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Question:
Why is there a real bright light before a light bulb burns out? burbank school



Replies:
When a flaw develops in the filament of the light bulb, the resistance of the filament increases. When this happens, more heat is produced by the current traveling through the filament. This increased temperature causes resistance to increase more in a viscous cycle. The resistance goes up, the heat produced goes up, and on and on. Very soon, the filament is very hot, burns very brightly, and melts, causing the light bulb to burn out.


Actually, I think the flash (and the melting) cause the resistance to go DOWN, not up - because a light bulb is not a constant current device but a constant voltage device, so if resistance went up, the current would simply drop. I believe the reason the resistance goes down is because the filaments are long curled up pieces of metal, and any melting can cause a short circuit between the loops, thus lowering the resistance. I am not sure if it is melting, or something else that creates the initial short circuit though.

Arthur Smith



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