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Polarization

4 important questions on Polarization

What does alpha stand for in the tiny dipole moment in an atom? And what to do when the dipole is not just in one direction?

It stands for atomic polarizibility. Its value depends on the detailed structure of the atom in quetsion.

When it is not along an axis, you devide it in a perpendiculaar and a parallel part. When it is completely assymetrical , we ise the polarizability tensor whose values depend on the orientation of the axis you use

Was there always a dipole moment in the neutral atom?

No the the atom had no dipole moment to start with, it was induced by the appled field. Although, some molecules do have built-in, permanentdipole moments. Thes e molecules are called polar molecules.

What happens when polar molecules are placed in an electric field?

The force on the posotve end will exactly cancel the force on the negative end, however there will be a torque . THus the dipoe p in an unfirom field experiences a torque. N is in such a direction that p and E line up parallelt o eachother.

When the field is nonuniform. Then F- will not cancle F+ completely resulting in a net force on the dipol in addition to the torque. (however E must change rather abrublty for this to happenn o it is not a significant variation. ) see formula in picture.

So what happened to a piece of dielectric when p;aced in an electrica field?

When the substance cpnsist of neural atoms, the field wil induce in each tiny dipole moments, pointing in he smae direction as t he filed.

If the material is made up of polar molecules each permanent dipole will experience a torque, tending to line it up along the fled direction.

However thesemechanisms produce the same basic result: a lot fo little dipoles along the direction o the field. And thus the material becomes polarized.

The question on the page originate from the summary of the following study material:

Electrostatistcs

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