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!

Energy in Electromagnetic fields

  1. May 2, 2009 #1
    Iv been trying to prove that the energy reisdes in the magnetic field in a good conductor and equally in both electric and magnetic for an insulator. My problem lies in the time averaging part of the problem...i cant seem to find out how they do it!

    The time averaging formula used is:

    For conductor:
    <1/2 [tex]\epsilon[/tex] E[tex]^{2}[/tex]>/<1/2 [tex]\mu[/tex] H[tex]^{2}[/tex]>

    is worked out to this:

    [tex]\epsilon[/tex]E[tex]^{2}[/tex]/[tex]\mu[/tex] H[tex]^{2}[/tex]

    The epsilons and mu's look like superscripts but there just multipled in!

    any thoughts?
  2. jcsd
  3. May 2, 2009 #2


    User Avatar
    Science Advisor
    Gold Member

    As I recall the energy density is dependent on the integral of the summation of the squares of the magnitudes of the electric and magnetic field, not the ratios. In a good conductor the imaginary part of the permittivity is very very large which would greatly decrease the contribution of electric field, leaving the magnetic contribution to dominate (if that is the case).
  4. May 2, 2009 #3


    User Avatar
    Science Advisor

    The ratio of [tex]E^2/H^2[/tex] in a good conductor is proportional to
    [tex]\omega\mu/\sigma[/tex]. (The \mu is the relative permeability.) The sigma in the denominator is why E^2 is negligible.
    The 1/2 in the time average of the squares is just the time average of sin^2(wt).
  5. May 2, 2009 #4


    User Avatar

    Staff: Mentor

    When you want to use LaTeX "inline", i.e. inside of text, use "itex" and "/itex" tags, not "tex" and "/tex". You might as well do the whole equation at once, while you're at it: [itex]\epsilon E^2 / \mu H^2[/itex] (click on an equation to see the code).
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Energy in Electromagnetic fields
  1. Electromagnetic field (Replies: 5)