# Electric or magnetic field at point P in presence of dielectric/magnetic material.

by yungman
Tags: electric, field, magnetic, material, point, presence
 P: 3,898 I want to confirm either the electric field at a point P in space in an $\vec E_{ext}$ field with the presence of a dielectric material is the sum of the $\vec E_{ext}$ and electric field $\vec E_{p}$ from the dielectric material due to polarization cause by the $\vec E_{ext}$. And this is also true of the magnetic field at a point P in space with $\vec B_{ext}$ and magnetic material. Thanks
P: 3,898
 Quote by granpa whats your question?
I just want to verify my assertion. Books are not very clear on this.

P: 2,258
Electric or magnetic field at point P in presence of dielectric/magnetic material.

 Quote by yungman I want to confirm either the electric field at a point P in space in an $\vec E_{ext}$ field with the presence of a dielectric material is the sum of the $\vec E_{ext}$ and electric field $\vec E_{p}$ from the dielectric material due to polarization cause by the $\vec E_{ext}$ And this is also true of the magnetic field at a point P in space with $\vec B_{ext}$ and magnetic material. Thanks
you are asking if the field due to the polarization due to the external field itself causes secondary polarization?
 Sci Advisor P: 3,593 Yes, yungman, that's true, at least for the electric field and/ or in magnetostatics. In full electrodynamics, there is also a contribution of the change of P to the magnetic field. In fact, in optics one sets often M=0, so that all effects of the medium are due to P alone.
P: 3,898
 Quote by granpa you are asking if the field due to the polarization due to the external field itself causes secondary polarization?
No, I just want to verify that the field experienced at a point P with a piece of dielectric material close by, is the sum of the $\vec E_{ext} + \vec E_{P}$ where $\vec E_{P}$ is the field from polarizing of the dielectric sitting somewhere in space( somewhere close to P but not touching P).

Same as in the case of magnetic with a piece of magnetic material close by.
P: 3,898
 Quote by DrDu Yes, yungman, that's true, at least for the electric field and/ or in magnetostatics. In full electrodynamics, there is also a contribution of the change of P to the magnetic field. In fact, in optics one sets often M=0, so that all effects of the medium are due to P alone.
Thanks

Do you mean if P is at some conducting material where very small free current density $\vec J_{free}$ created by even static magnetic field. But if P is just a point in space ( empty space) there should be no more changes.
HW Helper
PF Gold
P: 2,015
 Quote by yungman I want to confirm either the electric field at a point P in space in an $\vec E_{ext}$ field with the presence of a dielectric material is the sum of the $\vec E_{ext}$ and electric field $\vec E_{p}$ from the dielectric material due to polarization cause by the $\vec E_{ext}$. And this is also true of the magnetic field at a point P in space with $\vec B_{ext}$ and magnetic material. Thanks
Yes to both cases, but the polarization need not be "caused" by the field. In ferro cases, there mayi not even be an external field.
P: 3,593
 Quote by yungman Thanks Do you mean if P is at some conducting material where very small free current density $\vec J_{free}$ created by even static magnetic field. But if P is just a point in space ( empty space) there should be no more changes.
I don't know what you mean with P being just a point in space. In the simplest cases, P corresponds to a dipole density and, e.g. a rotating dipole will lead to a magnetic field. Hence the $$\partial P/\partial t$$ term on the rhs of Ampere's law.
P: 3,898
 Quote by DrDu I don't know what you mean with P being just a point in space. In the simplest cases, P corresponds to a dipole density and, e.g. a rotating dipole will lead to a magnetic field. Hence the $$\partial P/\partial t$$ term on the rhs of Ampere's law.
What I meant P is just a reference point in space. It is like books always talk about fields experienced at a point from a source some distance away.

Anyway, thanks for all the replies to confirm my understanding.

Alan

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