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Similarities and analogies between the $E, P$ and $D$ fields with the $B, M$ and $H$ fields and their limitations

Physics Asked on July 11, 2021

In the course of my learning electromagnetism, I’ve noticed there are a striking amount of symmetries in electrostatics and magnetostatics, almost down to replacing divergence operators with curl operators. For instance,

$$vec P = epsilon vec E$$
$$vec H = mu vec B$$

For linear media, and

$$vec D = epsilon_0 vec E + vec P$$
$$vec B = mu_0 (vec H + vec M)$$

And where $vec H$ is defined, at least in Griffiths, in a completely analogous way to electric displacement, save for the typical curl operator that is usual for magnetostatics and a current density instead of charge density.

I am tempted to say that the effects of polarization and magnetization are totally analogous, that “$vec H$ is basically the magnetostatic equivalent of $vec D$” since I have far more trouble visualizing magnetization than I do polarization so if I can get away with thinking this way it’d make my learning easier I think. Do I have it wrong? Am I justified in thinking things in terms of analogizing from polarization? Am I oversimplifying things massively? Be as pedantic as you’d like.

One Answer

$vec{H}$ is basically the magnetostatic equivalent of $vec{D}$” since I have far more trouble visualizing magnetization than I do polarization so if I can get away with thinking this way it’d make my learning easier I think. Do I have it wrong?

All these analogies are limited and context-dependent.

Sometimes $mathbf H$ is analogous to $mathbf E$, since in static scenario they both obey Coulomb's law, have similar equations, similar "lines of force"; bar magnet' H field is similar to electrically polarized bar's E field.

But sometimes $mathbf B$ is analogous to $mathbf E$, for example in diamagnetics (copper, bismuth), the magnetized medium decreases $mathbf B$ field inside, just as polarized dielectric decreases $mathbf E$ inside.

There is no generally valid analogy, these are 4 different quantities with unique properties.

Answered by Ján Lalinský on July 11, 2021

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