1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    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!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Energy carried by an EM wave
  1. Energy from an EM Wave (Replies: 3)