Hic wrote:...
2 magnets in attraction it is the magnetic energy that is converted into kinetic energy
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It's the energy potential magnetic, linked to the starting position, which is transformed into kinetic energy, in the same way as in a fall, our gravitational potential gravitational energy is transformed into kinetic energy.
Conversely, when we want to return to starting positions, we must spend at least as much energy as we had gained, either against magnetism when we remove magnets, or against gravity when we go up. In a cycle: zero balance.
These potential energies are unrelated to the energy of the fields. The energy density of a magnetic field B is U = B² / (2 * μ) where μ is the permeability of the medium. It is the minimum energy (given the yield, it will take much more in practice) that will have to be provided to magnetize the mass of the material of which one wants to make a magnet, in general the electrical energy supplying the bank capacitors that are discharged into the coil inside which the material is placed.
It can thus be seen that the energy of a magnet is ridiculous, incommensurate with the mechanical energy it provides when it is attracted by a ferromagnetic mass or another magnet. Two attracting magnets are like an elastic, a vector of transformation: energy is provided by spreading them apart, as one elongates, and the elastic is then restored. An elastic does not provide intrinsic energy.