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Homework Help: Poynting Vector Energy Delivered

  1. May 11, 2009 #1
    1. The problem statement, all variables and given/known data

    If B0 = 5*10^-7 [T], relative permittivity = 2 and relative permeability = 1.15 how much energy is delivered to an area of 1m^2 by the wave in one second?

    Where B0 is a magnetic field measured in teslas.

    2. Relevant equations

    < P > = ((B0^2)*c*(1)) / (2*(vacuum permeability))

    (1 is the area given to me)

    3. The attempt at a solution

    My problem here is that I don't know how to use the relative permittivity and relative permeability. I've been googling for a few hours to try and find a relation between them and the poynting vector but i can't find anything at all.

    I know c = 1/root(vacuum permittivity*vacuum permeability) but that doesn't really help me.

    I'm pretty sure that's the equation I have to use but i'm thinking i need to *modify* it. My best guess is that maybe i should use c/n instead so the equation would be:

    < P > = ((B0^2)*c*(1)) / (2*(vacuum permeability)*n)

    n = root(relative permittivity * relative permeability)

    Can anyone confirm if this is right and if not how would you use the relative permittivity and relative permeability?


    edit: c = 1/root(vacuum permittivity*vacuum permeability) (forgot the 1 over originally whoops)
    Last edited: May 11, 2009
  2. jcsd
  3. May 11, 2009 #2


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    Homework Helper

    Close, but you missed one thing: when "converting" EM equations from vacuum to a material, you just convert the permittivity and permeability from their vacuum values to their in-material values everywhere they occur in the equation. So if you start with
    [tex]\langle P\rangle = \frac{B_0^2 c A}{2 \mu_0} = \frac{B_0^2 A}{2 \mu_0\sqrt{\mu_0\epsilon_0}}[/tex]
    then in the material, your equation becomes
    [tex]\langle P\rangle = \frac{B_0^2 A}{2 \mu\sqrt{\mu\epsilon}} = \frac{B_0^2 c A}{2 n \mu}[/tex]
    where [tex]\mu = 1.15\mu_0[/tex] and [tex]\epsilon = 2\epsilon_0[/tex].

    So basically I think you're right about inserting the factor of n, but you also need to add a factor to the permeability that appears in the denominator.
  4. May 11, 2009 #3
    Thanks so much! This was confusing me for so long but i get it now, you're a star ^_^
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