What am I doing wrong

[Carultch]

So I guess ohms law is giving the resistance when the bulb is hot? That doesn't make sense to me

Ohm's law is the idealized case that resistance is independent of electrical operating conditions. It is a property of size/shape, material identity, and temperature. As long as temperature isn't significantly changed, you can expect resistance to remain unchanged.

Due to the fact that an incandescent light bulb has to heat up to 10 times the absolute temperature in order to work, resistance will indirectly depend on the voltage. But it really is depending on the temperature of the filament. Suppose you heat it up to its operating temperature with a separate source, instead of powering it with electricity. You will find its resistance is close to what you'd expect when operating at the rated voltage.

 

[GoldDigger]

A resistor in series with the bulb while it is not supposed to be lit will not directly limit inrush, but it can preheat the filament enough that the inrush when the resistor is bypassed will be much smaller than the cold filament inrush. The circuit to bypass the resistor could end up putting the resistor in parallel with the bulb, though there would be no specific benefit to doing that.

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[Besoeker3]

Aren't we all going the LED route?

 

[ptonsparky]

I drew

A resistor in series with the bulb while it is not supposed to be lit will not directly limit inrush, but it can preheat the filament enough that the inrush when the resistor is bypassed will be much smaller than the cold filament inrush. The circuit to bypass the resistor could end up putting the resistor in parallel with the bulb, though there would be no specific benefit to doing that.

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I drew it out to understand how the lamp and resistor would have gone from series to parallel. Doable, but way simpler to parallel a contact with the resistor.

 

[charlie b]

So I guess ohms law is giving the resistance when the bulb is hot? That doesn't make sense to me

A better way to say it is that Ohm's Law is in effect at all times. What changes as the bulb heats up is the resistance, and therefore the watts. In other words, using the value of 100 watts in an equation is invalid until the filament heats up. Instead, if the resistance is 12 ohms before you energize the bulb, then the moment you turn it on the power is equal to V2/R, or 14,400/12, or 1,200 watts.

 

[kwired]

So I guess ohms law is giving the resistance when the bulb is hot? That doesn't make sense to me

Already answered, bulb is rated in watts, when a rated voltage is applied.

200206-2249 EST

Dennis:

I have referenced my plots and data many times on this forum. See http://beta-a2.com/EE-photos.html .

Variation of resistance with temperature is something you should have learned about in high school physics.

When you put several bulbs in series across the line you reduced the voltage on each bulb. Thus, a change of the resistance of a bulb vs with one bulb across the line.

There is much more to this discussion, but I won't go in to that.

A resistor in parallel with a bulb won't reduce the bulb inrush. It has to be in series.

.

resistor in parallel won't reduce inrush, but placing a little to no resistance path in parallel AFTER getting some preheating of the filament going on will. I think that is what was being described.

 

[kwired]

Though commonly used dimmers are not resistors, similar effect applies, effective voltage seen by the lamp is reduced and that lessens the current inrush, especially if a design that always first turns on at the lowest level then progresses to higher level.

Incandescent lamps used on dimmers typically have much longer life than same lamp not used on a dimmer, even if you typically run them near full output.

I put recessed lights in exterior soffits on my home built about 15 years ago. They are on photo control and also on dimmers and serve not only decorative effects but are also security lighting. They are usually dimmed to at least 50% but can always be adjusted if you have outside activity and want more light. Never changed any lamps since installed and they have run nearly every night for 15 years and a couple times all day when photo control had failed.

 

[iceworm]

Dennis -
I'm not adding anything new - following the rest of the posts:

From your thread https://forums.mikeholt.com/threads/ohms-law.147562/page-4#post-2315506
Feb 1, 2019

A typical 100W bulb would have a resistance of 12 ohms at 20C, and a resistance of 144 ohms at 2550C. The attached graph shows the effects of temperature on the filament resistance

As mentioned, about the only to measure the hot resistance is to use an ammeter and voltmeter and calculate it.

 

[GoldDigger]

Most people have a flawed understanding of Ohm's law. E=I R is not Ohm's law, it is simply the definition of resistance. Ohm discovered experimentally that for the substances available to him at the time the value R is (nearly) constant over a wide range of current and voltage.

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[iceworm]

... E=I R is not Ohm's law, it is simply the definition of resistance. ...

... Ohm discovered experimentally that for the substances available to him at the time the value R is (nearly) constant over a wide range of current and voltage.

For experiments, he initially used voltaic piles, but later used a thermocouple as this provided a more stable voltage source in terms of internal resistance and constant potential difference. He used a galvanometer to measure current, and knew that the voltage between the thermocouple terminals was proportional to the junction temperature. He then added test wires of varying length, diameter, and material to complete the circuit. He found that his data could be modeled through the equation

​

where x was the reading from the galvanometer, l was the length of the test conductor, a depended only on the thermocouple junction temperature, and b was a constant of the entire setup. From this, Ohm determined his law of proportionality and published his results.​

​

(note: emphasis is mine) Reference: http://purehistory.org/ohms-law/

Considering that "b" is proportional to the measurement/lead resistance, Ohm' "law of proportionality" certainly looks like "E=IR".

Yes, there are issues with:

statistical fluctuations, (Johnson–Nyquist noise),​

breakdown under strong enough electric fields​

non-ohmic under weak electric fields​

None of these really affect me - except maybe breakdown, which I would likely call "flashover" The rest are lab grade stuff - nothing I can measure with a fluke 87.

I'm liking my flawed understanding of E=IR

Minor off-subject quote from the worm: I had someone describe my job as "herding electrons". I corrected them, "Nope, amps maybe - never seen or measured an electron."

 

[synchro]

Most people have a flawed understanding of Ohm's law. E=I R is not Ohm's law, it is simply the definition of resistance. Ohm discovered experimentally that for the substances available to him at the time the value R is (nearly) constant over a wide range of current and voltage.

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And also when a meter is in the "Ohms" setting it does not measure resistance directly, but instead it applies a voltage and then measures the resulting current. The meter then utilizes Ohm's law by using the ratio of applied voltage to the measured current to indicate a resistance value.

Now when an analog Volt-Ohm-Meter is set to one of the "ohms" measurement ranges, the meter moves in direct proportion to the measured current Im and not 1 / Im as would be needed to directly display the resistance since R = V / Im. So the meter is really responding to the conductance which is 1 / resistance, and not the resistance itself. So in order to display resistance, they just label it with a nonlinear scale using the resistance values that correspond to measured conductance values. That's why maximum resistance is on the left with a minimum amount of meter movement, and zero ohms is on the right with maximum meter movement. So effectively Ohm's law is being utiized with this "re-mapping" of the displayed values.
Of course DVMs have active analog and digital signal processing and so this is not an issue, and they can apply Ohm's law more directly.

 

[LarryFine]

I've always been under the impression that Ohm's Law states that one volt is capable of pushing one amp through one ohm.

In other words, the equation should technically be written I = E / R, as the current is the result of the other two parameters.

 

[iceworm]

I've always been under the impression that Ohm's Law states that one volt is capable of pushing one amp through one ohm.

In other words, the equation should technically be written I = E / R, as the current is the result of the other two parameters.

Sure. In fact that is in the quote I used in post 30.
However, as synchro said, one measures the voltage and current and then calculates the resistance. I don't know of any instrument that measures resistance directly - yes, even a bridge doesn't.

 

[LarryFine]

Sure. In fact that is in the quote I used in post 30.
However, as synchro said, one measures the voltage and current and then calculates the resistance. I don't know of any instrument that measures resistance directly - yes, even a bridge doesn't.

Still, it's the resultant current that the meter responds to.

Whether voltage the source is internal (ohmmeter) or external (voltmeter or ammeter), the meter movement responds to the current through its coil. What is displayed is a matter of scale markings.

(yes, I know I'm describing analog meters, not electronic/digital)

 

[Carultch]

I've always been under the impression that Ohm's Law states that one volt is capable of pushing one amp through one ohm.

In other words, the equation should technically be written I = E / R, as the current is the result of the other two parameters.

It's written in a form similar to the way people learn Newton's second law (Fnet = m*a). The order of variables being written as "cause = object property * effect". The formula formats are written in this manner, not because there is a convention of how cause and effect should be arranged, but rather because it eliminates division for simplicity. Multiplication doesn't care about the order, but division does.

V = I*R eliminates division, and gives the same information as both of the following:
R = V/I would put it in the form of the definition of resistance.
I = V/R would rephrase it as "effect = cause / object property", to see the equation the way the universe solves it.

Ohm's law is that R is independent of both V&I for most materials, as long as other factors (such as temperature) are constant. The formula itself is a rearrangement of the definition of resistance.

 

[Carultch]

Still, it's the resultant current that the meter responds to.

Whether voltage the source is internal (ohmmeter) or external (voltmeter or ammeter), the meter movement responds to the current through its coil. What is displayed is a matter of scale markings.

(yes, I know I'm describing analog meters, not electronic/digital)

Given that resistance having any meaning depends on both voltage and current being non-zero, I don't know of any way you could measure resistance without using a sample voltage and the response current. R = V/I is an indeterminate form (zero divided by zero) when both voltage and current are zero. We only assume it would still remain the same as it was with our sample voltage measurements, because it shows no trend of changing with diminishing voltage.

 

[ramsy]

The fact that an incandescent lamp is close to a short circuit when you first turn it on is why switches for incandescent lamps must be "T" rated.
From the UL Guide Information for "Snap Switches" (WJQR)

Frequently find 15A snap switches used for 1/3 HP to 3/4 HP kitchen disposals.
Until now, never understood why those disposers didn't burn up 15A switches w/ no horsepower rating.

 

[ggunn]

I've always been under the impression that Ohm's Law states that one volt is capable of pushing one amp through one ohm.

In other words, the equation should technically be written I = E / R, as the current is the result of the other two parameters.

All three variables are just arbitrary constructs, models that we only comprehend in terms of their relationship to one another.

Everything is everything, man.

 

[LarryFine]

IDIC

 

[jaggedben]

Years ago on Myth busters show. There was a debate as to if it makes sense to turn the lights off or leave them on based on the inrush current and the normal running current. What was the break even point. I think it came out to 15 seconds or something close. If you were going to turn the light off and then turn it on in 15 seconds or less you were better off leaving it on. If you were going to leave the room for 15 seconds or longer turn them off.

Here's the episode https://www.vanderbilt.edu/sustaina...s-test-the-effects-of-turning-off-the-lights/

I remember that episode and it wasn't even close to 15 seconds (I believe it was 2.7) and that was only for florescent bulbs. Think of the time it takes a typical florescent to come on fully after you flip the switch. Any other bulb type, they basically found that you'd be hard pressed to make the bulb consume more energy from inrush than from leaving it on even if you flipped a switch on and off as fast as you could. They judged the myth busted.

Now you can watch the video and see how good my memory is.