Uncovering the Mystery of Hot Glass and Light Bulbs

[Physicsissuef]

Why when sometimes touch the glass balloon around the light bulb, it is hot?

 

[||spoon||]

the filament of the globe is extremely hot. I suppose it is just heat transfer from this.

 

[Physicsissuef]

the filament of the globe is extremely hot. I suppose it is just heat transfer from this.

Isn't just visible light released from the filament? Where the IR light comes from?

 

[||spoon||]

the light from the filament is caused by the random excitation of electrons due to the heating of the filament due to the current passing through it. This is why the light voming from a lightbulb is incoherent.

 

[Physicsissuef]

the light from the filament is caused by the random excitation of electrons due to the heating of the filament due to the current passing through it. This is why the light voming from a lightbulb is incoherent.

There is light because of the collision of the electrons with the atoms of the filament. So that's how there is light. But where the heat come from??

 

[CompuChip]

Isn't just visible light released from the filament? Where the IR light comes from?

Ordinary light bulbs are very inefficient. In fact, only between 2 and 10% of the output is actually visible light, the rest is heat. I think in modern light bulbs the efficiency is somewhat higher, up to 30% and maybe even more. Still this means that over half of the energy is "wasted" (of course, you can calculate how much you can turn down the heating if you have a couple light bulbs burning).

Basically what happens in an ordinary light bulb is that a material (usually tungsten, aka wolfram) is heated up by pumping electricity though a very thin wire. Almost as a side effect, the wire emanates light (just like any metal does when heated, compare molten iron).

 

[CaptainQuasar]

Forget that the filament gives off light for a moment. The filament is an electrical resistor, just like the heating coil in an electrical heater. When electricity is passed through it its resistance creates heat. This heat radiates outward from the filament and increases the temperature of the bulb glass and the other components of the bulb.

⚛​

 

[||spoon||]

what causes the electrons to be of such a high energy to release light?

Heat does. The filament has a high resistance so when a current is passed through it a lot of heat is generated. This causes electrons to become excited... Moving between discrete energy levels. When they drop from a higher energy level to a lower a certain frequency of light is emitted.

 

[russ_watters]

The principle was mentioned by description, but not by name: The filament of a light bulb is a black body radiator. So it gives the entire spectrum, with a center around a frequency corresponding to its temperature. That's the reason that incandecent lights are so inefficient: they give off way more heat than visible light.

 

[Claude Bile]

Also, glass is quite opaque beyond a wavelength of 2 microns, and therefore absorbs most of the heat being emitted by the bulb.

Claude.