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? Thanks!
Based on H_{sun} 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.
Hi Pepealej! A is the area that is radiating, ie the surface area of the whole sun (4πr^{2}). (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).