Why do light bulbs burn out just as they’re turned on? [Madscience]

Light bulbs reside on borrowed time, but why do their timers are likely to expire just as you switch on a mild, rather than flickering out randomly?

Everyone has had the experience of coming home from a protracted day, turning on the lights, and having the light bulb sputter at them as it flickers out. Fewer people have seem them randomly fizz out in the course of the evening, when they’ve been on for some time.

Light bulbs give off light by pumping electric current through a thin tungsten filament. The filament heats and offers off light. Over the years, the filament oxidizes and becomes progressively more brittle, until it breaks apart and the bulb goes out. Since the oxidation occurs gradually and builds up, the light bulb should give out randomly, at any time. As anyone who lives in a house with electricity knows, that’s not the case.

Tungsten gains resistance as it heats. Resistance is the volume of ‘push back’ a cloth has against an electric current. The only real thing that heats tungsten in a light-weight bulb is electric current flowing through it. Imagine if a rubber hose gained strength only after water flowed through it. After some use, it might be ready to handle a heavy stream of water. At the beginning, though, it might bulge and strain like a water balloon before regaining its shape. Unless the rubber is in fine condition, this may snap. Inside the same way, a tungsten filament is overloaded with current inside the first few seconds after being turned on. The heat causes it to expand, and the filament experiences thermal stress, the tension of the fabric seeking to expand because of sudden changes in temperature. Unless it truly is in good shape, it snaps.

What’s more, over the years, the filament becomes uneven. At certain points along the filament, the tungsten evaporates, thinning the filament an increasing number of. At other points, the coils of the filament get pushed close together. When the high level of current surges through a stretch of wire even thinner than the remainder of the filament, the heat builds up even faster than the remaining of the filament. When it heats a piece of coils pressed close together, the heat between them can’t dissipate as quickly as it does within the rest of the bulb. The filament breaks or burns or simply melts.

It turns out that the typical light bulb isn’t designed to be turned on. It’s meant to already be on. The initial stress on the tungsten filament is much higher than the light bulb can handle. People who are looking to prolong the lives of their bulbs for ecological or sentimental reasons can pre-warm them before turning them on to a comfy 2,000 degrees before switching them on.

Top image by Andrew Price .

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This post is tagged: coming home, electricity, rubber hose, sudden changes, thermal stress, tungsten filament, water balloon