Ohmic and non-Ohmic conductors

[FS98]

Do ohmic conductors obey ohms law at high voltages?

Also, I’ve seen it explained that some conductors are non-ohmic because the temperature caused by the current changes the resistance in the circuit. If that’s the case, isn’t ohms law still being obeyed, just with a varying resistance.

 

[phinds]

...I’ve seen it explained that some conductors are non-ohmic because the temperature caused by the current changes the resistance in the circuit. If that’s the case, isn’t ohms law still being obeyed, just with a varying resistance.

Yes. Ohm's law is valid at each instant in such a situation.

 

[sophiecentaur]

Do ohmic conductors obey ohms law at high voltages?

Also, I’ve seen it explained that some conductors are non-ohmic because the temperature caused by the current changes the resistance in the circuit. If that’s the case, isn’t ohms law still being obeyed, just with a varying resistance.

There is a lot of confusion between R = V/I and the full statement of Ohm's Law which implies the V/I ratio is constant at constant temperature.
Take a Tungsten wire in the Lab and keep it in a water bath. It will follow Ohm's Law because it is a metal and the temperature is constant. Put a thin tungsten filament in a light bulb and its temperature will change as the current varies (of course). I would say that the Ohm's Law conditions are no longer there so the lamp doesn't have a constant resistance. This could be indistinguishable from a different (non-metallic) substance / component that happens to have the same V:I characteristic as the tungsten filament. Who could then say that "Ohm's Law" tells us the resistance is V/I"?
If we look at Boyle's Law, we are happy to say P/V for gas in a cylinder is constant at constant temperature. Some simple gases do follow Boyle's Law but we would never say that gas in a Stirling Engine is following Boyle's Law. To my mind, using the definition of Resistance as a statement of Owm's Law is just sloppy. For this I blame the (often highly competent) Electrical Engineers for bad usage of words. (Now stepping into my fallout shelter. )

 

[Vanadium 50]

Ohm's law is valid at each instant

I'm not sure how far this abstraction takes you. Any circuit element at any instant has a voltage across it and a current through it, so one can calculate an instantaneous resistance. The fact that this is possible does not necessarily mean it is useful.

I look at it differently. Ohm's Law is the definition of an ideal resistor. Whether this is or is not a useful model for a physical object depends on the situation. Just like with frictionless planes, stretchless ropes, etc.

 

[phinds]

I'm not sure how far this abstraction takes you. Any circuit element at any instant has a voltage across it and a current through it, so one can calculate an instantaneous resistance. The fact that this is possible does not necessarily mean it is useful.

No argument there. I really meant it (and should have said so) for resistors experiencing modest changes due to temperature.

 

[sophiecentaur]

I look at it differently. Ohm's Law is the definition of an ideal resistor

Ohn's Law doesn't even require the word "resistance" to be used at all. It just states that the ratio V/I is constant at constant temperature. Calling the V/I 'instantaneous Resistance" implies some time dependence. Perhaps "Effective Resistance" would do better and then you could do Kirchoff calculations to reveal things about a circuit with some non Ohmic components. (This is what we do, ain't it? And it is "useful".)

 

[FS98]

I'm not sure how far this abstraction takes you. Any circuit element at any instant has a voltage across it and a current through it, so one can calculate an instantaneous resistance. The fact that this is possible does not necessarily mean it is useful.

I look at it differently. Ohm's Law is the definition of an ideal resistor. Whether this is or is not a useful model for a physical object depends on the situation. Just like with frictionless planes, stretchless ropes, etc.

But aren’t there ways to find resistance without measuring it? So at any instant, the value of R would have to be a single value based on resistivity, length and area? The statement seems stronger than just saying that there exists some current and some voltage.

 

[phinds]

But aren’t there ways to find resistance without measuring it? So at any instant, the value of R would have to be a single value based on resistivity, length and area? The statement seems stronger than just saying that there exists some current and some voltage.

I think that the problem is that when you apply this to non-ohmic elements you get a result that isn't really helpful. That's Vanadium's point in post #4

 

[ZapperZ]

But aren’t there ways to find resistance without measuring it? So at any instant, the value of R would have to be a single value based on resistivity, length and area? The statement seems stronger than just saying that there exists some current and some voltage.

The ONLY thing I can think of is that you have to go back and look at the microscopic details of charge transport in the material itself. This is NOT EASY, because you either have to go back to solving for the Boltzmann transport equation, or worse still, go back to the band structure and figure out the scattering rate of the charge carrier. Each one of these still requires experimental intput!

I don't understand how this is now related to your original question in this thread. Has that question been answered and you're going off on a tangent from it?

Zz.

 

[David Lewis]

If you operate resistors inside their safe voltage and temperature range, they're ohmic conductors for practical purposes (with few exceptions, such as thermistors). But if resistance depends on voltage then you deal with dynamic or incremental resistance, and the term non-ohmic alerts you that circuit analysis is not as simple. It doesn't imply that a Law of Nature is being violated.

 

[CWatters]

Do ohmic conductors obey ohms law at high voltages?.

Define high voltage?

"Ideal" ohmic conductors would. "Real" ohmic conductors would require more information. All real world conductors have some limitations. For example too much power and copper wire melts. The resistance of molten copper is an order of magnitude higher than solid copper.

 

[CWatters]

Also, I’ve seen it explained that some conductors are non-ohmic because the temperature caused by the current changes the resistance in the circuit. If that’s the case, isn’t ohms law still being obeyed, just with a varying resistance.

The definition of ohmic conductor as I understand it is that the relationship between voltage and current is linear.

In...

V=I*R

R can't be a function of V or I and remain linear.

 

[CWatters]

That said we can ometimes model non-ohmic conductors (like diodes) using a piecewise linear model. Eg for small changes about a fixed operating point the relationship between I and V is sufficiently linear to be useful.

 

[sophiecentaur]

The definition of ohmic conductor

I don't think there really is one. It's a term that's used as shorthand and it never needs to be used in actual circuit analysis.