If light fixtures are powered through electricity, why do light bulbs need to be replaced?
In: Engineering
Have you ever taken a piece of metal like a wire and bent it and then unbent it, and then re-bent it, back an forth, until the metal simply broke apart? The reason that happens is because of mechanical stress being placed on the metal. Even smaller stresses, if given enough repetitions and time can cause a similar failure.
When you light an incandescent bulb you’re heating a metal filament to thousands of degrees. Heat causes the metal to expand and when the light is switched off the metal cools and contracts, this repeatedly stresses the metal much like bending and unbending.
The filament doesn’t just fall in half from stress though. Instead it begins to fail at it’s weakest part, but it can still form an electrical circuit. However this failing area starts to pull apart, forming a much thinner passage for the electricity to run through, which greatly increases it’s resistance, which due to the flow of electricity causes it to heat up even more than the white hot temperature it was already at an WOOSH, the light bulb gets very bright for a moment and then burns through the filament and it goes out.
This is why they tend to fail when turning on, not while they are already on, and why they get very bright at the moment of failure. For a few hundredths of a second, it’s like you swapped their relatively dim, beefy, filaments, with a much thinner, brighter one that burns out instantly.
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