Turning lightbulbs on and off makes it blow faster False or true?

[naab]

When you were little you were always told not to "play" with light, by turning it on and off. This was because the light bulb would "blow up" faster..

But is this true?

I have learned that the tungsten filament in a light bulb wears up because of the heat that makes the tungsten "vaporize" slowly.. So I don't see how turning the light on and off would make it "blow up"/"burn over" faster...??

explain please.. is this myth true or busted? :P

 

[rewebster]

true

but not the 'right' reason---

turning the light 'on' causes a immediate surge that makes the filament vibrate/jump with the potential to break the more often it is 'turned on'

 

[HallsofIvy]

Except for the rather minor effect that rewebster mentioned, no. The idea that turning on an incandescent light "harms" it more than just having it on is an "old wives tale". It is, however, true that the ballast in neon lights are affected more by turning on the light than letting the light run.

 

[naab]

I also thought it was an "old wives tale" haha..

Like taking a swim right after you have eaten..

 

[rewebster]

watch at around 41 secs

 

[Doc Al]

I'm not convinced that it's an "old wive's tale". I'd think that the repeated expansion and contraction of the filament would make stress fractures much more likely, making weak spots more likely to burn out. But I can't quantify how much of an effect that would be. Call the mythbusters!

 

[russ_watters]

They'll need to structure the experiment in such a way as to separate the repetive stress factor from the stress of rapid heating because it certainly is true that most general use incandescents fail when turned on.

 

[naab]

Heey...! when you turn the light on the filament vibrates.. so maybe there is something about it after all:



around 40 secs...

 

[turbo]

In our old house, we had a dimmer switch on the chandelier over the kitchen table and that fixture at light bulbs like candy. The power filtered through that dimmer was really "notchy" and if it was quiet in the house, you could hear the filaments humming as you dimmed the lights.

 

[rewebster]

...you could hear the filaments humming as you dimmed the lights.

I think in that case, it was probably the contacts humming (arcing bulb to socket) from the AC current and resonating through the bulb.

--at some harmonic maybe too.

 

[naab]

rewebster, I think so too... the filaments vibrate so rapidly so their hum would be almost ultrasound I think.. on the video you can barely see them shake and it's slowed down 40 times...

 

[rewebster]

OK naab---I have a thought WHY the filament 'vibrates/moves/shakes'---

--why do you think it does?

 

[Meir Achuz]

Do the experiment.
Use a timer (or your old wife) for one bulb, and leave the other one on.

 

[naab]

I think it vibrates because of the moving electrons, but I'm not sure.. but you agree that the frequency of a possible sound from the filament would be high right.. so if you could hear the filament vibrate it would be a high pitch sound.. right?

 

[rewebster]

I think it vibrates because of the moving electrons, but I'm not sure.. but you agree that the frequency of a possible sound from the filament would be high right.. so if you could hear the filament vibrate it would be a high pitch sound.. right?

I think from the frequency of the AC current adds a component (which MAY create a sound which MAY add another component), but a DC lightbulb's filament vibrate/jump/move when first turned on too...so...

do you have another thought?

(that was a big hint...)

------------------------------------------

OK--here's a bigger hint to what I think causes the movement


http://amasci.com/elect/poynt/voltagef.gif

 

[naab]

I'm not familiar with DC and AC because I'm from Europe.. Is DC = and AC is the variable current? or the other way around..

please the answer to why it vibrates.. :P it's late, I can't think..

 

[rewebster]

The filament is not perfect and sets up variations in heat and magnetic fields along the length of the filament. As the wire first heats up and the magnetic field is initiated, variations in these two cause the filament to bend AND to attract/repel for a while setting up a movement/harmonic vibration/jerking/twisting motion. Some of the more expensive bulbs are longer lasting because the filaments are made with less imperfections in material and consistency in diameter/shape.

 

[naab]

I wish I was as smart as you.. how old are you? :D

 

[rewebster]

I wish I could say
I worked with Faraday.

 

[naab]

:P I just turned 18 last Thursday and I'm in my second year of high school out of three.. :D
you?

 

[rewebster]

Age doesn't matter ...



(that's what we older folk say)


just keep reading and asking questions

 

[naab]

:D I read/hear/watch physics almost every day, it's so addictive... I'm a total physics junkie :D

 

[rewebster]

Be The Lightbulb


------

but watch out for the imperfections

 

[naab]

Yeah, but everytime I ask a question I turn on, and I might blow up someday if I ask too much :P

No.. If you ask a question you might feel stupid for 5 minutes, but if you don't ask, you might feel stupid your whole life..

 

[rewebster]

Well post another one of your questions then...


and find out what will happen.

 

[turbo]

rewebster, I think so too... the filaments vibrate so rapidly so their hum would be almost ultrasound I think.. on the video you can barely see them shake and it's slowed down 40 times...

No. AC in the US is at 60 cycles and if you use a dimmer to interrupt that the sine wave gets really notchy with a squared-off wave form. There is nothing ultrasonic about a 60 hz signal. One of the bars that I used to play guitar in produced a horrible buzz through our amps - it was the dimmer for the lights over the back bar.

 

[lightarrow]

When you were little you were always told not to "play" with light, by turning it on and off. This was because the light bulb would "blow up" faster..
But is this true?

Maybe there is another effect to consider, but I don't know if it's true.
We know that electric resistance of the filament varies a lot with temperature, specifically, it reduces much from room temperature to ~ 3000°K. So the electric current is higher when the filament is cold. Maybe the fact the filament doesn't heat up immediately, gives the current the time to reach high values and so to heat up, then, the filament to an higher temperature than at stationary conditions.

 

[rbj]

I'm not convinced that it's an "old wive's tale". I'd think that the repeated expansion and contraction of the filament would make stress fractures much more likely, making weak spots more likely to burn out. But I can't quantify how much of an effect that would be. Call the mythbusters!

i am not either convinced it's a old wive's tale.

in fact, mostly for the reasons given, i am quite certain that a common incandescent light bulb that has the full voltage cycled on and off repeatedly (say being on 10 seconds, then off for ten seconds) will, more often than not, burnout earlier than an identical light bulb that has been left on the whole time.

 

[turbo]

in fact, mostly for the reasons given, i am quite certain that a common incandescent light bulb that has the full voltage cycled on and off repeatedly (say being on 10 seconds, then off for ten seconds) will, more often than not, burnout earlier than an identical light bulb that has been left on the whole time.

Yes - there is a price to pay (duty-cycle) for on-again-off-again usage that is mitigated by continuous use. It's not just in light bulbs. If I was looking at a two-year old vehicle with 100K miles on it vs the same vehicle with 25K miles, I would really love to know who was driving those vehicles and under what circumstances. A person shuttling their kids back and forth to school/daycare and enduring stop and go traffic on their daily commute would kill a vehicle a whole lot quicker than someone who did a lot of long distance driving to get from job to job. I would WAY rather own a high-mileage used vehicle used by a salesman/tech service rep with a large territory than one owned by a soccer mom.

 

[drpizza]

I think Doc Al's reasoning is correct. Should be a relatively simple experiment to set up... on for 5 seconds... off for 5 seconds... repeat. If it really does have an effect on the life expectancy of a bulb, I think it'd be significant enough so that you'd get the bulbs to blow within an hour or so under such conditions.

 

[rbj]

Yes - there is a price to pay (duty-cycle) for on-again-off-again usage that is mitigated by continuous use. It's not just in light bulbs.

oh yeah, we know it's not just light bulbs. another example issue is the one between people who leave a computer (or perhaps some other electronic device) on always and people who turn it off every time they're done with it. I'm in the middle category - i turn on the computer the first time of the day that i use it and leave it on until i am done with it for the last time that day, then i shut it down. i just think that is better than leaving the hard drives spinning for 10+ hours at night without any use and sometimes i hear the read/write heads rachetting around when the computer just sits there.

 

[rbj]

I think Doc Al's reasoning is correct. Should be a relatively simple experiment to set up... on for 5 seconds... off for 5 seconds... repeat. If it really does have an effect on the life expectancy of a bulb, I think it'd be significant enough so that you'd get the bulbs to blow within an hour or so under such conditions.

it would take longer than an hour. could take weeks.

 

[cesiumfrog]

it would take longer than an hour. could take weeks.

What do you base that on?

I mean, the "one hour" was a complete guess. Order of magnitude at best. There was no justification given whatsoever, so no reason to think it meant specifically one actual hour. But since you think that guess is out by a specific amount, I've got to ask, what makes your refined estimate any more trustworthy?

 

[rbj]

What do you base that on?

i pulled it out of my butt (and I've replaced a lot of light bulbs in my 5 decades, you get an idea how long they last).

I mean, the "one hour" was a complete guess. Order of magnitude at best. There was no justification given whatsoever, so no reason to think it meant specifically one actual hour. But since you think that guess is out by a specific amount, I've got to ask, what makes your refined estimate any more trustworthy?

it's hardly refined. in fact it's very broad ("weeks" which could mean two or fifty) and qualified ("could"). but check out

http://www.gelighting.com/na/home_lighting/ask_us/faq_defective.htm

now if a typical light bulb might last for about 1000 hours (about 4 weeks) when simply left on (100% duty cycle), you might, under decent circumstances, expect it to last for 8 weeks at 50% duty cycle. but that's something like on for a few hours and off for another few hours. i really doubt it would last as long as that where the frequency of power cycling is 1/10 Hz.

what ballpark figure were you thinking of? one hour? one year? "weeks" is somewhere sloppily in between.

 

[Mk]

Didn't Mythbusters do this one?