Enthusiastic layperson here, curious about magnetism:

Scenario 1:

Imagine that I lift a piece of iron with my arm from a tabletop a fixed distance upwards, then place it back on the table.

In this scenario, the energy required to lift the mass against gravity comes from the nutrition that I eat. Furthermore, if I repeatedly lift the weight without replenishing those calories expended by my lifting muscles I will eventually reach a point where there is no more energy available to make the lift. Of course I'll probably have starved by then, but hopefully I've gotten my point across: a supply of energy is required for my muscles to lift the iron.

Scenario 2:

Imagine now that instead of lifting the iron with my muscles, I hover a magnet over it from the same height. Just like my muscles, the magnet lifts the iron against gravity.

I know where the muscular energy required to lift the iron comes from. But where did the energy required to lift the iron by the magnet come from?

I then remove the iron from the magnet, place the iron back on the table, and repeatedly lift the iron using the magnet. My understanding is that the magnet will continue to repeatedly lift the iron without losing its strength.

How is the magnet resupplied with energy so that it can continue to lift the iron?

  • $\begingroup$ How are you hovering the magnet above it? $\endgroup$ – probably_someone Jul 16 '19 at 17:08
  • $\begingroup$ I suppose that I could use a variety of methods to hover the magnet: hold it in my hands, construct some sort of mechanical contrivance to position it, and so on. $\endgroup$ – Stu Smith Jul 16 '19 at 17:11
  • $\begingroup$ "...I then remove the iron from the magnet." Hint: your arms could get tired of doing that too. $\endgroup$ – Solomon Slow Jul 16 '19 at 18:16
  • $\begingroup$ @SolomonSlow I'm not very good at riddles. Are you saying that the act of prying the iron away from the magnet somehow resupplies the magnet with energy so that it can pick up the iron again? $\endgroup$ – Stu Smith Jul 16 '19 at 18:40
  • $\begingroup$ I am not a physicist, so I can't say where the energy is stored with any authority, but yes: By prying an iron armature away from a permanent magnet, you are increasing the potential energy of a "system." It is very similar to how, by lifting a weight (i.e., by prying it away from Earth's gravity) you increase the potential energy of that "system." $\endgroup$ – Solomon Slow Jul 16 '19 at 19:07

I know where the muscular energy required to lift the iron comes from. But where did the energy required to lift the iron by the magnet come from?

Just like a gravitational system has gravitational potential energy (that can be released by allowing an object to fall within it), a magnetic system has magnetic potential energy.

In your scenario 2, the magnetic potential energy is lower after the iron has lifted onto the magnet. The lifting process has reduced the magnetic potential energy and increased the gravitational potential energy. The reverse happens if the iron is moved from the magnet to the desk.

Here's another way to think about it. Instead of a magnet, imagine you have a very dense piece of matter that is held above your desk. It's gravitational pull can lift the book so that instead of sitting on the desk, it "sits" on the object.

The energy to do this lift comes from the fact that the potential energy of the book being near this object is lower than the book being near your desk.

How is the magnet resupplied with energy so that it can continue to lift the iron?

Once lifted, no energy is required to keep the object in place, any more than a shelf requires energy to hold a book up against gravity. To remove the object requires increasing the magnetic potential energy. This energy is supplied by whatever force pulls the iron away (such as your arm).

  • $\begingroup$ Interesting! The comparison to gravity helps greatly. However, it (happily!) creates new questions for me. I'll create a new post shortly when I've thought them through. $\endgroup$ – Stu Smith Jul 16 '19 at 20:52

You are asking where the energy of a permanent magnet comes from.

Now these objects are made of usually metals, that have electrons in them that are on this site cite two ways:

  1. loosely bound

  2. delocalized

Now the magnetic energy of the magnet comes from the magnetic moments of their electrons.

Ordinarily, the enormous number of electrons in a material are arranged such that their magnetic moments (both orbital and intrinsic) cancel out. This is due, to some extent, to electrons combining into pairs with opposite intrinsic magnetic moments as a result of the Pauli exclusion principle (see electron configuration), and combining into filled subshells with zero net orbital motion. In both cases, the electrons preferentially adopt arrangements in which the magnetic moment of each electron is canceled by the opposite moment of another electron. Moreover, even when the electron configuration is such that there are unpaired electrons and/or non-filled subshells, it is often the case that the various electrons in the solid will contribute magnetic moments that point in different, random directions so that the material will not be magnetic.

Now with normal matter, these magnetic moments cancel out.

Sometimes, either spontaneously, or owing to an applied external magnetic field—each of the electron magnetic moments will be, on average, lined up. A suitable material can then produce a strong net magnetic field. The magnetic behavior of a material depends on its structure, particularly its electron configuration, for the reasons mentioned above, and also on the temperature. At high temperatures, random thermal motion makes it more difficult for the electrons to maintain alignment.

You were asking about where the energy of the magnet comes from, and the answer is that in the case of the magnet, the magnetic moments of the electrons align.


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