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Wave amplitude

  1. Mar 5, 2012 #1
    1. The problem statement, all variables and given/known data
    from a line source, if amplitude of a wave at a distance 'r' is'a'.then the amplitude at a distance '4r' will be what?







    3. The attempt at a solution
    i could not understand the pattern of the spreading of the wave from the line source.
     
  2. jcsd
  3. Mar 5, 2012 #2

    tiny-tim

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    hi altamashghazi! :smile:
    the line is a cylinder

    the wave-fronts spread out cylindrically :wink:
     
  4. Mar 5, 2012 #3
    what would be change in amplitude with distance. will it obey inverse square law?
     
  5. Mar 5, 2012 #4

    tiny-tim

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    draw it!! (from above) :rolleyes:

    what happens to the field lines? :wink:
     
  6. Mar 12, 2012 #5
    i think they obey this law. am i right?
     
  7. Mar 12, 2012 #6

    tiny-tim

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    no!

    the line is a cylinder

    the wave-fronts spread out cylindrically ​

    draw it!! (from above) …

    what happens to the field lines?​
     
  8. Mar 12, 2012 #7
    wave fronts will be elliptical. then also it will follow inverse square law. bcoz area of ellipse is
    π*a*b
     
  9. Mar 12, 2012 #8

    tiny-tim

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    why?? :confused:
     
  10. Mar 12, 2012 #9
    sorry wavefronts will be cylinderical out wards but i couldn't guess the pattern of change in amplitude.
     
  11. Mar 12, 2012 #10

    tiny-tim

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    yes! :smile:

    ok, then how is the surface area of a cylinder related to radius? :wink:
     
  12. Mar 12, 2012 #11
    it is directly proportional to r.
    A=2πr(h+r)
     
  13. Mar 12, 2012 #12

    tiny-tim

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    ok :smile:, so if you draw the field lines, they'll all be radially outward,

    and the number going through any wavefront will be proportional to r …

    so what is the loss in strength?​
     
  14. Mar 12, 2012 #13
    it will be inversely proportional to r.
    i mean answer should be a/4
     
  15. Mar 12, 2012 #14

    tiny-tim

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  16. Mar 12, 2012 #15
    but its wrong the answer is a/2. therefore i m confused. pls help.
     
  17. Mar 12, 2012 #16

    tiny-tim

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    oh, i'm sorry, i thought a was the intensity :redface:

    the intensity (as shown by field lines) is proportional to 1/r,

    and the amplitude is proportional to the square-root of the intensity :smile:
     
  18. Mar 12, 2012 #17
    thank u very mucch
     
  19. Mar 12, 2012 #18
    thank u very much
     
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