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Homework Help: Electromagnetic Wave

  1. Apr 10, 2007 #1
    1. The problem statement, all variables and given/known data
    A plane electromagnetic wave, which is polarized in the positive y-direction, propagates in the negative z-direction. What is the direction of the magnetic field of this wave?

    2. Relevant equations
    My choices are:

    +z direction.
    -y direction.
    -x direction.
    +y direction.
    +x direction.
    -z direction.

    3. The attempt at a solution

    I don't understand polarization much. Also, from what I know about electromagnetic waves the magnetic and electric fields point in two directions so I can't just answer one answer it would have to be two of the choices above but I don't have that option.

    Please Help! I Would really appreciate it! It's due tomorrow!!!
  2. jcsd
  3. Apr 10, 2007 #2


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    Homework Helper
    Gold Member

    HINT: Magnetic field components of a E-M wave, will be at right angles to the electric field components.
  4. Apr 11, 2007 #3
    So, from what I can gather, the answer is +x or -x, it can be either one can't it?
  5. Apr 11, 2007 #4
    Nevermind. I got it and it was positive x. How is that so, though? I swear it can be both positive and negative x. I even drew it out!
  6. Apr 11, 2007 #5


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    Gold Member

    The equation that connects the direction of travel (k) and the directions of the E and B fields is -

    [tex] \vec{k} = \vec{E} \times \vec{B} [/tex]

    That's a vector cross product - see here http://mathworld.wolfram.com/CrossProduct.html
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