How do light bulbs go out?
In: Chemistry
If you mean how does a light bulb blow, it’s when the brightly glowing wire (or ‘filament) inside them breaks.
When you first switch on, the most stress is on the filament, and then as the filament heats up, the stress on it becomes less.
Think of it like a tug o war, at first there’s a huge amount of stress on the rope, but before long, the people participating get tired, and the strain on the rope gets less.
As long as the rope can put up with that initial real hard tug, it’s probably not going to break. The light bulb filament is the same.
As long as it can cope with that initial switch on stress, it’ll likely be fine. This is why when light bulbs blow, they tend to when you first turn them on.
If you want the more advanced reasoning behind it, it’s this:
Light bulb filaments are what’s called a resistive load, so as they warm up, their resistance increases.
If you know about Ohm’s Law, that means for a given voltage, and resistance, a certain amount of current can flow. If you reduce that resistance, more current can flow.
As mentioned above, the filament resistance is lowest when it’s cold (perhaps even as little as 1/10 the resistance of when it’s hot) so at that point it can pass the largest amount of current (10x as much as normal operating conditions). That means there’s more energy going through the filament in those first fractions of a second than when it’s warmed up, and therefore, if it’s going to fail it’s most likely to immediately when turned on and when it does, it’ll be with a -very- bright flash, because there’s an awful lot more energy involved than when the bulb is operating normally.
Assuming it survives that, as it heats up it’s resistance increases and that lets everything settle down to normal operating power (which might be, say, 60 watts).
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