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Homework Help: Very basic Inverse square law problem

  1. Mar 20, 2008 #1
    The intensity of solar energy from the sun arriving at the earth's surface is aproximatly 1400 joules, per square meter, per second.

    The sun is 1.5 x 10^11m away from earth. At what rate is the sun radiating energy? per meter squared on its surface.

    The soloution is
    1400 = E/(1.5* 10^11)^2

    and that becomes 1400 / (1.5*10^11)^2 = E

    THe inverse square law is I = 1/d^2. However this equation replaces 1 with the E. Could someone explain why this is done?
  2. jcsd
  3. Mar 20, 2008 #2


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    Gold Member

    This is not right. Can't you see that the units are wrong ? The correct expression is I = A/d^2 where A has the appropriate dimensions.
  4. Mar 20, 2008 #3
    oh, umm... what do you mean the units are wrong? sorry if the question is completely childish, i'm new to physics.

    Would this question be better answered in the HW "introductory physics" forum?
  5. Mar 20, 2008 #4
    Dw i got it
  6. Mar 20, 2008 #5
    i thick it comes from the two sphere surface area ratio,
    one is the big ball centering the sun and touching the earth, the other is the solar surface itself.
  7. Mar 20, 2008 #6
    true, the surface area of a sphere is given by 4pi(r)^2. And so, the surface area is related to the distance from the center of the sphere (the radius).
  8. Mar 21, 2008 #7
    Inverse Square Law, General

    Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law. This comes from strictly geometrical considerations. The intensity of the influence at any given radius r is the source strength divided by the area of the sphere. Being strictly geometric in its origin, the inverse square law applies to diverse phenomena. Point sources of gravitational force, electric field, light, sound or radiation obey the inverse square law. It is a subject of continuing debate with a source such as a skunk on top of a flag pole; will it's smell drop off according to the inverse square law?

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