Light bulbs are living on borrowed time, but why do their timers tend to expire just as you turn on a light, instead of flickering out randomly?
Everyone has had the experience of coming home from a long 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 middle of the evening, after they've been on for a while.
Light bulbs give off light by pumping electric current through a thin tungsten filament. The filament heats and gives off light. Over time, the filament oxidises and becomes more and 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, this is not the case.
Tungsten gains resistance as it heats. Resistance is the amount of "push back" a material has against an electric current. The only thing that heats tungsten in a light bulb is electric current flowing through it. Imagine if a rubber hose gained strength only after water flowed through it. After some use, it would be able to handle a heavy stream of water. At first, though, it would bulge and strain like a water balloon before regaining its shape. Unless the rubber is in good condition, it will snap. In the same way, a tungsten filament is overloaded with current in the first few seconds after being turned on. The heat causes it to expand, and the filament experiences thermal stress, the strain of the material trying to expand due to sudden changes in temperature. Unless it is in good condition, it snaps.
What's more, over time, the filament becomes uneven. At certain points along the filament, the tungsten evaporates, thinning the filament more and more. 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 rest of the filament, the heat builds up even faster than the rest of the filament. When it heats a section of coils pressed close together, the heat between them can't dissipate as quickly as it does in the rest of the bulb. The filament breaks or burns or simply melts.
It turns out that the average light bulb is not designed to be turned on. It's meant to already be on. The initial stress on the tungsten filament is far higher than the light bulb can handle. Those who wish to prolong the lives of their bulbs for ecological or sentimental reasons can pre-warm them before turning them on to a cosy 1000C before switching them on.
Top image by Andrew Price.