1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Heat current from the sun

  1. Feb 3, 2013 #1
    I'm recently studying heat transfer processes. I saw that the heat current emited by a radiating body is given by:

    [tex]\frac{dQ}{dt}=H=A e \sigma T^4[/tex]

    I was wondering how to calculate the H from the sun to a unit area here on earth. How should I do it? I've seen exercises where they use this formula and for A they use the area of the body in question, but they never do it like:

    What H from the sun do we recieve here on earth? I guess it's not the same being on the surface of the earth and being on that of the sun, right?

  2. jcsd
  3. Feb 3, 2013 #2


    User Avatar
    2017 Award

    Staff: Mentor

    Based on Hsun and the surface area of the sun, you can calculate the total power of the sun. In a distance of 1 AU (=the distance earth-sun), this power is distributed over a sphere with a radius of 1 AU, which allows to calculate the intensity.

    Simplified: ##H_{earth}R_{earth}^2=H_{sun}r_{sun}^2## where R is the orbital radius of earth and r is the radius of the sun.
  4. Feb 3, 2013 #3


    User Avatar
    Science Advisor
    Homework Helper

    Hi Pepealej! :smile:

    A is the area that is radiating, ie the surface area of the whole sun (4πr2).

    (see http://en.wikipedia.org/wiki/Thermal_radiation#Radiative_power)

    e (or ε) is the emissivity factor, always less than 1, because the sun is not a perfect black body radiator …

    I don't know where to find the value of ε.

    That gives you the total radiated power …

    at the earth's surface, that covers the whole sphere round the sun at the distance of the earth, so you have to divide by the surface area of that sphere, and multiply by the area of the bit of earth you're interested in (usually 1 square metre). :wink:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook