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: 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

    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    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

    User Avatar
    Homework Helper
    Gold Member

    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:
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted