Why dielectrics get polarized instead of directly ionized?

[torito_verdejo]

From what I could understand, without external perturbation the nucleus and its cloud of electrons are in an energetic balance. Nevertheless, when they are put in an electric field, the nucleus moves in its direction and the cloud in the opposite. Now, this electric field apart from a torsion produces a polarisation. If we simplify poles as point charges with equal magnitude but opposite side, ##q## and ##-q##, then the bigger the field, the bigger the gap between the point charges. My question, maybe stupid, is why don't they simply snap?

Coulomb law implies that the force mutually exerted by the positive pole and the negative pole of the atom is smaller the greater is the distance between them. How then does the force between ##q## and ##-q## increase in order to stay in balance, even now that the poles are at higher distances?

My question is actually very fundamental. I have the impression that it is comparable as asking why the normal force is proportional to the weight, but I'm not even able to tell. I'd be great if you could, apart from answering my question, tell me what's wrong with my reasoning.

 

[anorlunda]

Summary:: Why, since the electric force is weaker the further the nucleus is from the electron, do atoms get polarized without actually getting ionized?

Coulomb law implies that the force mutually exerted by the positive pole and the negative pole of the atom is smaller the greater is the distance between them. How then does the force between qqq and −q−q-q increase in order to stay in balance, even now that the poles are at higher distances?

You can't evaluate an atom using classical physics. It requires quantum mechanics.

 

[torito_verdejo]

You can't evaluate an atom using classical physics. It requires quantum mechanics.

I can understand that, but couldn't you explain a bit more how the force between the electron cloud and the nucleus increases to compensate the polarization?

 

[Dale]

Summary:: Why, since the electric force is weaker the further the nucleus is from the electron, do atoms get polarized without actually getting ionized?

Nevertheless, when they are put in an electric field, the nucleus moves in its direction and the cloud in the opposite.

This is not the effect that matters for dielectrics. A dielectric has a molecule (not an atom) which is polar, meaning that one part of the molecule is positively charged and another part is negatively charged. The presence of an external electric field aligns the molecule with the field.

My question, maybe stupid, is why don't they simply snap?

They can snap. That molecular bond has a certain strength, it is related to the binding energy. If the field strength is lower than the binding energy then it will stay intact, if it is larger then the bond will break.

 

[torito_verdejo]

They can snap. That molecular bond has a certain strength, it is related to the binding energy. If the field strength is lower than the binding energy then it will stay intact, if it is larger then the bond will break.

What I can´'t get is why they can get a little apart without doing it totally. And I´'m talking about atoms. How does the force increase between electrons and the nucleus in order to compensate the increase distance between them.

 

[Dale]

And I´'m talking about atoms.

That is not relevant to dielectrics. However, a similar principle applies:

There is a certain atomic binding energy called the ionization energy. If the electric field is stronger than that binding energy then you get ionization. If the electric field is weaker then you do not get ionization.

 

[Lord Jestocost]

How then does the force between qq and −q−q increase in order to stay in balance, even now that the poles are at higher distances?

This is a wrong picture. Regarding electronic polarizability, you assume that the electron cloud is a uniformly charged sphere and the nucleus is a point charge. You then have to calculate the force between the point charge which is shifted with respect to the center of a uniformly charged sphere around it.
https://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_3/backbone/r3_2_2.html

You have to consider the electrical field inside a sphere of uniform electrical charge.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elesph.html

 

[torito_verdejo]

This is a wrong picture. Regarding electronic polarizability, you assume that the electron cloud is a uniformly charged sphere and the nucleus is a point charge. You then have to calculate the force between the point charge which is shifted with respect to the center of a uniformly charged sphere around it.
https://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_3/backbone/r3_2_2.html

OK, I was thinking this in a very stupid way. I see that you cannot treat the problem as two point particles. When you represent the problem by a charged ball inside of which is a positive particle, the bigger the distance from the center, the stronger the electric field.

 

[Mister T]

You can't evaluate an atom using classical physics. It requires quantum mechanics.

In the early 1990's I attended a series of lectures on atomic physics given by Dave Church, who later moved on to Lawrence Livermore to build one of the first atom traps. He was quite fond of announcing that one can do a lot of atomic physics without quantum mechanics or relativity. He would tell us that there was not a single ##h## or ##c## appearing in the equations he happened to be using for those lectures.

 

[vanhees71]

One should also note that a dielectric is a "dielectric" only if the external fields are not too large. Particularly when you use the usual linear constitutive relations (like ##\vec{E}=\epsilon \vec{D}## and the like) they are only valid for the case that the external electromagnetic field is much smaller than the intrinsic fields holding the matter together. The reason, why this "linear-response approximation" works so well is that these intrinsic fields are quite strong at the location of the charged particles (electrons and atomic nuclei), because the microscopic length scales are pretty small compared to the distance to the sources (charges and currents) making up the external field.

If you make the external field strong enought, first the "linear-response approximation" breaks down and you have to go to higher orders (leading among other things to the fascinating subject of "non-linear optics"). If you make them even stronger, you can indeed break up the dielectric, which then becomes a conductor and may even "vapourize" finally to a plasma ;-)).

 

[Lord Jestocost]

This is not the effect that matters for dielectrics. A dielectric has a molecule (not an atom) which is polar, meaning that one part of the molecule is positively charged and another part is negatively charged. The presence of an external electric field aligns the molecule with the field.

With all due respect, orientational polarization is only one type of polarization mechanism in dielectrics.
There are other types of microscopic polarization mechanisms:

Electronic polarization
Ionic polarization
Interface or space-charge polarization (in case there are mobile ions)

 

[Dale]

With all due respect, orientational polarization is only one type of polarization mechanism in dielectrics.
There are other types of microscopic polarization mechanisms:

Electronic polarization
Ionic polarization
Interface or space-charge polarization (in case there are mobile ions)

Yes, very good point. Maybe expand on those different mechanisms for the OP?

 

[Ibix]

If you make the external field strong enought, first the "linear-response approximation" breaks down and you have to go to higher orders (leading among other things to the fascinating subject of "non-linear optics"). If you make them even stronger, you can indeed break up the dielectric, which then becomes a conductor and may even "vapourize" finally to a plasma ;-)).

A non-linear optics guy showed me a non-linear crystal he, personally, had ruined by running slightly too high a laser power through it. Its non-linearity in this case manifested in self-focussing behaviour, and it had focussed his slightly over-power beam to a hair thickness that had waaay exceeded the energy density the crystal could handle, leaving a beautiful straight white line right through his otherwise clear crystal.

 

[Dr_Nate]

Summary:: Why, since the electric force is weaker the further the nucleus is from the electron, do atoms get polarized without actually getting ionized?

From what I could understand, without external perturbation the nucleus and its cloud of electrons are in an energetic balance. Nevertheless, when they are put in an electric field, the nucleus moves in its direction and the cloud in the opposite. Now, this electric field apart from a torsion produces a polarisation. If we simplify poles as point charges with equal magnitude but opposite side, ##q## and ##-q##, then the bigger the field, the bigger the gap between the point charges. My question, maybe stupid, is why don't they simply snap?

I think the problem lies with your classical model. If you instead consider an electron cloud, then as one portion gets farther from the nucleus another portion gets closer, thus it is possible for it to stay bound to the nucleus.