Poynting vector in static electromagnetic field

  1. zql

    zql 5

    There is a situation, we have an electric field and a magnetic field, both are static. And we know the density of energy is u=E·D/2+B·H/2, so dU/dt=0, but Poynting vector S=ExH is not zero, which means energy is flowing. This confused me. Static field also has energy flux?
  2. jcsd
  3. DrDu

    DrDu 4,635
    Science Advisor

    Yes, correct. Look also for "hidden momentum".
  4. zql

    zql 5

    can you give me more details?thanks.
  5. clem

    clem 1,276
    Science Advisor

    The static magnetic field is produced by a constant electric current. That means there is resistance, and energy is flowing into matter. "Hidden" momentum is not involved.
  6. Andy Resnick

    Andy Resnick 6,141
    Science Advisor
    Education Advisor

    The relevant expression, from the conservation of energy, is:

    [tex]\frac{\partial u}{\partial t} + \nabla \bullet S = -J \bullet E[/tex].

    Does this help?
  7. zql

    zql 5

    I think I got it, thanks.
  8. zql

    zql 5

    And how about "hidden momentum"? I didn't know about this.
  9. DrDu

    DrDu 4,635
    Science Advisor

    Probably the best source on these matters is still Jackson's book on Electrodynamics.
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