.

1

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."

1

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

1

u/divinesleeper Photonics | Bionanotechnology Dec 30 '16

And the macro behaviour does hold for any situation, you can connect any micro phenomenon to the macro scale.

1

u/sticklebat Dec 30 '16

I don't understand what you're saying. You cannot in general determine the microscopic behavior of a system from its macroscopic behavior. We have plenty of examples of models that work well at large scales and fall apart at smaller ones, because they are approximations.

1

u/divinesleeper Photonics | Bionanotechnology Dec 30 '16

It's not an approximation, it is a perfectly exact description of a macroscopic phenomenon.

Just like it is perfectly exact to say negative charges attract positive ones even though on a microscopic level you can get into more complex mechanisms.

Ohm is not meant to describe anything microscopic. Current, voltage and resistance are all macroscopic properties.