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I’m an amateur trying to grapple around this problem of what sources the magnetic energy in a bar magnet…

We know that the source of the magnetic force that a bar magnet exerts is due to its magnetic field. What I’m trying to understand is what (if indeed it is true) is the source behind such a magnet to stay as being able to exert this magnetic force over an extended period of time (days/months/years).

My questions basically are as;

  • Is it true that a bar magnet (assume not connected to any EM source) with a magnetic field B is undecayed in time and continues to possess the same field intensity?
  • If it is indeed true, what can enable a piece of matter to continue to have such energy?

The following stumped me as I was trying to explain to my 5 year old that magnets when attracting(or repelling) each other are doing work and therefore have to use energy…. To which the immediate question in my mind was what provides this undampened energy to this body (magnets)

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2 Answers 2

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Energy is the ability of a system to perform work on another system. One can indeed convert the energy of a permanent magnetic field into useful work by demagnetizing the magnet, but then the useful magnetic energy will be gone.

What we are usually doing is to extract work from the system by moving a small piece of magnetic material into the field. This causes a force on this magnetic body along a displacement, hence it "performs work". But now we have modified the magnet because it has a little bit of magnetic material hanging on it, which weakens the external magnetic field. If we do this over and over, again, then these small magnetic "test particles" will weaken the magnetic field more and more (i.e. they will be shielding the magnet) and eventually we can't extract any more work. The only way to use our magnet again, is to remove the magnetic materials hanging on it, which takes (at least) the same amount of work that we got out of the system in the beginning. Combine this cycle with a magnetic coil and electric and mechanical energy source and sinks and you have an electric machine called a generator or electric motor.

It is correct that an ideal magnetic material does not show a loss of magnetic field over time. In real life applications heat and external mechanical forces can and do demagnetize "permanent" magnetic materials slowly. They can be "refreshed" by exposing them to a very strong external field that restores the original magnetization.

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In addition to the good answer already provided I want to point out that simply exerting a force does not require any energy if the force does not act through any displacement. So a magnet could stick to your fridge, or hold a door closed etc. for years and never need to be re-energized. This is the same as a bookshelf that can keep holding up books, which requires a force, but doing so does not drain any energy from the shelf.

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