Lightbulb filament failure

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Lightbulb filament failure
« on: 30/11/2005 10:35:00 »
I recently read the following question in the N** Scientist magazine:

"Why do lightbulbs more commonly fail when first switched on,
   rather than when they have been on for some time ?"

Here is a possible (?) answer, perhaps readers can post alternative explanations.

The filament of a typical incandescent lightbulb is a coiled wire:  

so it is effectively a miniature solenoid.
When electrical current passes through a solenoid a magnetic field is generated, (the coil becomes an electromagnet). This magnetic field creates forces, (mechanical stress), in the coils of the solenoid,
 (the bulb filament).

The strength of this magnetic field is directly proportional to the electrical current passing through the coils. The electrical resistance of the (coiled) wire is inversely proportional to its temperature.

When the bulb is initially switched on its temperature is at its lowest , so its electrical resistance is at its lowest, so the current is initially high, therefore the magnetic field is initially high, so the mechanical stress in the filament caused by the magnetic field is at its highest the instant the bulb is switched on, so the filament is most likely to break then.

Comparing the filament when the bulb has been on for some time with the initial conditions:-
the filament is now (2000 C) hotter,
its electrical resistance is higher,
the electrical current is therefore lower,
the magnetic field is lower,
so the mechanical stress on the filament due to the magnetic forces is lower than at initial "switch on".

« Last Edit: 30/11/2005 11:25:37 by ROBERT »


Offline daveshorts

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Re: Lightbulb filament failure
« Reply #1 on: 30/11/2005 13:28:17 »
Many things tend to fail when they are turning on or off, mostly due to thermal expansion and contraction, which will move everything slightly and so trigger something which was almost broken to break now. The solenoid effect is probably part of the effect too, especially as when you just turn it on the rate of change of current could be very large so applying the biggest magnetic forces to the filament.


Offline gsmollin

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Re: Lightbulb filament failure
« Reply #2 on: 03/12/2005 13:26:31 »
Those answers are both true. The cold resistance of a tungsten filament is about 1/10 its hot resistance, so there is a powerful surge of current at turn-on. This is called "inrush". It can also cause fuses in the branch circuit to fail. The inrush causes the filament to heat at a very high rate. The rapid change in temperature causes mechanical stresses in the filament from differential expansion. This is known as "thermal shock". Most of the time the filament breaks at one end, where the thermal shock stresses are the highest. The high current also causes mechanical stress from the Lorentz force on the conductors due to magnetic fields. In small bulbs, the former effect is most important. In large bulbs, the latter becomes important.

"F = ma, E = mc^2, and you can't push a string."
"F = ma, E = mc^2, and you can't push a string."