confuse about linear dielectric

athosanian

hello, I am learning electromagnetism and read the polarization in dielectric. The text book tell me that in linear dielectric exists a simple relation:
P=[itex]\chi[/itex]*ε₀*E
where [itex]\chi[/itex] is the susceptibility, ε₀ is the permittivity of vacuum and E is total electric field in the dielectric. It is noted that E is the total field, namely, applied external field E_0 plus depolarization field E_d in the media.
But when I search this topic on the wiki, I find it says polarization is proportional to the applied external field (link below). So I have a confuse about meaning of E, which is right? I hope any help. Thanks.

http://en.wikipedia.org/wiki/Ferroelectricity

DrDu

Your text book is right.
I suppose you refer to the following phrase:
"When most materials are polarized, the polarization induced, P, is almost exactly proportional to the applied external electric field E"
This statement is also true, however, the constant of proportionality will vary with position and depend on the geometry of the sample and external field. Consider for example a charge in front of a half plane of some dielectric. The true electric field lines will break on entering the material while the external field lines radiate unbroken from the charge. Nevertheless both E and E_ext will be proportional to the charge q.

athosanian

Thanks! Drdu, I see. Because E is proportional to E_ext, P is proportional to both E and E_ext.