Say there’s an iron nail sitting on the table and I bring a permanent magnet close enough above it that the nail comes off the table and sticks to the magnet. Now it appears a force has been exerted on the nail to move it over a distance (the definition of work, measured in units of energy). Where did the energy come from?
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There is a potencial energy associated to that magnetic field. A uniform Magnetic Field can only exert rotational work over a dipole (or a system of dipoles like a nail). If this field is not uniform (like the one generated by a magnet), the dipoles are not rotating at the same speed generating a net force that performs traslational work
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