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Q & A: Bohr atom and radiation

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Most recent answer: 09/26/2013
Q:
According to Bohr, electrons do not emit energy when they move in the orbits, but how did he postulate so? And doesnt this defy the universal law that "charged particles lose energy during motion"
- Hullas (age 16)
New Delhi, India
A:

Since every charged particle is "moving" in most reference frames, it wouldn't make sense for motion itself to cause radiation. The "universal law" describes the radiation from accelerating charged particles.

At any rate, an orbiting Bohr electron would be also accelerating, so the problem you raise is important.  Bohr had no theoretical way of making sense of his postulate. He just invented it to match up with the fact that the low-energy atom states last forever. In other words, he had the courage to make a sort of illogical, incoherent mixed theory, which may have been been a necessary step toward a more coherent theory.

In the more modern theory, the lowest-energy electron state is not changing in time, so there's no reason to radiate. The state is always spread out spatially, rather than being the average of some little dot dashing about to different positions.

Mike W.


(published on 07/22/2013)

Follow-Up #1: radiation from electrons

Q:
Firstly, I didn't understand that why does a charged particle radiate energy when it is being "accelerated"? They should simply continue their motion and because of the Coloumbic forces, the path should be circular (as celestial bodies have). Secondly, according to the law, why doesn't an electron radiate energy? (I didn't understand what you meant by "In the more modern theory, the lowest-energy electron state is not changing in time, so there's no reason to radiate").
- Hullas (age 16)
New Delhi, India
A:

The basic argument for why accelerating charged particles radiate is given in Purcell's book on Electricity and Magnetism. Unlike for uniformly moving particles, one cannot pick a special-relativistic reference frame in which the accelerated particle is stationary. The field lines at a distance point toward where the particle would have been if its motion hadn't changed. The closer field lines point toward where it is. Continuity of the field requires a transverse component, and this is the radiating field. 

On the quantum picture, the change from what you have in mind is larger. Electrons (and other particles) are not dots at particular places. They are spread-out quantum states. For a hydrogen atom, to pick a simple example, that quantum state keeps the same shape and position around the nucleus. It doesn't go anywhere at all.

You might have a look at this earlier answer: 

Mike W.


(published on 07/23/2013)

Follow-Up #2: radiation from quantum atoms

Q:
hi,we read in physics elcetromagnetic that a electron when have a Acceleration it should emit electromagnetic waves,while in atom electron dont emit wave ??if dont emit cant i talk elctromagnetic is wrong!why in mechanic quantom electron should not emit wave?if it is true,new science how to able Justify this?thanks
- nima (age 22)
iran
A:

Hi Nima- we think we've addressed this question in the preceding answer. Let us know if more follow-up is needed. Mike W.


(published on 09/26/2013)

Follow-up on this answer.