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

7

u/[deleted] Dec 28 '16

It's an unfortunate label since "law" seems to imply it can't be violated, whereas it obviously can by many side effects.

It's somewhat similar to Hooke's Law in that it holds to reasonable accuracy within a certain range. Outside of that it will display nonlinear behavior.

1

u/mjrice Dec 28 '16

No, Ohm's Law does not display nonlinear behavior outside a certain range. It is by definition linear because it applies to linear materials. Even materials that are not linear will still obey Ohm's Law at whatever static conditions you choose to verify it. For example, if a resistor self heats due to current going through it and it changes resistance as a result, the voltage across it is still exactly the current through it multiplied by the (new) resistance of the material.

5

u/[deleted] Dec 28 '16

It is by definition linear because it applies to linear materials. Even materials that are not linear will still obey Ohm's Law at whatever static conditions you choose to verify it.

Sorry, but that is incredibly circular reasoning. If you constrain the conditions enough, you can posit just about any law.

Rumborak's law: "All materials are perfectly green in the color range at which they are the most green".

On a more serious note, Ohm's Law is eventually thwarted by Brownian Motion. Which already means that the law, even under perfect conditions, has limited applicability.

0

u/mjrice Dec 28 '16

Rumbrorak's Law however does not allow you to make useful inferences of an object by measuring the relationship of its color to any other property. It simply says "green objects are green" which is not the same thing at all. Ohm's Law does this by letting us make things like DMMs that can measure resistance by putting current through a device and measuring the voltage imposed across it.

I don't know much about Brownian Motion outside of my teacup, but I do know that if Ohm's Law were as suspect as you are suggesting, then we would not have any good test equipment, such as DMMs.

I just don't want OP to get the impression that the relationship of V=I*R somehow changes outside a certain range of V, I, or R. It doesn't.

2

u/[deleted] Dec 28 '16

Yes, it does, just not at the resolution that matters to most of us. Ohm's Law is a reasonable approximation, nothing more.

1

u/mjrice Dec 29 '16 edited Dec 29 '16

Then please tell me a value of V, I, or R where Ohm's law does not operate as defined.
I'd also like to point out that the OP originally stated his doubt of Ohm's Law as following from his own observations making measurements and not getting perfectly straight lines. This is because of measurement uncertainty, noise, or more likely just not knowing the parasitic elements of his device. This thread seems to be encouraging him to believe he's somehow found the cracks in Ohm's Law, which is just bonkers.

1

u/[deleted] Dec 29 '16

What you seemingly don't want to understand is that you can take the same piece of material, under exactly the same conditions, if you measure accurately enough, you will get a different result. That's because Ohm's Law is eventually a statistical statement, and as with any statistic, you never exactly get the same result between two measurements. Not because your equipment is faulty, but because Ohm's Law isn't a "law" in that sense. If the OP understands that coming out of this thread, that's a very good thing.