Physics Help Forum

Physics Help Forum (
-   Electricity and Magnetism (
-   -   Maxwell's law (

harve Nov 21st 2014 01:38 AM

Maxwell's law
I have a query:why rotating charges (i.e. the elecrons in a spool,the orbital electrons,the common electrons of two atoms etc) are not producing EM waves? Their variable electric and magnetic field is not propagated farther than the static.

ChipB Nov 21st 2014 05:48 AM

In classical EM a charge in a circular orbit would indeed give off EM radiation. But in QM the electron is not considered to be in a circular orbit, and since giving off radiation would mean emitting a photon and dropping to a lower energy level, this is not possible if the electron is already on the lowest orbital. So in a nutshell: the electron does not emit electromagnetic radiation because quantum mechanics forbids it.

MBW Nov 21st 2014 08:13 AM

The old image of electrons in circular orbits around the nucleus is a grossly misleading analogy.
More modern analogies tend to refer to electron shells, but these are still just analogies.
The nucleus and the electrons co-exist in a state of mutual balance, called an atom.

I don't think it is correct to suggest that an electron "moves" within the atom at all.
(except perhaps when it absorbs or emits a photon and changes energy levels)

Jerromyjon Nov 21st 2014 10:16 AM

How would you describe electrons shared between atoms as in molecular bonds? Are they just "suspended" between atoms?

MBW Nov 21st 2014 10:52 AM

Note that this is just my personal interpretation,
Dabbling in physics is just my hobby, the proper physicists out there may shoot me down on this one.

I feel a lot of the misconceptions of QM come from considering the "elementary particles" as points.
This then requires that these points have to move about in order to encompass their region of influence.
However these entities are NOT particles as we might imagine very small motes of existence when we extrapolate down from snooker balls through dust particles, etc...
Also an electron in a molecule is a very different beast to a free electron in (for example) a cathode ray tube.
When we get down to an electron around a molecule, it's existence does not occur at a precise location.
The electron is the energy field binding the molecule together.

harve Nov 22nd 2014 10:30 AM


Originally Posted by ChipB (Post 27866)
In classical EM a charge in a circular orbit would indeed give off EM radiation.

Does a coil fed off DC produce EM waves?
Why the alternative magnetic field of a magnetic stirrer or the electric field of a charge moving in a loop (circular conductor) are not spread further away from the static (before rotation)?

MBW Nov 25th 2014 09:51 AM

Once again I am rather guessing...

The EM waves are produced by the change in momentum of the electrons.

If the speed of the electrons around the coil is modest,
the change in momentum of the electrons will be modest,
and the energy dissipated as EM waves will be small.

I have no idea what the typical electron speeds in such a circuit might be,
so I have no idea if my guessed idea is valid.

harve Dec 2nd 2014 09:23 AM

How much the speed of district current is? Even thought is a big coil fed of high amperage DC it's not give off significant radiation? Why?

ChipB Dec 2nd 2014 10:26 AM

In general the speed of electrons moving on a copper wire under DC voltage is pretty slow - generally less than 1 meter per hour. This is too slow to cause any significant amount of EM radiation.

harve Apr 24th 2017 12:55 AM

Can you find a formula giving the wavelength(λ) as a function of amperage(I)
and induction(L)?

All times are GMT -7. The time now is 12:30 AM.

Copyright © 2016 Physics Help Forum. All rights reserved.
Copyright © 2008-2012 Physics Help Forum. All rights reserved.