# Energy conservation of permanent magnets engine

How to explain the fact that magnet can attract an object (apply a work W) without losing a (significant) part of its internal energy?

How to apply the energy conservation principle?

Please think to a purely permanent magnet motor (without any electricity supply).

• That seems to be a duplication of Conservation of Energy in a magnet Mar 1, 2015 at 2:18
• aandreev, I coudn't find the answer and I couldn't write a comment (it needs 50 reputation change). Sorry Mar 1, 2015 at 7:22

Magnet's energy doesn't go anywhere. It always stays $\frac{B^2}{2\mu}$ (in some conditions). So where does the work come from, the energy to pull metal to the magnet?
If you put piece of iron, or better a wire with flowing current $j$, on distance $d$ from source of magnetic field $B$, then there will be force between two: $\vec F=\vec j \times \vec B(d)$ (see Lorentz law). If you hold this wire in place, then there will be potential energy $U=-\int_{0}^{d} Fdx$. If you let this wire go, potential energy will turn into kinetic energy of movement $E=\frac {p^2}{2m}$.