So by that definition a copper wire is non-ohmic since its resistance changes with temperature and a current through the wire will change its resistance.

Hello,

The most concise description of Ohm's Law is that it is not a true "Law". There's nothing that can obey a low that isnt really a law. We inject subjectivity to what we call ohmic and non ohmic and that comes with a set of assumptions as usual. If we abandon all assumptions, then nothing is ohmic because there is no law of nature that demands that there is. It's just an observation that serves us well in applications where it is accepted.
This is a very concise and perfect view of what Ohm's Law is and what it means, and it may take a lot of deep thought to understand fully because we take so many things for granted.

We do have certain things that we classify as ohmic or non ohmic because of the general overall operation of the things. We always have to remember that there will always be times when we have to look deeper because nature is ever present and always influencing the assumed simpler behavior. For wires we usually assume a constant temperature over a range of currents even though it wont be, but for light bulbs we never assume a constant temperature over a range of currents.

An extreme and funny view:
We can make Bozo the Clown's head look ohmic if we connect one lead to each ear and control the fluids in his brain

I invite anyone to show a regular light bulb that is ohmic.

 

I guess I look at things differently. To me......if the current is in time(phase) with voltage........it is ohmic. It does not matter if the resistance is not constant......if current remains in phase.......ohmic.

And I believe that heat generates the light.......not current. The heat sets the intensity....not the current. One can not change intensity without changing heat. If filament temp is held constant.....intensity should remain constant.....not matter the current.

 

I guess I look at things differently. To me......if the current is in time(phase) with voltage........it is ohmic. It does not matter if the resistance is not constant......if current remains in phase.......ohmic.

And I believe that heat generates the light.......not current. The heat sets the intensity....not the current. One can not change intensity without changing heat. If filament temp is held constant.....intensity should remain constant.....not matter the current.

Hi,

Yes and by that i take it you mean that if it has no reactance then it is "ohmic".
But the better term to use is "resistive". If we assume that ohmic=resistive then there's no reason to have the term "ohmic". Using "ohmic" in this way means resistive, and then there's no way to describe something that is resistive that also obeys Ohm's Law (for the most part).

So if you run across a quantity that you want to call "ohmic", then you wont know for sure if it follows Ohm's Law or not. If you reserve "ohmic" for things that obeys the law then you can use resistive, non ohmic for those that dont obey the law and resistive, ohmic or just ohmic for those that do.
I mentioned something about this previously, where "ohmic" can be taken to either mean "has ohms" or "obeys Ohm's Law", but i believe that the latter is the better way to handle that.

I know there are people who like to call things resistive ohmic, but the trend i see around the web is that ohmic should be reserved for things that obey Ohm's Law. There's going to be confusion in any case i think at one time or another so more info will have to be added anyway to a conversation about resistances.

 

I guess I look at things differently. To me......if the current is in time(phase) with voltage........it is ohmic. It does not matter if the resistance is not constant......if current remains in phase.......ohmic.

Pure nonsense. You can have a square-law device in which the current is proportional to the square of the current but still have the current in phase with the voltage. This is clearly non-ohmic.

And I believe that heat generates the light.......not current. The heat sets the intensity....not the current. One can not change intensity without changing heat. If filament temp is held constant.....intensity should remain constant.....not matter the current.

Totally irrelevant to the discussion -- whether light is being emitted and/or how it is being emitted is not even remotely related to whether it is ohmic or not.

 

Pure nonsense. You can have a square-law device in which the current is proportional to the square of the current but still have the current in phase with the voltage. This is clearly non-ohmic.



Totally irrelevant to the discussion -- whether light is being emitted and/or how it is being emitted is not even remotely related to whether it is ohmic or not.

Hi,

I think he is taking "ohmic" to mean, "the quantity is measured in Ohms".
I dont agree with that anymore though. I believe "ohmic" should be reserved for something that follows Ohm's Law over a reasonable operating range. We already have the noun "resistive" which is what he is really referring to.

I think the confusion comes from the fact that we have Ohm's Law and we have Ohms the unit of measure, and then we have to put up with "ohmic".
One point though is that we call inductors inductive we dont call inductors "Henryic"
but then again there's no law called Henries Law i dont think.
"My light bulb showed signs of becoming Faradic"

 

Hello,

The most concise description of Ohm's Law is that it is not a true "Law".

Whether you use the word "law" or "theory" or "lemma" or something else is immaterial as those terms have no rigid distinction. Many high school science books like to pretend that they do when trying to distinguish various concepts, but the reality is that the term that gets used is far more often dictated by the phrase that was coined at some point in some paper and it stuck.

Call it Ohm's Guess, if you want. It changes absolutely nothing. It is a claim that the voltage is directly proportional to the current. If this is true to an acceptable approximation over the range of interest under a specified set of conditions, then the material's behavior is ohmic under those conditions. If it isn't, then it's not.

 

Sorry MrAl.......what is your point. I've read your post several times......I can't make sense of it.

Did I mis-use a term or mis-define something?

 

So...if I have a DC circuit and current thru a resistance...........and I vary the resistance at a cube rate.......then it ceases to be ohmic?

 

So...if I have a DC circuit and current thru a resistance...........and I vary the resistance at a cube rate.......then it ceases to be ohmic?

Hi,

Well yes that appears to be the more accepted way of looking at it.

If we look at Ohm's Law we have:
E=I*R

but if we look at the cube law then we see:
E=I*R^3

Clearly there is something different about the two, and we want to be able to describe that without so many words. Note both resistances will still be measured in Ohms.

 

Whether you use the word "law" or "theory" or "lemma" or something else is immaterial as those terms have no rigid distinction. Many high school science books like to pretend that they do when trying to distinguish various concepts, but the reality is that the term that gets used is far more often dictated by the phrase that was coined at some point in some paper and it stuck.

Call it Ohm's Guess, if you want. It changes absolutely nothing. It is a claim that the voltage is directly proportional to the current. If this is true to an acceptable approximation over the range of interest under a specified set of conditions, then the material's behavior is ohmic under those conditions. If it isn't, then it's not.

Hi,

Well there still appears to be a distinction between what is a Law and what is not a Law.
A Law is taken to be something that is true no matter what. It does not matter if it can be disproved some time in the future, but for the day it is called a Law it remains a Law until disproved.
If you want to call it a theory instead of a Law, then it is taken to be true no matter what until such time that is is disproved.
I guess the best way to say it is that there is no substance in the universe that obeys Ohm's Law so it cant be taken to be a Law of any kind. Hubble's Law on the other hand, i think is a real Law, but we can look at other laws too and compare. How about E=m*c^2.

 

OK...MrAl......does ohm's law work on a logarithmic pot? Does ohm's law fail while moving wiper?

What If I build a resistor......that changes resistance with current. Does ohm's law fail?

I don't look at ohm's law as an equation. That's the problem here. It's a principle. That principle is that current is proportional to voltage(IN TIME)......depending on the load. If I change the load....that proportionality changes.

I won't argue over math definitions, they are all incomplete and for mathematicians to straighten out.

I go by phase. And it wasn't on homework so I thought I would comment. Didn't mean to upset anyone.

I think the TS left.

 

Hi,

Well yes that appears to be the more accepted way of looking at it.

If we look at Ohm's Law we have:
E=I*R

but if we look at the cube law then we see:
E=I*R^3

Both of those satisfy Ohm's Law.

In either case, if you double the current, the voltage doubles, regardless of what the initial voltage and current happen to be.

I cube law device would look like

E = Eo * (I/Io)^3 = I^3 * (Eo/(Io^3)) = k I^3

Making the voltage proportional to the cube of the current. THIS is non-ohmic.

Clearly there is something different about the two, and we want to be able to describe that without so many words. Note both resistances will still be measured in Ohms.

Not in this universe.

In the first one, R has units of V/A or ohms.

In the second one, R has the units of (V/A)^(1/3), which is cube-root-ohms.

 

Hi,

Well there still appears to be a distinction between what is a Law and what is not a Law.
A Law is taken to be something that is true no matter what. It does not matter if it can be disproved some time in the future, but for the day it is called a Law it remains a Law until disproved.
If you want to call it a theory instead of a Law, then it is taken to be true no matter what until such time that is is disproved.
I guess the best way to say it is that there is no substance in the universe that obeys Ohm's Law so it cant be taken to be a Law of any kind. Hubble's Law on the other hand, i think is a real Law, but we can look at other laws too and compare. How about E=m*c^2.

YOU are imposing a definition on the usage of a word that YOU would like it to have.

Consider Newton's Laws of Motion. They do NOT hold true no matter what, so by your definition they are not Laws. But they are still called Newton's Laws of Motion and will continue to called that probably as long as humans are around. Whether you call them a law or not has absolutely zero relevance to when they do and do not apply.

Did you notice that you used the same definition for a law and a theory? In both cases, you state this it is taken to be true no matter what until disproved.

 

OK...MrAl......does ohm's law work on a logarithmic pot? Does ohm's law fail while moving wiper?

What If I build a resistor......that changes resistance with current. Does ohm's law fail?

I don't look at ohm's law as an equation. That's the problem here. It's a principle. That principle is that current is proportional to voltage(IN TIME)......depending on the load. If I change the load....that proportionality changes.

I won't argue over math definitions, they are all incomplete and for mathematicians to straighten out.

I go by phase. And it wasn't on homework so I thought I would comment. Didn't mean to upset anyone.

I think the TS left.

Do you understand what "proportional" means?

If A is proportional to B, then if you scale A by a factor k, then B is scaled by that same factor k.

It's that simple.

 

Hahaha, @#12 tried to keep us on the rails but we simply cannot help ourselves!

Regarding the filament experiment discussed earlier, there is a simpler proof than trying to immerse a filament and control it's temperature while current is passing through it. Hold a filament in an oven at it's typical operating temperature within a lightbulb. That temperature can be determined from it's spectrum and probably in some other ways. You'll need an oxygen-free chamber, of course, to prevent oxidation of the filament. The experiment could be run at a range of temperatures representing various dimming levels of the bulb.

With virtually no current flowing through the filament, only enough for the meter to get a signal, you'll find that the measured ohms match with the calculated ohms based on the voltage and power rating of the lightbulb. Within experimental errors, of course.

That means that, once corrected for temperature, the current changing from zero (in the temperature controlled furnace) to the rated current (in the lit bulb) has no effect on the measured resistance. The filament is therefore "ohmic", R is independent of I. In calculus we would say that the partial derivative ∂R/∂I at constant T = 0.

I tend to think of physics like this in terms of calculus and frankly it's hard not to see things that way. So to me, ∂R/∂I = 0 means ohmic, period, even if the real-world lightbulb isn't.

 

For goodness sakes, I am going to have to whack you all on the knuckles, send you to sit in the corner and lock down this thread.
This discussion has been way too pedantic.

This forum is All About Circuits. We discuss electronic circuits. We go to the parts store and buy a resistor. We apply a voltage across the resistor and expect the resistor to conduct a certain amount of current as stated by Ohm's Law.

We can calculate the current using the formula I = V/R.

Let's be pragmatic about this.

Time out for everyone. Thanks for all your input.