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Why are P/M and D/H defined oppositely in Electromagnetism

  1. May 18, 2015 #1
    The definitions of D and H are:

    ##D=\epsilon_0 E+P##
    ##H=B/\mu_0-M##


    ##P=\epsilon_0 \chi E##
    ##M=\chi H##

    I was wondering, if E and B are the fundamental field relating to all charges/currents, why is the definition of the polarisation the opposite for each of them? So why is H in the definition of M and not B, when B is the actual physical field.

    Thanks
     
  2. jcsd
  3. May 18, 2015 #2

    TSny

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    You can trace the difference in sign to the relations

    ##\vec{\nabla} \cdot \vec{P} = -\rho_{bound}## and ##\vec{\nabla} \times \vec{M} = +\vec{J}_{bound}##

    where ##\vec{P}## and ##\vec{M}## are electric and magnetic polarization vectors.
     
  4. May 19, 2015 #3

    rude man

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    I would venture that H is more fundamental than B, in the sense that B is H modified by magnetic material, just as D is E modified by dielectric material.
    E.g. you have a solenoid with current thru it: the B field is one thing if the core is air and another if the core is iron. But H does not change. Ampere's law is most simply stated as ∫H ds = I.

    But that's just a venture. :smile:
     
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