Does Ohm's Law work for Light Bulbs?

[malawi_glenn]

By the way, back in 2016 I wrote an Insights Article (with help from the late Jim Hardy), Learn Why Ohm’s Law Is Not a Law.

No idea why that link did not work for me, but I found it here
https://www.physicsforums.com/insights/ohms-law-mellow/

 

[vanhees71]

Well, Ohm's Law indeed can be derived. Historically the first was Drude's theory, which has been refined quantum-mechanically by Sommerfeld. It's one of the first applications of quantum statistics in condensed-matter physics.

 

[anorlunda]

That derivation can be found here:
https://en.wikipedia.org/wiki/Drude_model#DC_field
It's also discussed in that Insights article.

@vanhees71 has spoken favorably about the Drude model several times on PF. Nevertheless, I fear citing it because of some comments I've seen on PF that make Drude sound deprecated like relativistic mass.

The simple classical Drude model I described above is only a crude heuristic model!

In 1900, there was something called the Drude model, which attempted to do this. It got Ohm's Law as an output (sort of) and pretty much got everything else wrong.

 

[sophiecentaur]

Summary: Does Ohm's Law work for Light Bulbs? I did a simple experiment where it doesn't seem to.

this is the only explanation I can think of off the top of my head of why Ohm’s law does not work for a lightbulb.

Exactly. Ohm's law describes the behaviour of metals at constant temperature - same as Boyle's Law assumes constant temperature and Charles' Law assumes constant pressure.

I never understand why people refer to the definition of Resistance as Ohm's law. (Actually, I can understand why; it's because sloppiness propagates when it's convenient). Everywhere you look, they say things like "By Ohm's Law, R=V/I" which is nonsense, when you think about it. The ratio of V/I is defined as R to enable all those circuit calculations to work and any element that is actually conducting a current can be said to have a 'Resistance' for any particular V across it. That's basically an 'Effective Resistance' and can't be relied on from one moment to the next - except when someone has gone to the trouble of making a resistive component with a very low temperature coefficient of resistance. You could say that it does better than Ohm's Law because it stays at '220Ω' even when it gets (slightly) hot.

I can't change the usage and so PF will forever be getting questions about it and students will lose sleep.

 

[Albertgauss]

Hi all,

Some background:

I’m not intending to “disprove” Ohm’s Law. From the experience of this post that I didn’t know beforehand, I will think better to reword phrases like “Ohm’s Law doesn’t work for...”

The materials I use here are all standard, cheap, physics education stuff on the shelf of any storeroom for a classroom, which you all know from when you were students going through the physics introductory series in college. No, I am not claiming any new physics here.

I am doing something similar to drmalawi steps 1,2,3,4

Basically, I prepared some activities for students in a class with regular resistors and Ohm’s Law works great for them. Then, thinking that the demo light bulbs we have on the shelf wouldn’t be demo light bulbs unless they conformed to Ohm’s Law, I was quite surprised how much they did not adhere to Ohm’s Law as how the resistors did. I was going to use the light bulbs in an activity as “real-world” devices as examples of Ohm’s Law; but obviously I can’t do that because they behaved much differently than I expected. The point of this post was to see if I overlooked something major on Ohm’s Law since I had been so sure the light bulbs should have had an easy dependence on Ohm’s usual Law, V = IR. I realize now that Ohm’s Law applied to demo light bulbs is not straightforward.

I don’t want to get technical if Ohm’s Law is a “Law” or “Rule” or “theory”. I know it is used for at least demo resistors and that’s is really what I am using it for.

I am quite satisfied with simple answers such as: 1) Light bulbs are far from ideal resistors, 2) The light bulb is non-linear, 3) Light bulb resistance is a function of temperature (without the math), 4) Ohm's law describes the behaviour of metals at constant temperature. A consensus of these reasons by the group as reasons to explain my experiments is perfect enough for me to say the post is completely answered.

For this post, I am not concerned with AC anything, any kind of diodes, MOVs, gas discharge tubes, some polymers, no Maxwell differential calculus, I’m not doing a continental power grid either, no quantum, we won’t be doing Taylor series for this. Such points are surely correct and valid, but I’m not going to into these topics at this time Ohm’s Law.

I do actually appreciate everyone giving attention to this; I did learn a lot of useful things I did not know before.

 

[sophiecentaur]

I’m not intending to “disprove” Ohm’s Law. From the experience of this post that I didn’t know beforehand, I will think better to reword phrases like “Ohm’s Law doesn’t work for...”

Ohm's Law works just fine. It would even work for a filament lamp if you could circulate the envelope with distilled water and kept the temperature constant, even when the bulb is dissipating its 100W - or whatever. It would work because Tungsten is a metal. (It would not glow much at 300K, though)

I have to apologise for the sloppy way you (and most of us) were taught how to mis-use Ohm's Law. But we're stuck with things the way they are.

 

[Herman Trivilino]

I would say Ohm's law defines resistance.

That is not my understanding of Ohm's Law. $R=\frac{V}{I}$ defines $R$. The assertion that $V$ is proportional to $I$ is Ohm's Law.

 

[anorlunda]

$$\sum_{(x_2,x_3)\in{\{0,1\}^2}}$$

Then, thinking that the demo light bulbs we have on the shelf wouldn’t be demo light bulbs unless they conformed to Ohm’s Law, I was quite surprised how much they did not adhere to Ohm’s Law as how the resistors did.

Admittedly, this thread is overly long and drifts away from your question. However, you still haven't gotten the point.

Having a constant value of R is not part of Ohm's Law.

@berkeman said it well: $V(t) = I(t)*R(t)$ is a valid expression of Ohm's Law.

 

[JT Smith]

Just for the heck of it I thought I'd see if R for a 60W incandescent light bulb would be relatively constant at low voltage. I measured the resistance with an ohmmeter to be about 18Ω. Then I applied 1.4V DC and the current was 53mA. That means 26Ω. At 3.3V it was 39Ω. Even at these low voltages there was significant self-heating.

Given that I've seen the same sort of thing with thermistors I suppose it shouldn't have surprised me. I guess you need a water bath...

 

[bob012345]

Here is a slightly more technical historical discussion of Ohm's work which discusses his experiments and apparatus.https://www.researchgate.net/profile/Madhu-Gupta-8/publication/3052303_Georg_Simon_Ohm_and_Ohm%27s_Law/links/5becd0db92851c6b27bf6ace/Georg-Simon-Ohm-and-Ohms-Law.pdf

 

[jrmichler]

Just for the heck of it I thought I'd see if R

... and I did the same thing. I connected a power supply to a small 6 volt bulb, and measured the current while stepping up the voltage. I started at 0.007 volts, then 0.085 volts, eventually increasing in one volt steps until the bulb failed. The voltage and current were measured using the power supply display, so the voltage is the voltage at the power supply, not at the bulb. The bulb was dim red at 1.5 volts, dull red at 1.8 volts, bright at 5.8 volts, dazzling at 10.8 volts, and failed at 13.8 volts. The plot of results:

The slight departure from a smooth curve at the lowest voltage is explained by the one digit precision of the display - 0.007 volts and 0.004 amps.

Ohm's Law correctly describes the relation between voltage, current, and resistance at all voltages. The resistance is different at each different voltage, but that does not invalidate Ohm's Law. It just means that the resistance is a function of voltage, or more correctly, the filament temperature.

I once tried to make a sensitive and fast responding air temperature sensor out of a light bulb by smashing the glass bulb. It was sensitive and fast responding to changes in air temperature, but self heating from the ohmmeter made it difficult to use.

 

[sophiecentaur]

That is not my understanding of Ohm's Law. $R=\frac{V}{I}$ defines $R$. The assertion that $V$ is proportional to $I$ is Ohm's Law.

And do you include or exclude the 'constant temperature' requirement?

It just means that the resistance is a function of voltage, or more correctly, the filament temperature.

Yes and it's nothing to do with Ohm'/s Law, because Ohm's Law only applies under constant temperature. Resistance is defined (as always) as V/I whether it's a resistor, filament, thermistor or anything. It is just a ratio. Compliance with Ohm's law implies that the temperature is constant.

Re: a bulb filament. Break the glass and allow the filament to cool or, better still, dip it in circulating distilled water and your 'non-ohmic' component starts to behave more like an ohmic component. A regular bulb will appear more 'ohmic' for very short voltage pulses.

 

[JT Smith]

A regular bulb will appear more 'ohmic' for very short voltage pulses.

I was thinking of that. It's what I do with thermistors to minimize self-heating in air: turn them on, measure the voltage drop, and then shut them back off. It's maybe 100µs that they are energized. But it's a less visual demonstration and a bit harder to rig up.

 

[DaveE]

I once tried to make a sensitive and fast responding air temperature sensor out of a light bulb by smashing the glass bulb.

Sounds more like a hot-wire anemometer to me. Air is a poor heat conductor, so the filament power would need to be kept very low to avoid heating the air you're trying to measure. But you probably did that. Metals aren't as temperature sensitive as other things like thermistors. The issue with incandescent bulbs is the incandescent part, they get really, really hot.

 

[Herman Trivilino]

And do you include or exclude the 'constant temperature' requirement?

$R$ has to constant for $V$ to be proportional to $I$. A constant temperature keeps $R$ constant.

 

[sophiecentaur]

$R$ has to constant for $V$ to be proportional to $I$. A constant temperature keeps $R$ constant.

Preeeecisely. And that is Ohm’s Law.

 

[Opera]

A friend had a gas welding mixer that combined C)2 and argon which went out of calibration. It used a hot wire bridge, one in CO2 and the other looking at the mix. Over time the CO2 one corroded slightly and wouldn't calibrate. Factory said he had to buy an entire new unit. I just changed the plumbing so they saw the other gas. Then it would calibrate. Amazing how the hot wire is so sensitive it can see the difference in heat transfer of a slightly mixed gas. I use lamps to limit current when repairing electronics with shorts. Saves fuses which have become quite expensive. The lamp with nearly a 7 to one change in resistance keeps current to a safe level so in circuit testing can be done. I'm saving every old lamp I can find as these are quickly disappearing.

 

[bhobba]

We need a good insight article on this.
I have been taught it's a law from the early 80's onward.

Well, F=M*A is a definition of force. R = E/I is a definition of resistance (and its extension to impedance). In mechanics, the concept of force has proven crucial, and in analysing electrical circuits, the concept of resistance has proven valuable. Neither is a law in the usual sense ie an experimentally verifiable statement - you can't verify a definition. It is a law in the sense that Newtonian Mechanics is a statement about nature that, as John Baez expresses it, says - get thee to the forces. Ohm's law is a law in the same sense - a statement that says in analysing circuits - get thee to resistance. A device is usually called a resistor if it, to a good degree of accuracy, can be considered to have constant resistance. Such simplifications are valuable in analysing circuits. Of course, a 'real' resistance changes with temperature etc but if such can be neglected it helps in analysing circuits.

Thanks
Bill

 

[Herman Trivilino]

I have been taught it's a law from the early 80's onward.

It is a law. All laws have limits of validity.

 

[fxstsb]

Try measuring resistance at different temperatures.

 

[fxstsb]

Summary: Does Ohm's Law work for Light Bulbs? I did a simple experiment where it doesn't seem to.

Does Ohm’s Law, V = IR work for light bulbs? It appears not to from my simple experiment below.

In the figure below, I measured the resistance of a lightbulb and found that resistance to be 2.6 ohms.

View attachment 304144

However, when I connect this lightbulb into the circuit where I measure the voltage across the lightbulb and the current through the lightbulb simultaneously, the entire simple circuit sourced by a power supply, then I get a derived resistance of 18 ohms.

View attachment 304145

I say derived because the voltage I measure is 3 Volts across the lightbulb and the current through the lightbulb is 165 mA. By Ohm’s law, 3 Volts divided by 165 mA is equal to 18 ohms. If the resistance of the lightbulb with current flowing through it is indeed 18 ohms, this certainly does not match the resistance of 2.6 Ohms measured directly by the resistance meter of the multimeter. From such an experiment, I would conclude the Ohm’s law does not work for light bulb.

Can anyone confirm that Ohm’s law does not work for a light bulb?

Obviously, the lightbulb heats up, and that heat could change the resistance dramatically when the lightbulb has current flowing through it versus when there is no current flowing; this is the only explanation I can think of off the top of my head of why Ohm’s law does not work for a lightbulb.

I actually measured the resistance of all the wires, the ammeter, etc and their total resistances still came to around 3 ohms; thus, the high resistance of 18 Ohms measured by the lightbulb when On cannot be attributed to the resistances of other components in the circuit.

Try measuring resistance at different temperatures.

 

[Opera]

Ohm wasn't too happy how it worked out either. He was ridiculed and lost his job as a teacher and had an utterly miserable life as a result. The detractors kept adding more batteries in series and the current wouldn't change. They never took into account that the batteries used had very high internal resistance. I can get up to 10 times change in resistance depending on what the lamp is filled with to carry off heat. 6 or 7 is about normal for most lamps. For UL we need the internal temperature of a relay sometimes. Measuring the resistance of the copper coil at 25C and then after it has been on and stabilized, the internal temperature can be calculated. I have a 6 1/2 digit ohm meter. Touching one leg of a resistor with a finger I can see the resistance reading change. It is not the resistance changing, but thermoelectric effect from two different metals. Resistance can be tricky. It always follows the rules.

 

[bob012345]

It is a law. All laws have limits of validity.

Yes, it's the law so resistance is futile.

 

[sophiecentaur]

Ohm wasn't too happy how it worked out either. He was ridiculed and lost his job as a teacher and had an utterly miserable life as a result. The detractors kept adding more batteries in series and the current wouldn't change. They never took into account that the batteries used had very high internal resistance. I can get up to 10 times change in resistance depending on what the lamp is filled with to carry off heat. 6 or 7 is about normal for most lamps. For UL we need the internal temperature of a relay sometimes. Measuring the resistance of the copper coil at 25C and then after it has been on and stabilized, the internal temperature can be calculated. I have a 6 1/2 digit ohm meter. Touching one leg of a resistor with a finger I can see the resistance reading change. It is not the resistance changing, but thermoelectric effect from two different metals. Resistance can be tricky. It always follows the rules.

The results from ancient scientific measurement were often dodgy. That, I can accept. My problem is more with the ignorance that's around these days, about what Ohm's law actually says.

 

[sophiecentaur]

Resistance can be tricky. It always follows the rules.

Actually, resistance follows no rules - until you describe or include the conditions of the measurement.

 

[bob012345]

The results from ancient scientific measurement were often dodgy. That, I can accept. My problem is more with the ignorance that's around these days, about what Ohm's law actually says.

I think Ohm was a careful and thorough experimenter as the paper I linked to earlier suggests. I hope nobody here assumes his work was dodgy.

 

[Opera]

It is amazing work considering there were no resistance meters and a volt meter couldn't be made until resistance was understood. All you had was a swinging magnet, like shaped bars of different metals and batteries that seemed to do something though you didn't know what. This was flying blind.

 

[bob012345]

It is amazing work considering there were no resistance meters and a volt meter couldn't be made until resistance was understood. All you had was a swinging magnet, like shaped bars of different metals and batteries that seemed to do something though you didn't know what. This was flying blind.

My point is that Ohm was not flying blind. His experiments were sophisticated and clever. He knew exactly what he was doing. For example, at one point he used a thermocouple between boiling water and ice to generate a fixed potential.

https://www.daviddarling.info/encyclopedia/O/Ohm.html