Electric & Magnetic Fields at Point P with Dielectric/Magnetic Material

[yungman]

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

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

 

[granpa]

whats your question?

 

[yungman]

whats your question?

I just want to verify my assertion. Books are not very clear on this.

 

[granpa]

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?

 

[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.

 

[yungman]

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.

 

[yungman]

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.

 

[Meir Achuz]

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.

 

[DrDu]

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.

 

[yungman]

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