Where do the energy come from?

[fargoth]

lets say i got by a chance a bunch of atoms, when which happens to have 50% up and 50% down spins when i activate a magnetic field.
now, some of the down (the excited ones) would decay into an up state and emmit a photon.

where does the energy for light came from?
the bunch of atoms didnt have potential energy until i brought the magnet, and i don't think it will be physically harder to push a magnet under the sample as a function of number of atoms in it (more atoms emmit more light)...
does it because they cancel the magnetic field and reduce its intensity?
i can take the magnet back and the magnetic field would get to its normal strength...
will it be harder for me to take the magnet away from the sample as a function of number of atoms in the sample?
will the magnet itself lose its magnetism?

 

[RandallB]

Seems more like a general physics question but then errors in thinking seem more in Particle physics.

Changing from up spin to down spin does not come about from a decay, I would think it would not require an energy release. And magnetism wouldn’t be involved.

If you have a moving charged particle it could be deflected a bit by a magnetic field, causing the magnet to be moved a very tiny bit (it is rather massive by comparison). So there you have some energy going into the magnet from the change in motion of the charged particle.

Then you did say you had some excited atoms – I assume as in moving a lot with Kinetic Energy, you might say they are hot. If they where to cool down or become less excited, losing some of that Kinetic Energy to become colder. Emitting that energy as EM radiation, maybe as infrared heat of whatever frequency suits your particular atoms. See black body radiation. Don’t need the magnet here, more a action - reaction kind of thing really. Energy into your atoms they react getting hotter. They toss off some energy they slow down to become cooler

 

[fargoth]

sorry, i missused the term decay...

when you got a particle in rest within a magnetic field, it is excited when its spin is opposite to the magnetic field - the energy difference between the up and down spins is given by larmour frequency which is proportional to the field's strength.

when your system changes its state from 50%-50% to something like 10%-90% (depending on the temperature), light at the larmour frequency is emmited.
thats the mechanism behind NMR.

hope i made myself clearer.