Paul Meyer wrote:
Btw, in a stastics class I learned that light bulbs don't fail because of
filament
failure at switch-on. The failure rate is constant with time (I forget which
distribution
models this, but whichever is a "memoryless" distribution) which indicates that
the cause for failure is most likely external, i.e. power surges on the line.
From http://members.misty.com/don/bulb1.html
How light bulbs burn out
Due to the high temperature that a tungsten filament is operated at,
some of the tungsten evaporates during use. Furthermore, since no light
bulb is perfect, the filament does not evaporate evenly. Some spots will
suffer greater evaporation and become thinner than the rest of the filament.
These thin spots cause problems. Their electrical resistance is greater
than that of average parts of the filament. Since the current is equal
in all parts of the filament, more heat is generated where the filament
is thinner. The thin parts also have less surface area to radiate heat
away with. This "double whammy" causes the thin spots to have a higher
temperature. Now that the thin spots are hotter, they evaporate more
quickly.
It becomes apparent that as soon as a part of the filament becomes
significantly thinner than the rest of it, this situation compounds
itself at increasing speed until a thin part of the filament either
melts or becomes weak and breaks.
Why bulbs often burn out when you turn them on
Many people wonder what goes on when you turn on a light. It is often
annoying that a weak, aging light bulb will not burn out until the next
time you turn it on.
The answer here is with those thin spots in the filament. Since they
have less mass than the less-evaporated parts of the filament, they heat
up more quickly. Part of the problem is the fact that 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 would be
running at, 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.
This means that the thin spots, which run too hot anyway, get even
hotter when the thicker parts of the filament have not yet fully warmed
up. This is why weak, aging bulbs can't survive being turned on.
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