The typical resistance of the tungsten filament of a 60 watt incandescent light bulb when it is turned off is 24 ohms and its resistance when operating from a 120 volt power source is 240 ohms. Generally, an incandescent light bulb filament is most likely to “burn out” when it is first turned on than at any other time. Explain why? HINT: Calculate the power dissipated when first turned on.

Question

The typical resistance of the tungsten filament of a 60 watt incandescent light bulb when it is turned off is 24 ohms and its resistance when operating from a 120 volt power source is 240 ohms.  Generally, an incandescent light bulb filament is most likely to “burn out” when it is first turned on than at any other time.  Explain why?  HINT:  Calculate the power dissipated when first turned on.

bobpursley · Answer

You can answer this the way the hint suggested: what is V^2/R when turned on as compared to later?

But the reason for failing is not this, it has to do with a metal problem mainly. Tungsten when hot then cooled crystallizes. After many times, the filiament when cold is brittle in spots, very unductile. This brittleness in spots offers high resistace in just those spots, so a lot of ohmic losses occur in those spots, and failure occurs. Yes, the surge currents cause it, however these spots are the ones which fail.

drwls · Answer

With less resistance at turnon, before the filament heats up to operating temperature, there is a current and power surge that stresses the delicate tungsten filament. Temperature gradients during rapid heating can cause thermal stress fracture.