# Magnet and needle

When I place a needle near a clamped magnet and let it go, it moves towards the magnet. Since magnetic field cannot work on individual charge particles, we must conclude that the needle loses its overall internal energy and hence cools down! Is this true?

• Why "must we conclude" the energy loss in the needle? – Kyle Kanos Nov 5 '15 at 0:36
• Dr, it is not the force between a magnetic field and a charge, but between two magnetic fields.en.wikipedia.org/wiki/… – user83548 Nov 5 '15 at 0:51
• @KyleKanos If you think of a needle as a collection of ions, isn't it clear that the individual speeds of the ions do not change in the magnetic field. But the needle accelerates as a whole system so it must lose internal energy. – user97646 Nov 5 '15 at 1:02
• think of each ion as a little magnet too – user83548 Nov 5 '15 at 1:22

• Magnetic field lines in space contain energy - $\frac12 \frac{B^2}{\mu}$ see for example this link. As the needle approaches the magnet, the shape of the magnetic field lines changes - while they will be closer together in some areas (thus higher energy per unit volume), other (larger) regions will have a lower magnetic field. The net result is that there will be less energy stored in the magnetic field. – Floris Nov 5 '15 at 1:30