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Radiation problem

  1. Aug 24, 2008 #1
    I was trying to work out a question where there is a little sphere inside a big sphere with a perfect vaccuum inbetween.the big sphere been 0.13m dia. and 1550 degrees kelvin hot. The little sphere .1m dia. and 1500 degrees kelvin. All of the radiation from the little sphere hits the big but some from the big misses the small and hits itself.What is the net radiation between them ? Do you think you can work out the percentage of radiation that hits the small sphere from the big is proportional to the amount of angle that hits the little sphere from a point on the big sphere divided by 180 degrees?Both spheres are black bodies.
     
    Last edited: Aug 24, 2008
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  3. Aug 24, 2008 #2

    russ_watters

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    The answer is zero. This is directly from the principle of black body radiation.
     
  4. Aug 24, 2008 #3

    Redbelly98

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    Russ, the spheres are at different temperatures. Shouldn't there be a net energy transfer (i.e. not zero)?

    It wouldn't work that way, for two reasons:

    1. We are dealing with solid angles here, with units of square degrees or steradians. "180 degrees" does not represent the solid angle of a hemisphere.
    2. The radiated intensity follows a cos(θ) distribution. So at larger angles from the normal, the intensity is less.
     
  5. Aug 25, 2008 #4
    Oh I see I did not know about the cos(x) distribution that is very interesting. I was hoping on my incorrect thinking of even distribution that there would be a net transfer from the cold body to the hot (yes I know this is suppose to be impossible) But we do a lot of things today that 100 years ago seemed impossible. Cheers. Perhaps if the hot body was a star shape with the cold body in the middle away from the perpendicular radiation.
     
    Last edited: Aug 25, 2008
  6. Aug 25, 2008 #5

    russ_watters

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    Sorry, misread - thought it said 1550 for both.

    Anyway, you don't really need angles to do such a problem, you just need the Stefan-Boltzman equation: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/stefan.html
     
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