Radiation Problem: Balancing Net Radiation of Two Spheres

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Discussion Overview

The discussion revolves around a problem involving two spheres, one inside the other, with differing temperatures and a vacuum in between. Participants explore the net radiation transfer between the spheres, both of which are considered black bodies. The conversation touches on concepts of radiation intensity, solid angles, and the implications of the Stefan-Boltzmann law.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant poses a question about calculating the net radiation between a small sphere and a larger sphere, both at different temperatures, and suggests a proportional relationship based on angles.
  • Another participant asserts that the net radiation transfer is zero, referencing the principle of black body radiation.
  • A third participant challenges the zero net transfer claim, arguing that differing temperatures should lead to a net energy transfer.
  • Concerns are raised about the initial assumption regarding angle calculations, specifically mentioning solid angles and the cosine distribution of radiated intensity.
  • A later reply reflects on the misunderstanding of the cosine distribution and expresses curiosity about the implications of a non-standard shape for the hot body.
  • One participant suggests that the problem can be approached using the Stefan-Boltzmann equation, indicating an alternative method to analyze the situation without relying on angles.

Areas of Agreement / Disagreement

Participants express disagreement regarding the net radiation transfer, with some asserting it is zero while others believe there should be a net transfer due to the temperature difference. The discussion remains unresolved with multiple competing views on the topic.

Contextual Notes

Participants highlight limitations in the initial assumptions about angle calculations and the distribution of radiation intensity, indicating a need for clarity on solid angles and the application of the Stefan-Boltzmann law.

philrainey
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I was trying to work out a question where there is a little sphere inside a big sphere with a perfect vacuum 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.
 
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The answer is zero. This is directly from the principle of black body radiation.
 
Russ, the spheres are at different temperatures. Shouldn't there be a net energy transfer (i.e. not zero)?

philrainey said:
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.

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

1. We are dealing with http://en.wikipedia.org/wiki/Solid_angle" 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.
 
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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.
 
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