1. Dec 1, 2013

### subzero0137

Hi,

I have to plan and carry out an experiment on thermal radiation as part of my physics lab module. The description of the experiment is as follows: Investigate the radiation from a bulb filament as a function of input power, using a pyrometer to measure the temperature of the filament.

I'm guessing the purpose of this experiment is to verify the Stefan-Boltzmann law, which states that P=AσT^4 where P is the power radiated from the object, A is the surface area of the object (filament or light bulb?) and T is its temperature in Kelvin, and σ is the Stefan-Boltzmann constant. So, plotting P against T^4, and dividing the gradient of the resultant graph by A should give me a value close to Stefan's constant. But I have 2 questions:

1) Should I measure the surface area of the bulb, or the filament?
2) Can I assume input power = P, so that I can calculate the values of P using a multimeter in a circuit?

Any ideas would be appreciated.

2. Dec 1, 2013

### haruspex

If you kept the filament the same but made the bulb ten times the diameter, would the power from it change?
Seems fair.

3. Dec 1, 2013

### subzero0137

I don't think it would.

4. Dec 1, 2013

### haruspex

Right, so which area matters?

5. Dec 1, 2013

### subzero0137

The surface area of the filament.

6. Dec 1, 2013

Yes.