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EM plane wave question

  1. Apr 20, 2009 #1
    This isn't a homework question so much as a general question about the equation I see used in the textbook:

    The E field of an EM wave is defined as E(x, y, z, t) = E0sin(kx - wt)y_hat and the B field of an EM wave is defined as B(x, y, z, t) = B0sin(kx - wt)z_hat.

    Why is there a minus sign in front of the wt term? I understand how kx (the offset in space) is positive; why wouldn't the time offset (wt) also be positive in this equation?

    Thanks!

    -Max
     
  2. jcsd
  3. Apr 20, 2009 #2

    Redbelly98

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    The minus sign just means the wave is traveling in the +x direction.

    You could have it be +wt. In that case the wave travels in the -x direction.
     
  4. Apr 20, 2009 #3
    I see... But, why is a +x direction a -wt term?
     
  5. Apr 20, 2009 #4
    At t = 0
    . f(k). ..........................................

    At t = t
    ..................... f(k-wt) ..................

    Where f(k) is a pulse

    see f(x-5) goes towards positive side and f (x+5) is at negative side or you can use the word "shift". This can understood good if you use one simple pulse rather than a complex sinusoidal function
     
  6. Apr 20, 2009 #5

    Redbelly98

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    Hmmm. Well, let me ask a somewhat more basic question. Do you understand why a graph of the function f(x-1) would look the same as f(x), except that it would be shifted by 1 unit in the +x direction?
     
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