# Homework Help: Blackbodies and power

1. Oct 12, 2008

### Quelsita

The Tungsten filament of an incandescent lightbulb is a wire of diameter=0.080mm and length=5.0cm at a temperature of 3200K. What is the power radiated by the filament?

We Know:
Power=Watt/time=Energy/time'

E=h(nu)
nu=5.088E10 Hz/K

I'm not sure how to use the diameter and length to find the power. Since it's a filament, it would be in the shape of a cylinderso we could use the information to find the area, but I don't know how that applies.

Any help is appreciated!

2. Oct 12, 2008

### nasu

Use Stefan-Boltzmann equation. It gives the power radiated by unit area.

3. Oct 12, 2008

### cepheid

Staff Emeritus
No. Power is measured in watts.

Yes.

Yes, but not that relevant to the problem.

How did you arrive at this number? No energy is given. Also, frequency is measured in just hertz. NOT hertz/kelvin.

As for the approach, have you heard of the Stefan-Boltzman law?

4. Oct 13, 2008

### Quelsita

Sorry, it should be multplied by T which would cancel the Kelvin units.
I have but I was unsure what to do with the diameter and length information as they are not needed in the Stefan-Boltzman law.

Since S-B law is power/unit area, could you just multiply the Sv by the area to get power?

5. Oct 13, 2008

### cepheid

Staff Emeritus
That's sort of beside the point. The unit error was not the main thing I was getting at. The main thing I was getting at was that E = hν is totally irrelevant to the problem! The fact that you have *somehow* mysteriously arrived at a number for ν makes absolutely no sense to me because 1. you don't have the information required in order to be able to calculate it (no energy is given) and 2. you are not *asked* to calculate it. I'm sorry, but your posts just aren't making a whole lot of sense.

Ummm.....really? You don't know what to do with that information? Continue reading...

YES! Now tell me something...just how exactly do you expect to compute the area without the "diameter and length information." Now do you see why that is given?