# Rough Estimate about electric light bulb

1. Feb 14, 2006

### darkar

Well the question asked us to make a rough estimate of the temperature of the filament in an ordinary electric light bulb which emits white light. And if it has radiated power of 100 W, make a rough estimate of the surface area of filament.

Well, the thing is how u make a rough estimation on the temperature?

2. Feb 14, 2006

### chroot

Staff Emeritus
The color of the light determines the temperature of the radiating body, as described by Wien's Law. A body which is radiating white light has a peak wavelength of emission about in the middle of the visible spectrum.

After you know the temperature, you can use the Stephan-Boltzmann law to find the radiating area, knowing that the radiated power is 100 W.

- Warren

3. Feb 14, 2006

### Tide

The gas pressure inside a typical light bulb is reduced so you can probably ignore thermal conduction as a dissipation mechanism and take the thermal energy as being radiated away - more or less as a blackbody.

4. Feb 14, 2006

### darkar

Right, so assuming it is a black body, but will it be better to use Planck Spectral Distribution diagram to estimate it? But then the temperature is quite high, about 5500 K, is this possible? Really this hot?

5. Feb 14, 2006

### chroot

Staff Emeritus
That's about right, darkar. A blackbody at about 5500K produces white light. The Sun, for example, is such a blackbody. Approximating the light bulb filament as a blackbody is obviously an oversimplification, but it's all you can do in this context.

- Warren

6. Feb 14, 2006

### darkar

Wow, didnt know the light bulb is that hot. But if thats so, wouldnt the glass melt?

7. Feb 14, 2006

### chroot

Staff Emeritus
darkar,

No, because the inside of a light bulb is a (partial) vacuum. As Tide says, the only mechanism by which heat can leave the filament is EM radiation, and glass is nearly transparent to the majority of the EM radiation thus produced.

- Warren

8. Feb 14, 2006

### darkar

I see, thanks a lot everyone !

9. Feb 14, 2006

### Tide

And that is precisely why Thomas Edison had so much difficulty finding the right material and configuration for his lightbulb filaments! :)