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Homework Help: Energy carried by an EM wave

  1. Mar 13, 2007 #1
    1. The problem statement, all variables and given/known data energy density in a laser beam is 9.5x10^-8 J/m cubed. the laser beam is cylindrical, and it delivers a power of 15W to a certain area. What is the radius of this circular area?



    2. Relevant equations
    Intensity :
    S=cu and S=Power/Area
    Area of a circle (or possibly the area of a cylinder?)



    3. The attempt at a solution
    first i find the intensity by multiplying the speed of light by the energy density in the laser: (2.994x10^8)x(9.5x10^-8)=28.44
    Then i find the Area using S=P/A: 28.44/15W=1.89m
    Using the area of a circle: 1.89=pi*radius^2. According to my calculations, my radius would be .78 meters. The correct answer is supposed to be 0.41 meters. Can someone please tell me where i am screwing this one up:mad:
     
  2. jcsd
  3. Mar 13, 2007 #2

    hage567

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    Homework Helper


    You made a math error. You accidently calculated S/P for the area, when it should be P/S. So it should be 15W/28.44 instead of what you have.
     
  4. Mar 13, 2007 #3
    ohhhhh...that would seem to be a problem. thanks!
     
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