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Question:
Is it more likely for a light bulb to burn out if one turns it on and off very quickly several times? If so, why?



Replies:
I recall reading some research on vacuum tube filaments that was done about 50 years ago. The results indicated that heating and cooling reduced the life of a filament more than the length of time it was on. As a result the military chose to leave radios on even when not in use. I don't know if any more recent research has been done.

Larry Krengel


I cannot quantify this answer, but when you ask if it is more likely I would have to say yes. The light we see when we turn on the switch represents the heating of a thin filament with the light 'bulb'. This heating causes the filament to glow brightly. Any time anything is heated, there are stresses involved; the same applies as an item cools. If the length of time over which an item is heated is increased--that is, if we heat the object more slowly, there is less thermal stress involved, mostly because the item heats more evenly and there are not certain areas which are very hot and other parts of the same item which have just begun to heat. When we heat things suddenly, it takes a bit of time, depending on the nature of the item, for heat to be transferred throughout the item. As a child, I once wanted some melted ice cream, so I put some into a glass and then placed the glass on a stove burner. Even though the electric stove heated more slowly than perhaps a gas flame might, there was still a great temperature gradient across the glass, with one side being very cold, and the other very hot. Somewhere between the two the molecular movement caused by the increasing temperature was not quickly transferred across the entire glass, and the glass shattered. In another 'experiment', my mom once accidentally left a clear glass plate on the same electric stove, not realizing it was there. While attempting to reheat some coffee, she noticed a bright glow on one of the burners. The plate had heated slowly enough so as not to break, and the heat was sufficiently transferred completely across the plate so that, while there was certainly thermal stress, it was not sufficient to shatter the plate. Had she left the plate on the burner and gradually turned the heat back down, the plate might have been saved. Instead, realizing the problem, she turned the burner off (problem 1) and then moved the eplate (carefully) from the burner. By attempting to help cool the plate, she provided the setting for greater stresses during the cooling than were experienced during the heating. That is, the temperature of the plate would drop much more rapidly. Of course, the temperature did not drop rapidly over the entire part of the plate. The outermost glass of the plate, exposed to room air cooled more quickly than glass situated more interior on the plate. This induced stress due to the inefficient transfer of heat across the glass, and, the molecular motion difference between the two surfaces lead to...you guessed it....a shattered plate. I can mention that even if the plate had been allowed to be brought back to room temperature slowly by keeping it on the burner and slowly reducing the temperature, it is not certain that the plate, even though perhaps unshattered, would have been undamaged. What usually happens is there is the introduction of stress damage which will render the item more 'delicate' in the future. When, in the future, we accidentally tap the plate or set it too hard upon a hard surface, what normally might not damage the plate at all might cause it to shatter, since the weakness, though invisible, has already been done.

Now, back to your question. :)

When you turn the light on and off very rapidly, you create and compound stresses caused by the rapid heating and cooling of the filament. A light bulb only has so many effective 'burning' hours, because, even though the metal filament conducts heat effectively, there is still a thermal stress involved which, over time, introduces weaknesses into the filament which will cause it to 'break'. You can try an experiment, but it would be a lengthy one.....compare bulb burn times for a light left on continuously versus one where it is turned on and off, say once per minute, once every 30 seconds, once every 10 seconds etc. The theory of thermal stress would dictate that the more rapid heating/cooling experienced by the filament, the greater the thermal stress and the shorter the filament life. As I mentioned, this could be a long experiment...I recall hearing about a light which had been turned on only once which was burning for years and years and years.

One final note.....I work in the field of the manufacture of integrated circuits. During the processing of silicon wafers which are eventually cut into the chips used in our modems, pc's and cellular phones, there are furnace operations where various films are deposited or grown on the surfaces of the wafers. These are usually done at high temperatures. The wafers must enter the furnace at a low temperature, and the temperature must be increased quickly enough for throughput considerations, but slowly enough so there is not the introduction if thermal stress in the wafers, which then might be expected to shatter at that time or later in the manufacturing process. I have in the past seen wafers which need to be perfectly flat exit a furnace looking like Pringles potato chips because of too great a heating or cooling over too short a period of time. Thanks for using NEWTON

Ric Rupnik


One possible reason is the thermal stress you put on the light bulb by doing this. The light bulb is lit by a thin little wire strung from one holder to another. When it is hot, it expands, as it cools (when it is turned off) it contracts. Not much, but a bit. So if you turn the power on and off, you are asking the little wire to expand, shrink, expand etc, and eventually it may break. There may be other reasons... I don't know.



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