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Before a light bulb burns
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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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Update: June 2012
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