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Name: amj
Status: N/a
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993


Question:
What happens during the last 10 seconds before a light bulb burns out?



Replies:
From my experience, light bulbs generally seem to go out right after you turn the light on - actually within a second, so there is not question of waiting 10 seconds there. Light bulbs contain a narrow wire (the filament) and as this wire heats up, it becomes more resistive. Since current is the ratio of voltage to resistance, more current than normal flows in the light bulb during the first fraction of a second, until the filament has completely heated up. This is the time of greatest stress on the wire, and the power (which is proportional to the square of the current) can be considerably higher than what the bulb uses under normal conditions. Over time (even if the bulb is never turned off) the metal atoms on the filament evaporate and eventually weaken the wire sufficiently that it will break if subjected to stress. If left on long enough, the wire can break even without any stress, but then it does not do anything spectacular, I do not think. Basically what happens is the metal gradually evaporates and at some point the wire breaks, and the bulb is burnt out. If you look inside a burnt out unfrosted bulb you can usually see bits of the filament dangling from the two supports in there.

A. Smith


Generally, I would agree with the previous response, but there is another factor which is significant. As the metal evaporates at a point, or as a flaw develops (we all know bulbs often fail after shocks, especially when the shock happens while the bulb is lit) the resistance at that point will go up. This is because the resistance of a wire is inversely related to its thickness. As the resistance increases, the temperature at that point also increases due to I^2R heating. As the temperature goes up, the resistance at that point goes up even more (resistance is inversely proportional to temperature). This is a run-away situation. The increasing resistance drives temperature up, the increasing temperature drives resistance up, and so on. Eventually, the filament melts, and the bulb burns out. This whole process, once initiated, takes place very fast - it is very difficult to predict in advance when a bulb will burn out.

Obiwan



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