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Burning a light bulb
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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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Update: June 2012
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