Ohmic and non-ohmic devices

[rhubarbpf]

If an ohmic device is one where the current flowing through it is directly proportional to the potential difference across it, given that temperature remains constant, why is it that people are always giving an incandescent lightbulb as an example of a non-ohmic conductor?

Sure, the resistance of the lightbulb increases, as more voltage is applied, causing the temperature to increase. But that happens with ohmic conductors. How do we know if the lightbulb is ohmic or non-ohmic if the temperature increases as the voltage increases? What if we were to cool the lightbulb in order to hold the temperature constant?

 

[berkeman]

why is it that people are always giving an incandescent lightbulb as an example of a non-ohmic conductor?

What people? Can you provide a link please? Thanks.

 

[rhubarbpf]

Hmmm. How about this:
https://nigerianscholars.com/tutori... which obey Ohm's Law,will be a straight-line.

 

[Baluncore]

It depends on the definition, or your interpretation, of the term “ohmic device”.
https://en.wikipedia.org/wiki/Ohmic_contact

There was a time when a student's best known example of a resistive load was a light globe filament.

 

[berkeman]

Hmmm. How about this:
https://nigerianscholars.com/tutori... which obey Ohm's Law,will be a straight-line.

Do they ever give a quantitative definition of Ohmic Conductor? So far in looking at the link they just allude to small and large thermal coefficients of resistance. That seems pretty arbitrary to me.

The phenomenon of resistance changing with variations in temperature is one shared by almost all metals, of which most wires are made. For most applications, these changes in resistance are small enough to be ignored. In the application of metal lamp filaments, which increase a lot in temperature (up to about 1 000 ℃, and starting from room temperature) the change is quite large.

 

[Tom.G]

If an ohmic device is one where the current flowing through it is directly proportional to the potential difference across it, given that temperature remains constant, why is it that people are always giving an incandescent lightbulb as an example of a non-ohmic conductor?

Because they are ignoring the part of the definition that says "...temperature remains constant." The light bulb temperature definitely does NOT remain constant.

Just ask anyone that has tried to remove one from a socket while it is on. Ouch!

Cheers,
Tom

 

[Windadct]

Well to copypasta from Wikipedia "The device which follows ohm's law for all voltages across it is called as an ohmic device." --

I think what you are referring to is SOMWHAT subjective, since we learn most elements by looking at the ideal sense.

So is the Wiki Definition even possible? " ALL VOLTAGES" - of course not.

So look at it from an order of magnitude standpoint.

A lightbulb is very non-linear, so a VERY small change in voltage applied will look linear, but anything significant, even 1% of it's rated V will have a marked change in the resistance.

A conductor - especially operated well within its typical current limit, 1) will not have significant Temp rise and 2) have a less pronounced rise relative to temp. So the difference between 1% and 100% should be negligible.

We WANT light bulbs to heat up and glow, we do NOT want our conductors to.

So when we look at an actual resistor, it has a thermal characteristic, good high precision resistors are rated by their thermal characteristic. In some cases we want a higher rate of resistance change with temperature - like with thermistors.

ALL of these in some appropriate operating zone can be considered ohmic, but all of these can be used in a way they are not.

The trap is taking a definition and accepting it as absolute - in physics, nothing is absolute.

 

[rhubarbpf]

Windadct, you understood what I was asking and you have provided a great answer. Thank you very much.

 

[sophiecentaur]

What people? Can you provide a link please? Thanks.

Me, for one. The filament may be a pure metal and it would follow Ohm's Law if only it were not in its envelope. Treating the bulb as a component, it does not (cannot actually) follow Ohm's law - even if you immerse it in a water bath to keep its temperature constant.

The trap is taking a definition and accepting it as absolute - in physics, nothing is absolute.

10/10

 

[DaveE]

Exact definitions for words like this are often not worth the trouble. In practice, they are used, and intended to be, kind of sloppy. Ohmic in one application may be non-linear in another. I think the use of "ohmic" says as much or more about the application as the device. As if the speaker is saying "I don't care about and will ignore any non-linear effects", or maybe, "I don't want to make the effort to elaborate on the details of the V-I relationship".

 

[sophiecentaur]

ALL of these in some appropriate operating zone can be considered ohmic, but all of these can be used in a way they are not.

People seem to forget what Ohm's Law says. A vital part of it, which is seldom considered, is the constant temperature stipulation. Keep the 'metal' part of a component at constant temperature then it will obey Ohm's law fine. Semiconductors will not follow OL, even if you keep them at the same temperature.

HOWEVER, if you take any component and measure V and I, the ratio of them has the same units as the Resistance of an Ohmic component so you can assign it an effective 'resistance' at that point on the graph. But I wish someone would justify referring to that the ratio V/I as "Ohm's Law". That's not a Law - it's just a ratio.

 

[rhubarbpf]

Thank you sophiecantaur. I have just found something else that you wrote that is really helpful too. I will reference it here for completeness:
https://www.physicsforums.com/threads/difference-between-ohmic-and-non-ohmic-resistors.861778/
"Why don't people actually read Ohm's Law? It only applies for a metal at constant temperature. Even a lamp filament will 'obey Ohm's Law' if you can manage to keep the filament at a constant temperature but, of course, its resistance varies over a 10:1 [Edit 1:10] range as it warms up from room temperature to 2500K.
It is sloppy to use the term "Ohmic" in all cases because people get the wrong idea. The resistivity of some metals does not vary a lot over a practical range of temperatures so that's what to choose when you need to rely on the resistance, marked on the component.
As stated earlier - no electronic component is idea. We are lucky that the manufacturers have managed to produced devices that are usually 'near enough' to treat them as ideal in most circuits. Early devices were nothing like 'near enough'."

 

[anorlunda]

It is sloppy to use the term "Ohmic" in all cases because people get the wrong idea.

It can depend on what stage you are at. For students looking to become proficient at circuit analysis (CA), I think it is helpful to discourage all comparison to real physical devices. CA is just a fertile field of mathematics with no tie to the real world.

After CA is mastered, real life devices and circuits can be introduced.

The reason for this two step process is that some students get themselves all tied in knots wresting with non-ideal ideas while trying to learn CA at the same time. We see them all the time on PF asking questions like, "What happens if I connect the terminals of a V source with no resistor?"