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I Prove that solid angle of any closed surface is 4pi

  1. Jul 18, 2017 #1
    I googled a lot on proof of Gauss theorem and nearly every other proof (on web and so on books) state that solid angle of closed surface is 4pi but I can't find the proof of this nowhere !

    I tried setting up the integral but don't know how to proceed furthur :
    Ω=∫(cosθ/r^2)*dA

    Also The one more thing which confuses me is http://mathworld.wolfram.com/SolidAngle.html proves solid angle of some other 3d shapes , those shapes also have close surfaces then why their solid angle is not equal to 4pi sr?
     
  2. jcsd
  3. Jul 18, 2017 #2

    Nidum

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    I think that these calculations are for the solid angle subtended by just one facet on the 3D shapes .

    Add the solid angles subtended by all the facets on one of the shapes together and it comes to 4pi .
     
  4. Jul 18, 2017 #3
    @Nidum : Uh my bad, but how shall one prove that for any closed surface solid angle is always 4pi sr ?
     
    Last edited: Jul 18, 2017
  5. Jul 18, 2017 #4

    Nidum

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    The mathematicians can help you with formal proof but intuitively you can see why it is true by considering the arbitrary shape to be inside a sphere and the central point of the sphere to be inside the arbitrary shape . I'll leave you to think about that .
     
  6. Jul 20, 2017 #5

    olivermsun

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    By solid angle of closed surface, do you mean the solid angle subtended/covered by a surface, similar to the arc length subtended by a (plane) angle?
     
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