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Pardon me if my question sounds "out there", I've been out of touch with recent developments in physics, but this is something I've never really put deep thought into.

I know that when two pieces of magnets are bought with their like sides towards one another, they repel, with a force proportional to their intrinsic properties. So in principle you can make a contraption where two pieces of magnets act as a source of tensile strength, just like the electromagnetic forces that exist within metal wires that help them keep their shape under some tension.

My question is regarding the extent to this supply. Can these electromagnetic forces forever hold out the tensile forces against which they've been put to or do they decay over time (let's ignore environmental effects)? In other words, if I have a weight suspended on the top side of a two repelling magnet system, will that weight stay there forever? I can also ask it in another way:

If we have a piece of metal, we know that it has a large number of free electrons that help it maintain hardness. How long will it stay hard for? Do we have an estimate if there's a tendency for metals to lose hardness over time due to wearing of electrostatic repulsion between them?

Another way to ask the question would be if I replace magnets with superconducting magnets, which we know should in principle provide a source of magnetic force forever, is there a reason to believe that that will decay in time too?

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    $\begingroup$ In super conducting electromagnets, if the field performs work you lose energy from the magnet and it weakens if there is no additional power input. Kind of like a frictionless flywheel. All permanent magnets also demagnetize themselves over time, but that isn't really relevant to your question. Also, your examples often don't involve work. A table supporting a book, for example does no work so no energy needed. $\endgroup$
    – DKNguyen
    Feb 22, 2022 at 14:53
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    $\begingroup$ Good question, but your title seems to not match the question in the body. $\endgroup$
    – Steeven
    Feb 22, 2022 at 15:16
  • $\begingroup$ I agree with @Steeven the title has nothing to do with the body, I would replace the title since the body is a good question. $\endgroup$
    – Dale
    Feb 22, 2022 at 15:19
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    $\begingroup$ statically holding a tensile force doesn't require spending energy. no work is performed $\endgroup$
    – njzk2
    Feb 22, 2022 at 22:35
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    $\begingroup$ Potential energy does not increase if you are just holding a body in a field(here magnetic) without changing the position of the body. So no, even if the magnet didnt lose its property over time, it would still not lead to an increase in PE if you didnt push the magnets around(i.e., changed their positions wrt to each other) $\endgroup$ Feb 23, 2022 at 6:41

5 Answers 5

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My question is regarding the extent to this supply. Can these electromagnetic forces forever hold out the tensile forces against which they've been put to or do they decay over time (let's ignore environmental effects)?

They will decay over time.

The magnets were created by causing the atoms of the iron to line up when placed in a strong magnetic field. But over time, at any temperature above absolute zero, the atoms will move around and randomize their positions causing the magnet to slowly weaken over time. Of course, the process can be sped up by heating or dropping the magnet.

Hope this helps.

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    $\begingroup$ This answer does not seem to address OP's actual confusion. Even if magnets did not decay ever, that still would not require "infinite energy". $\endgroup$ Feb 23, 2022 at 3:19
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    $\begingroup$ @BlueRaja-DannyPflughoeft I don't understand your point. I suggest you post an answer of your own that you think better addresses the OP's confusion than my answer. $\endgroup$
    – Bob D
    Feb 23, 2022 at 3:47
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No.

First, potential energy can not be infinite - by definition.

Second, nothing is infinite. Everything will decay over time.

If you put something on top of a table and nothing happens around, then the "thing" will stay there as long as its lifetime ends - but that does not mean that the object has infinite energy.

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  • $\begingroup$ While I understand why you say potential energy cannot be infinite (a magnet having infinite energy is ridiculous), I don't see why it cannot be by definition. In classical theory, the self-energy of an electron is infinite. $\endgroup$
    – Jono94
    Feb 3 at 15:08
  • $\begingroup$ As I recall, you "set" the line integral to be zero at infinity, therefore even if the path closes at infinity, you'll get a finite number. I should check it to be sure, but I never saw an infinite potential energy :D $\endgroup$
    – gbon
    Feb 5 at 9:43
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Simply "holding something up" does not consume energy, therefore being able to hold something up forever does not mean an infinite supply of potential energy.

Consider a table in an everyday setting (i.e. near-Earth gravitational field). When you put a weight on the table, it stays there. The table supports the weight, while Earth pulls on the weight with gravity. The weight do not move because the net force on the weight is zero. Therefore, the table does not do work on the weight, thus the table does not transfer any energy to the weight. The table does not need "infinite potential energy" in order to hold the weight up.

The same applies to your question:

if I have a weight suspended on the top side of a two repelling magnet system, will that weight stay there forever?

Assuming the arrangement is stable and the magnets do not decay (other answers have already discussed ways in which magnets could decay), then yes, the magnets can support the weight forever. This does not mean that the magnets somehow have infinite potential energy in them.

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Potential energy does not changeif you are just holding a body in a field(here magnetic) without changing the position of the body. So no, even if the magnet didn't lose its property over time, it would still not lead to an increase in PE if you didn't push the magnets around(i.e., changed their positions wrt to each other). So no, no infinite PE.

And as Bob D said, the aligned atoms will lose their alignment with time(as they are always vibrating above absolute zero) and the magnetic properties will slowly decrease over time as more unaligned atoms means more of the "atomic magnets'" effects cancel out.

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Noting is infinite. This would violate the law of conservation of energy. Also, the magnet would decay and demagnetize over time.

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