Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: How to find emissivity of tungsten from ln graph?

  1. Nov 29, 2012 #1
    1. The problem statement, all variables and given/known data

    I'm doing a lab where I'm trying to find the emissivity of a tungsten filament. I measured voltage and current of a bulb with a tungsten filament and found the power. Then I plotted Power vs Temperature^4 and used the slope along with surface area and Boltzman's constant to find the emissivity. But I am expected to plot ln(P) vs ln(T) and find the emissivity and I can't quite figure out how to do that.

    2. Relevant equations

    I plotted it bas on this equation

    ln(P) = ln(a) +n ln(T)

    which was derived from the equation

    P = aT^n

    3. The attempt at a solution

    I'm lost for this part. I've found everything else I'm supposed to except for this. i found n to be close enough to 4 and can find a from the intercept, but I'm not really sure what a stands for. Is that the emissivity?
  2. jcsd
  3. Nov 29, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

  4. Nov 30, 2012 #3
    It looks like a=Aeσ. Finding the Surface area of the tungsten element might be tricky. Perhaps you need to measure the resistance of the element, estimate length and determine surface area.
  5. Nov 30, 2012 #4
    Ah of course that makes tons of sense. I have the surface area so now I have everything I need. Thanks!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook