If light fixtures are powered through electricity, why do light bulbs need to be replaced?
In: Engineering
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There is more to this question than just the actual physical reason for it. Light bulbs are probably the most prominent example of ‘planned obsolescence’, which means that they are designed to only last for a given amount of time. The reason for this, was that in the early days of the light bulb, companies were able to make light bulbs that could technically last for centuries. Some of those bulbs still exist and have been burning continuously for more than a century! However, if all light bulbs lasted that long, after the market had been saturated, nobody would ever need to buy new lamps. Thus, the companies who made lamps decided to jointly sign an agreement that they would all make their lightbulbs less resilient, so that they would eventually break after around 1000 hours. This pact is known as the ‘Phoebus cartel’, and included manufacturers like Philips, Osram and General Electric.
However, this was the case for incandescent light bulbs. Other types of light bulbs may have different light spans. In general, the ‘burning out’ has to do with the wear on the internal parts that eventually break the circuitry in the lamp. The wires can corrode, they expand and shrink constantly because of the heat, and over time, these factors make it so that the parts become more and more ‘brittle’, and eventually, usually when turning a lamp on, they fail or break.
It’s basically so companies who make light bulbs can make more money. Companies are capable of making them last much much longer, but then you wouldn’t be buying light bulbs as much.
Light bulbs give off light by pushing electricity through a thin tungsten filament. The filament heats up and produces light. Over time, the filament wears out and becomes brittle, until it breaks and the bulb dies.
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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