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EM plane wave question

  • Thread starter maxsthekat
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  • #1
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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
 

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  • #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.
 
  • #3
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I see... But, why is a +x direction a -wt term?
 
  • #4
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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
 
  • #5
Redbelly98
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I see... But, why is a +x direction a -wt term?
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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