Still puzzled about magnetism & energy.?

Question

OK, so a permanent magnet creates a force caused by the subatomic and quantum effects of the electromagnetic 'wavicles' in it, this much I get. BUT, if I fix my magnet under a shelf and put something magnetic or ferrous under it, it will at some point lift the item by attraction.
I also get that, once the item is held in place, there is no more use of energy, as if the item was on top of the shelf, but there has been an increase in the potential energy of the item as it was lifted.
SO, where did that energy come from? If it comes 'from' the magnetic field, what 'lost' energy to balance the increase as the field remains constant and, if I continue to remove items and get them lifted again, (notionally forever) what would the eventual result be. Something should be getting cold or something for this to happen.

Answer 1

"......what 'lost' energy to balance the increase as the field remains constant............"

That's just the thing, when you bring pieces of iron or steel near the magnet, the field DOES NOT remain constant. Rather, the presence of the iron causes the field to shift, spread out, and weaken. You can only use the magnet to attract and pick up a *finite* amount of material, therefore, the potential energy invested in the magnetic field is also finite.

Technically, the magnetic "energy density" is given:

dU = (1 / 8π) ||B||² dV

where the total energy can be found by integrating over the entire magnetized region.

"........ Something should be getting cold....."

You're right about this one.

Because of reasons of Thermodynamics, magnetizing and de-magnetizing an object is never 100% efficient. It requires a certain investment or expenditure of energy on top of that contained in the magnetic field. Usually this additional energy comes in the form of heat, or electrical resistance. This is known as "Hysteresis."
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/hyst.html

Magnetically "hard" materials, (such as iron(II) oxide,) tend to heat up when they are brought into a magnetic field, and cool down when the magnetic field is removed. The cooling effect comes because some of the object's heat energy is consumed by "scrambling" the object's internal magnetization.

Note: magnetic "hardness" refers to the object's tendency to remain magnetized after the external field is removed.

Answer 2

a magnet is magnetic because the electron motion is in limited paths, which are coincidental between atoms. when a magnet induces a magnetic field in another ferrous object, the electron spin of atoms in the new object are likewise limited in the possible paths. this change in electron path is where the energy comes from.
on an single atomic level an electron has rotational motion, and a spin in a random direction. if this is in a magnetic field at the time there is a force applied to the electron determined by the right hand rule relating electron flow in a field. that force applied results in a change in the direction of the electron, so you can theorize there is a kinetic energy change. then extrapolate to include the huge number of atoms per magnet and you can get a tiny energy change.
if in practice you bring the magnet/item up to a magnet fixed above it, the point at which it lifts will be very close to the fixed magnet, less than a couple of cm for the strongest magnets. that is a very small energy change that can be accounted for.
btw as you pull the magnet away your hand forces the electrons through a magnetic field increasing the energy in the magnet. if there is some residual magnetism, that is hysterisis.

Answer 3

The magnetic field in space is an energy field. When an object is lifted by the field broduced by a permanent magnet, the object may gain potential energy from the gravitational field but loses even more potential energy insofar as the magnetic field is concerned.

Furthermore, the presence of the object near the magnet reduces the strength of the field remote from the magnet and object, so the magnetic field is NOT constant when an object attracts it. To restore this "lost" energy, mechanical work must be done to pull the object away from the magnet again.

Answer 4

i have to star this one because now i'm confused and would like to know the answer as well, good question.