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Question about electromagnetic waves?

  1. Oct 15, 2011 #1
    Are the amplitudes of the electric and magnetic components of an electromagnetic wave proportional?
    Or is the amplitude of the electric portion unrelated to that of the magnetic portion?
  2. jcsd
  3. Oct 16, 2011 #2
    E=cB is the relationship that holds between amplitudes, where E is the amplitude of the electric component and B that of the magnetic component. c is the speed of light.
    If you want a bit more read the last 3 paragraphs of this wikipedia section http://en.wikipedia.org/wiki/Electromagnetic_wave#Derivation
  4. Oct 16, 2011 #3


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    Not quite accurate, actually. The E and H fields are related by

    Z0 = E/H
    where Z0 is the Impedance of free space (approx 377Ω)


    E/B = Z00

    I don't think your formula can be obtained by rearranging the above / substituting quantities.

    http://en.wikipedia.org/wiki/Impedance_of_free_space" [Broken] link shows it in more detail
    Last edited by a moderator: May 5, 2017
  5. Oct 16, 2011 #4
    I think you are contradicting yourself here. In the link you give and in the derivation in the section "Relation to other constants" it uses the fact that E/B=c. Z00 equals the speed of light in vacuum!!! (as read in section "exact value" in the link you give).
    Last edited by a moderator: May 5, 2017
  6. Oct 16, 2011 #5


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    What a plonker! It's right in there, amongst all the other ratios.
    Removes foot from mouth.

    otoh, the concept of Free Space Impedance is, interestingly, also related to the ratio of fields. I'd have got away with it if I hadn't actually said you were wrong.
  7. Oct 17, 2011 #6
    This is news for me, i corrected a science advisor of Physics Forums!!!( Ok ok you werent clearly wrong but rather self contradicting).
    I think probably you didnt read at all the wiki link i gave, it eplains it all there and the only extra assumption made to derive the amplitude relation E=cB is that the direction of the E-field is constant that is the wave is linearly polarized.
    Last edited: Oct 17, 2011
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