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Today a colleague and I had a heated debate over whether, when an external electric field is applied across a dielectric, the molecules in the dielectric actually move/rotate (flip to align with the electric field) in any way or whether they stay fixed in place and only the electron density is redistributed.

I believe I remember being taught in school that polar molecules will align themselves with the electric field, and that the rotational inertia (or maybe friction?) of the molecules is what causes the dielectric to "slowly" become polarized over time, with some time constant.

My colleague however completely refuted that idea, saying that the molecules don't actually move it is only the electron density that is redistributed.

The "molecules-flipping" explanation seems very much based on classical-mechanics and so I wouldn't be surprised if quantum-mechanics has something else to say about this.

If my old school-explanation is correct (I.e. that polar molecules flip to align with the electric field) then does that also happen when the dielectric is a single crystalline solid? or when the dielectric is a polycrystalline?

I am sure that the answer is out there but I wasn't able to find it, perhaps because I don't know the right words to search for. Sorry for wasting your time if this is a stupid question, but I appreciate any answers that can further my understanding.

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Most ceramic dielectrics have molecular subunits (unit cells) which are held in position with covalent bonds. This restricts those molecular subunits (SiO(x), for example) from physically rotating under the influence of an applied external electric field, to align themselves with it. However, externally-applied stresses will deform the unit cells by a very small amount and a net voltage will appear across the sample- without the unit cells themselves swiveling around.

Similarly, the application of a voltage across the sample will cause a tiny deformation of the unit cells, causing the whole sample to deform by a tiny amount- again, without any rotation of the unit cells.

The situation is different for polymer dielectrics where adjacent polymer chains are not cross-linked with directional bonds. In this case, the molecular subunits can flex in position slightly under the influence of an external field. In fact, for these materials it is possible to freeze the charge distribution in place by warming up the dielectric, applying an external field, and then cooling the dielectric down while maintaining the field. This yields a poled dielectric.

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    $\begingroup$ Piezoelectric materials certainly deform under electric fields as the unit cell deforms or part of the basis shifts position in the cell. $\endgroup$
    – Jon Custer
    Commented Sep 14, 2023 at 19:36
  • $\begingroup$ @jon custer, sorry, will edit. $\endgroup$ Commented Sep 14, 2023 at 20:44

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