Why Must an Incandescent Bulb Filament Have a Low Evaporation Rate?

[Simon Clement]

Hello here

Why is it necessary that the filament of an Incandescent bulb to have low evaporation rate below melting point?

 

[NascentOxygen]

What things do you picture might eventually happen if metal kept evaporating off the hot filament?

 

[Simon Clement]

Gets thinner

 

[Simon Clement]

Gets thinner

But the bulb should still be OK

 

[NascentOxygen]

Gets thinner

Would that be a problem?

 

[Simon Clement]

Resistance would increase and thus power consumed by bulb decreases. Bulb gets dimmer.

 

[NascentOxygen]

Can I say the resistance if the filament would increase?

Then that would make the power consumed by bulb brighter and hence bulb brighter.

If the bulb became thinner, its resistance would increase, certainly. This means it will draw decreasing current.

In reality, it won't become uniformly thinner, some areas will lose more metal than others.

What do you think will become of the atoms of metal that have evaporated off the hot filament?

 

[Simon Clement]

They would fall off from the metal. Most likely landing on the soft glass of the bulb

 

[NascentOxygen]

They would fall off from the metal. Most likely landing on the soft glass of the bulb

Condensing on the cool (i.e., cooler) glass surface, and as the layer of metal builds, the glass seems to darken.

 

[Simon Clement]

Ooh OK. So the bulb gets dimmer due to increased resistance and due to the to the evaporated black particles of the filament.

 

[NascentOxygen]

Ooh OK. So the bulb gets dimmer due to increased resistance and due to the to the evaporated black particles of the filament.

Yes, a combination of two effects.

But it's a decrease in the average brightness of the thinning filament, that we observe. If the filament becomes non-uniform in thickness, then we can expect it will have non-uniform resistance along its length. What might be a consequence of this?

 

[Simon Clement]

Nothing coming to my mind.

Except that the luminous Flux won't be same along the filament. But even if that's true, I'm guessing we won't be able to observe that variation.

 

[NascentOxygen]

We would not discern variations with the naked eye, but sensitive instruments should be able to pick up non-uniform emission. Also, as evaporation continued, some points along the filament might experience significant thinning. How would this influence the temperature at those points along the filament?

 

[Simon Clement]

Temperature at thinner points would be higher than less thinner points as the have higher resistance.

Since filament is tungsten, I don't think the temperature would be high enough to melt the filament even with the increased temperature.

I would say life of the bulb would decrease.

 

[NascentOxygen]

You should now be in a position to speculate on the sequence of events that marks the catastrophic end point in the life of an incandescent bulb.

 

[Simon Clement]

Thank you very much. I get it better now.