Question Corner: Incandescent bulb

An incandescent bulb exposed to the hot sun does not burst. Why?

S. KRISHNAMOORTHY

Chennai

Any incandescent bulb exposed to hot sun does not burst simply because there is no excess pressure build-up in side the bulb even when exposed to the hot-sun. This is in contrast to an air-filled balloon bursting when exposed to sun-light, especially in summers.

This difference is, in the case of a balloon there is already excess positive air-pressure inside which expands further excessively when exposed to sun resulting in bursting. Whereas in the case of the incandescent bulb there is only negative pressure inside (that is, inside pressure is only about 70 per cent of outside atmospheric pressure).

This takes care of pressure build-up from expansion of gases inside due to tungsten filament generating lot of heat upon lamp being switched on and also external factors like hot surroundings. The working of the lamp involves generating glow in the tungsten filament, an electrical resistor through which when electrical current passes generates lot of heat accompanied by the glow as seen as light.

In this process the filament is taken to high temperatures and the filament tends to evaporate, especially in presence of air or oxygen, leading to the attended problem of getting hot-spots eventually affecting the life of the filament. So to increase the life of the filament (hence the lamp) in operation, during lamp fabrication, usually the bulb is filled inside with inert gases like nitrogen, argon which are very effective in avoiding or minimizing evaporation losses of the filament.

In the design of these lamps adequate precaution is taken to avoid both explosion (due to pressure build-up inside) and also implosion ( due to pressure drop inside) as the lamp always has pressure comparable to surroundings with initial negative inside pressure gives sufficient allowance for expansion upon heating.

Another factor that is bothersome, is expansion of the bulb-glass shell due to heat. But atmosphere takes care this by virtue of the fact that rate of change in temperature is not too fast for other wise it might result in the glass shell developing cracks and splintering.

R. JAGANNATHAN

Luminescence Group

CECRI

Karaikudi, Tamil Nadu