I Does Ohm's Law work for Light Bulbs?

[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 resistence 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