# What force classically held electron in the orbit?

## Main Question or Discussion Point

Hey, what force classically held electron in the orbit? Also what force is it actually (quantum mechanics).

I think it was the electromagnetic force classically, I haven't been able to find a legit source that says it was directly that, but when I do calculations with electrons and atoms I'm often using the electric potential. And for quantum mechanics, I'm not really sure at all, just curious and want to know, if someone has some sources?

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Strictly speaking, there is no proper classical description of this system. But the only force you could actually think of classically is electromagnetic force, otherwise gravitational force is too weak and weak, strong interactions are purely quantum mechanical gauge theories that have no classical counterpart. However, the classical electromagnetism would cause problems due to radiative electrodynamics.

Quantum mechanically, we do not speak of forces in microscopic regime; we talk about potentials (or gauge particles in quantum fields). The electrons have bounded wavefunctions if the potential is larger than the total energy. For orbital electrons, the main contribution is electromagnetic potential. Again the gravity is too weak and length scale is too large for the nuclear interactions.

sophiecentaur
Gold Member
If you look up the Bohr Model (this link and many others) you will see an early Classical model, based on an actual charged particle in orbit around a nucleus - with the same equation as you will get for any non quantum system involving real forces. It doesn't work properly, though, because there are only certain, quantised energy levels ('orbit radii') that are found with atoms. The Shroedinger wave equation was the next step and that does a much better descriptive job.

DEvens
Gold Member
Depends on what you mean by "holds in orbit."

If you mean, what interactions does an electron in an atom have? Then the answer is electromagnetism is extremely close to the entire story. You can understand experiments exceedingly accurately using only E&M. And a finite sized nucleus, which arises from nuclear forces.

If you mean, why does the electron not drop out of orbit and cozy up to the positive charge in the proton, that's a different matter. It's not exactly an interaction. It's more to do with the fact that various quantum numbers that are conserved could not be conserved if that happened. It's more a lack of interaction that would be required for such a thing to move forward.

Thank you everyone, you have answered my question.
I appreciate it!