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: Units - Watts, meters & Kelvin

  1. Feb 28, 2005 #1

    tony873004

    User Avatar
    Science Advisor
    Gold Member

    Units -- Watts, meters & Kelvin

    I have an equation:
    [tex]L = \sigma T^{4} * 4 \pi R^{2}[/tex]

    Which I fill with values and units
    [tex]L=5.67*10^{-8} * (5860K)^{4} * 4 \pi * (6.96*10^{8}m)^{2}[/tex]

    and I get an answer:

    [tex]4.07*10^{26} K^{4} m^{2} [/tex]

    The correct answer is:

    [tex]4.07*10^{26} watts [/tex]

    I don't understand the relationship between meters, Kelvins, and Watts. How do I properly combine or cancel the units to arrive at Watts?
     
  2. jcsd
  3. Feb 28, 2005 #2

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Investigate the units on your [itex] \sigma [/itex]
     
  4. Feb 28, 2005 #3

    tony873004

    User Avatar
    Science Advisor
    Gold Member

    I was thinking the answer might lie there, but that might be a unitless constant. I think there is a Kelvin meter relationship that yields watts.
     
  5. Feb 28, 2005 #4

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

  6. Feb 28, 2005 #5

    tony873004

    User Avatar
    Science Advisor
    Gold Member

    Thank you. I get it now. You're both right. It does have units :redface:
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook