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


by yungman
Tags: electric, field, magnetic, material, point, presence
yungman
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#1
Nov16-10, 07:39 PM
P: 3,843
I want to confirm either the electric field at a point P in space in an [itex]\vec E_{ext} [/itex] field with the presence of a dielectric material is the sum of the [itex]\vec E_{ext} [/itex] and electric field [itex]\vec E_{p} [/itex] from the dielectric material due to polarization cause by the [itex]\vec E_{ext} [/itex].

And this is also true of the magnetic field at a point P in space with [itex]\vec B_{ext} [/itex] and magnetic material.

Thanks
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granpa
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#2
Nov17-10, 01:02 AM
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whats your question?
yungman
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#3
Nov17-10, 02:22 AM
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Quote Quote by granpa View Post
whats your question?
I just want to verify my assertion. Books are not very clear on this.

granpa
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#4
Nov17-10, 02:29 AM
P: 2,258

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


Quote Quote by yungman View Post
I want to confirm either the electric field at a point P in space in an [itex]\vec E_{ext} [/itex] field with the presence of a dielectric material is the sum of the [itex]\vec E_{ext} [/itex] and electric field [itex]\vec E_{p} [/itex] from the dielectric material due to polarization cause by the [itex]\vec E_{ext} [/itex]

And this is also true of the magnetic field at a point P in space with [itex]\vec B_{ext} [/itex] and magnetic material.

Thanks
you are asking if the field due to the polarization due to the external field itself causes secondary polarization?
DrDu
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#5
Nov17-10, 02:36 AM
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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
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#6
Nov17-10, 11:30 AM
P: 3,843
Quote Quote by granpa View Post
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 [itex] \vec E_{ext} + \vec E_{P} [/itex] where [itex] \vec E_{P} [/itex] 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
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#7
Nov18-10, 11:27 AM
P: 3,843
Quote Quote by DrDu View Post
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 [itex]\vec J_{free}[/itex] 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
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#8
Nov18-10, 10:05 PM
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Quote Quote by yungman View Post
I want to confirm either the electric field at a point P in space in an [itex]\vec E_{ext} [/itex] field with the presence of a dielectric material is the sum of the [itex]\vec E_{ext} [/itex] and electric field [itex]\vec E_{p} [/itex] from the dielectric material due to polarization cause by the [itex]\vec E_{ext} [/itex].

And this is also true of the magnetic field at a point P in space with [itex]\vec B_{ext} [/itex] 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
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#9
Nov19-10, 01:49 AM
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Quote Quote by yungman View Post
Thanks

Do you mean if P is at some conducting material where very small free current density [itex]\vec J_{free}[/itex] 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 [tex] \partial P/\partial t [/tex] term on the rhs of Ampere's law.
yungman
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#10
Nov19-10, 11:19 AM
P: 3,843
Quote Quote by DrDu View Post
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 [tex] \partial P/\partial t [/tex] 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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