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Homework Help: EM energy

  1. Apr 1, 2009 #1
    1. The problem statement, all variables and given/known data

    A certain star is 13 million light-years from Earth. The intensity of the light that reaches Earth from the star is 6 10-21 W/m2. At what rate does the star radiate EM energy?

    2. Relevant equations

    i know you have to convert lightyears to m. (1.23e17m) multiply this by 2 and multiply again by 4pi. Do you just multiply that answer by the intensity to get the answer?

    3. The attempt at a solution
  2. jcsd
  3. Apr 1, 2009 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Almost. You're correct except for the "multiply this by 2" part. Instead of multiplying by 2, you need the square of the distance to the star. (A sphere's surface area is 4πr2, not 2r·4π)
  4. Apr 1, 2009 #3
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