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Energy carried by an EM wave

  • Thread starter Dart82
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  • #1
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Homework Statement

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?



Homework Equations


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



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:
 

Answers and Replies

  • #2
hage567
Homework Helper
1,509
2

Homework Statement

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?



Homework Equations


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



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:

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.
 
  • #3
57
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ohhhhh...that would seem to be a problem. thanks!
 

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