This post includes a couple of intertwined questions… First, why do light bulbs only burn out at the moment I flip the switch on? I have NEVER been sitting in a room with a light on and it just spontaneously burned out. I know this is kind of like the tree-in-the-woods, but come on! Obviously, there must be some increased power demand when turning on a light, but inquiring minds need to know. Secondly, does the lifespan rating of a light bulb (the box says 1000 hours) factor in flips of the switch, or is that a best-case scenario (the bulb is never turned off and on)? Thirdly and thusly, if I turn on a new light bulb, sit in my chair next to it, never turn it off and never leave my chair, will it ever turn off?
(1) Yes, there is a small surge of electricity that that hits the light bulb (and all electrical equipment) when you first turn it on. Sometimes this is enough to burn through the weak part of the light bulb filament, and then on to its untimely demise.
(2) The stated lifetime of a bulb is actually an average of a number of test bulbs. In other words, half the bulbs will burn out by 1000 hours. This is based on continuous run; the more you turn a light off and on, the shorter its lifespan (this is also true for fluorescent lamps).
(3) Yes, eventually the light bulb will burn out. Even though there is very little air in the bulb, there is still enough to allow the filament to oxidize and burn out.
By the way, I have seen light bulbs burn out in the middle of a burn, rather than at the moment of switch-on.
Bulbs burn out because of thermal stress on the filament.
When you turn the bulb on the filament goes from room temperature to several hundred degrees very rapidly. The filament expands from the increased temperature and flexes. Do this enough times and one time it will break instead of merely flexing.
A bulb that is burning steadily has much more uniform stress on it. It doesn’t burn out because there is no reason for it to do so. There are very slow chemical processes occurring (they used to create a vacuum in the bulb – now I believe they use an inert gas as a filler) and they would eventually, I suppose, cause the filament to fail, but turning the bulb on and off is much more traumatic.
Let’s see. I lathered and I rinsed. But did I repeat?
This has to be the biggest coincidence of the day. After reading your post, I got up, looked closely (without touching), a clear bulb in the ceiling fan. I turn on the wall switch and the fucker burned out. All I can say is that the filament looked a little droopier than the others. Why was I focusing on that one out of the four? I had a strange feeling…
Well, here’s what I came up with. Similar to the pervious posts with the exception of filament evaporation. I just post everything I typed instead of editing it again.
Light bulbs will burn out when flipped on for a couple of inter-related reasons. The first - there is a small power surge experienced whenever an electrical device is powered-up. The second - failure due to thermal fatigue.
Thin spots in the filament have less mass than the less-evaporated parts of the filament, they heat up more quickly. Tungsten, like most metals, has less resistance when it is cool and more resistance when it is hot. This explains the current surge that light bulbs draw when they are first turned on.
When the thin spots have reached the temperature that they should be running at, roughly 4500 degrees F, the thicker, heavier parts of the filament have not yet reached their final temperature. This means that the filament’s resistance is still a bit low and excessive current is still flowing. This causes the thinner parts of the filament to get even hotter while the rest of the filament is still warming up.
All this causes thermal fracture.
In response to your final question, yes all bulbs will eventually burn out. The high temperature generated causes the tungsten filament to actually evaporate. The evaporation does not take place uniformly and areas of will generate a higher resistance, thereby creating a higher operating temp. This will accelerate the uneven evaporation of the filament until it finally breaks or melts.
As for the lifetime rating of a bulb, I can only guess. I suspect an average number of on-off cycles are included in the calculations, but I have no data to support that.
“A zebra does not change its spots.”
~Al Gore, 1992~
Just to make you feel safe the next time you fly – a similar process occurs in airliners. The cabin is pressurized at flight altitudes and de-pressurized at ground level. This cyclic flexing of the cabin can cause fatigue cracks in the structure.
This happened a lot in early passenger jets, notably the DeHavilland Comet, before they figured out what was going on and now they both design for it and inspect for problems. It’s not a common problem nowadays, although several years ago a Hawaiian Airlines jet lost the top of it’s fuselage because of this. The plane was used for inter-island hops. The short flights didn’t add many hours to the airframe but they did add cycles. This was roughly equivalent to flicking a light on and off.
So next time you fly don’t worry about the plane falling out of the sky once you get there. Only worry during takeoff and landing. Have a pleasant flight.
Afisher,
You asked:
There are several factors.
First, the predominant reason is that before you turn on the light, the filament is cold. When you flip the switch, the filament goes nearly instantly to white hot. The filament, which is a kind of brittle metal, undergoes some expansion forces. If the filament has started to fatigue over time, these expansion forces will cause the filament to break.
Second, the resistance of the filiment is higher when it is cold so the power consumption of the filament is higher in those first few microseconds when you flip the switch. If there is a micro-fracture in the filament, you can see a sort of fuse effect where the filament burns in two.
Third, table lamp bulbs burn out more frequently. This is because the filaments are sensitive to vibration. When you reach up to turn the lamp on or off, you set the filament vibrating and this can cause the filament to break, especially if it is already fatigued and is white hot.
This happens all the time, but is less frequent because usually the bulb goes due to one of the switch on effects first. In an amazing coincidence, last night I witnessed the bulb on a table lamp across the room burned out spontaneously. I took note because just before it went, it began a very high pitched squeal. I was looking around the room, trying to identify the source when the bulb just went in a high intensity flash. Interestingly, it is a 3-way bulb and the other two filaments are still working. This might seem like an obvious consequence, but it is counter to past experience. Usually, I notice, that 3-way bulbs burn out completely. I’ve not disassembled one to figure out why that is, but it seems to be the norm.
Also, it is not uncommon for bulbs to pseudo-spontaneously burn out in conditions of power fluctuations. Circuits where the lights frequently flicker, generally yield lower lifetimes for the bulbs. This is basically the same as the first and second cases, above, just without the switch.
No. The lifetime hours quoted are for a continuous burn.
No. At least not in your lifetime. Of course, given that you’re in your chair and can’t go to the toilet or refrigerator, your lifetime will be considerably shorter than the 41.67 days that your 1000 hour bulb will burn…
One reason for bulbs to burn out when they’re switched on is that metals expand when they’re heated, and when you switch a bulb on the filament goes from approximately room temperature to several hundred degrees in a very short amount of time. Sometimes if you look closely you can even see the filament vibrating as a result of this rapid expansion.
Modern quartz-halogen type bulbs don’t generally “burn out” because of oxidation specifically, but because the filament, which is thin to begin with, becomes thinner and more brittle over time as metal evaporates from it and deposits on the walls of the glass bulb itself. Eventually it can become so thin that the power surge when it’s switched on can physically break it.
One reason for bulbs to burn out when they’re switched on is that metals expand when they’re heated, and when you switch a bulb on the filament goes from approximately room temperature to several hundred degrees in a very short amount of time. Sometimes if you look closely you can even see the filament vibrating as a result of this rapid expansion.
Modern quartz-halogen type bulbs don’t generally “burn out” because of oxidation specifically, but because the filament, which is thin to begin with, becomes thinner and more brittle over time as metal evaporates from it and deposits on the walls of the glass bulb itself. Eventually it can become so thin that the power surge when it’s switched on can physically break it.
Light bulbs that get bounced around seem to have a shortened life span as well. The light bulb under the staircase needs to be changed much more often than the one in the bedroom closet. But the one at the top of the stairs that I turn off and on all the time needs changing most of all. Would it be worth it to just leave that one on all the time?
Oh, I’m gonna keep using these #%@&* codes 'til I get 'em right.
About Oxygen:
This info has been bouncing around my head for years, so maybe they don’t do this anymore.
In the manufacuring of light bulbs the filament assembly, is dipped in a liquid call “getter.” On first lighting, this stuff burns off, binding with any remaining oxygen in the bulb. You never see this since all bulbs are tested on the way to the shipping department.
There is a large surge of current when a tungsten-filament light bulb is turned on. The tungsten wire has a very low resistance to current at low temperatures and a higher resistance at high temperatures.
During that time when the temperature is increasing quickly, there are all kinds of stresses placed on the internal parts untill things equal out.
Mr’s Hewlett and Packard exploited this effect in their first product: An audio oscillator. This let their product have a very stable level over time and enabled Walt Disney to develop his Fantasia multi-channel sound track.
Light bulb Conspiracy?
A television program (I believe it was This Old House about the great Thomas Edison showed one of the rooms in his house where he had an electric light fixture which was burning bright with a nice yellow/orange glow. The curator of the house mentioned that those were the original (6 I think) bulbs from the 1880s!
This got me thinking that there are two possible explanations:
- We’ve had the capability to churn out bulbs which can last 115+ years, but a Vast Conspiracy of Lighbulb Manufacturers, not unlike OPEC, decided that it was in their best economic interest to withhold this crazy technology and sell bulbs which burn out after 1-3 years. Constant source of revenue and all.
Since this idea sounded kind of paranoid, I thought it might be,
- Those Edison SuperBulbs must contain some kind of expensive gas and/or metal which makes them unfeasable for the consumer market.
I don’t buy that, though, because even if they cost quite a bit more than the crap we’re buying now, it would STILL be far cheaper in the long run (sort of like those compact flourescents).
What’s the deal?
Hell is Other People.
- The curator is wrong.
I thought I read someowhere the original Edison bulb is still going? You can buy better bulbs, yes, commercially they often do.
There is supposedly a bulb that has burned non-stop for 100+ years in a firehouse in some small town. It’s the town’s tourist attraction. I’ll post the details when I find the article…
Believe it or not, once your lightbulb is dead, it may be possible to resurrect it.
You put the dead lightbulb into a lamp and turn it on. Then you jiggle the lamp around hoping to get the two pieces of broken filament to touch. If they do, they will weld together and the lightbulb will function for a while longer. (But not much.)
I have a chandelier which takes those small decorative bulbs. When one burns out I can sometimes bring it back to life by striking it by flicking my finger. While this works, the resurrected bulb is somewhat brighter than its companions.
history of light bulb life
flourescent
1938 1500 hours
1974 9000 hours
1989 24000 hours
today 100000 hours
incandescent
1879 14 hours
1881 600 hours
1910 1000 hours
today 1500 hours
halogen
1964 2000 hours
1974 3000 hours
today 4000 hours
from the january 2000 issue of popular science
what is essential is invisible to the eye -the fox
Sake Samurai,
My dad has some very, very old light bulbs that he rescued from a very, very old house once. They still function. One of them has a piece of paper inside with a date that was the late 1800’s (not sure exactly, I’ll check over the Christmas holidays). The most notable thing about this bulb, aside from the fact that it’s not frosted, is that it’s light is very weak. My guess is that the filament is made of a more durable material that doesn’t glow as bright… So there’s probably a tradeoff between luminance and bulb life.
Interesting - Because they were filming the program in the middle of the day with light pouring in through the windows (and other light fixtures were on) I couldn’t tell how luminant the bulbs were. All I know(besides what I wrote before) is that they were clear (not frosted) and they were incandescent (of course - look at that timeline).
Those old bulbs of yours sound like a great find. Please post again with the findings from that paper.
On a side note: How exactly did the Lighthouse at Alexandria function? I am presuming they had a large fire burning, but how was it fueled to prevent extinguishing and focused to cut through harsh weather?
Hell is Other People.