1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Use Poynting's Theorem to show E and B fields are out of phase

  1. May 9, 2009 #1
    1. The problem statement, all variables and given/known data
    In a Fabry-perot interferometer, light is reflected back and forth between 2 highly reflecting parallel mirrors, with a nonconducting medium inside. The waves of magnetic and electric field are 90o out of phase, unlike the case of a wave in free space where they in phase. Develop an argument using Poynting's theorem for why this should be so.

    2. Relevant equations

    3. The attempt at a solution
    I've thought of using E to calculate H and then say that they're out of phase, but then this method isn't based on Poynting's theorem.
    Then I've thought of using the eq.:
    E = E0exp((-kapper)z+i(kz-wt))
    B = B0exp((-kapper)z+i(kz-wt+theta))
    to calculate the Poynting vector, S = 1/mu * (E x B), then somehow show that theta is 90o, but then I don't really know how to do it.
  2. jcsd
  3. May 9, 2009 #2


    User Avatar
    Homework Helper

    I was under the impression that Poynting's theorem was the law of conservation of electromagnetic energy,
    [tex]\vec{J}\cdot\vec{E} + \frac{1}{2}\frac{\partial}{\partial t}\left(\epsilon_0 E^2 + \frac{1}{\mu_0}B^2\right) + \frac{1}{\mu_0}\vec{\nabla}\cdot(\vec{E}\times\vec{B}) = 0[/tex]

    Does that give you any ideas?
  4. May 11, 2009 #3
    Thanks. I just realised that I was in the wrong direction from the start, I've just now finished the argument using the law of conservation of electromagnetic energy instead.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook