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Question about intensity and area

  1. Jul 30, 2008 #1
    Hi everyone. Firstly, I know that intensity= Power/Area. But this question made me my mind confused.

    The question shows the sun shining upon the Earth and the soln is:

    Only the hemisphere facing the Sun receives energy at any moment.
    power received by Earth
    = intensity × area of Earth facing the Sun

    And it shows the area of the Earth facing the sun is projected to a circle (pi R square, where R is the radius of the circle)!!!!!!! But shouldnt the area be the half the area of a Earth (sphere)????

    Anyone can help me? Im really confused. Thanks.
  2. jcsd
  3. Jul 30, 2008 #2


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    Science Advisor

    Yes, the area is a hemisphere but, when viewed from the Sun's perspective, it looks like a circle with the poles at the top and bottom, and the Equator across the middle.
  4. Jul 30, 2008 #3


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    Science Advisor

    Another way to think of it is that, strictly speaking, your equation "intensity= Power/Area" is ONLY correct if the area is perpendicular to the direction of the radiation.
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