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Homework Help: Power Output and Intensity

  1. Dec 8, 2008 #1
    1. The problem statement, all variables and given/known data

    The Sun emits electromagnetic waves (including light) equally in all directions. The intensity of the waves at the Earth's upper atmosphere is 1.5 kW/m2. At what rate does the Sun emit electromagnetic waves? (In other words, what is the power output?)

    The answer is supposed to be in Watts, not kW.

    2. Relevant equations

    Surface area of sphere X Intensity = Power

    3. The attempt at a solution

    The surface area for a sphere = 4[tex]\pi[/tex]r2. The radius of the sun in meters is 695,500,000 m.

    The given intensity is 1.5 kW, which I converted to 1500 W.

    4[tex]\pi[/tex](695500000)2(1500) = Power output
    9.1e21 = Power output

    But this answer is coming up as incorrect. I'm not sure how to go about this.
  2. jcsd
  3. Dec 8, 2008 #2


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    The question gives you the power at the distance of the earth - what do you want to use for the radius ?
  4. Dec 8, 2008 #3
    I already tried to radius of the sun, so the radius of the Earth perhaps?

    The radius of the Earth = (6,378,100 meters).
  5. Dec 8, 2008 #4


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    You have a lightbulb at the centre of a sphere - you are working out the power at the edge of the sphere.

    Now picture the solar system; the sun is at the centre - at what distance is the earth?
  6. Dec 8, 2008 #5
    Would r be the distance from the sun to the earth?
  7. Dec 8, 2008 #6


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    Like maybe 150*106 km?

    That might work.
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