Problem
Why does a magnet attract iron nails but not copper coins? What happens microscopically in iron that does not happen in copper?
Solution
Iron is a FERROMAGNETIC material: inside it there are Weiss domains, small regions in which the atomic magnetic moments are already aligned with each other. In the presence of the magnet’s external field, these domains orient themselves in the direction of the field and those favourably oriented grow at the expense of the others. The nail becomes magnetised and itself becomes an induced magnet, with an opposite pole facing the magnet: this is why it is attracted.
Copper, on the other hand, is essentially non-magnetic (in fact weakly diamagnetic): its atoms do not have magnetic moments that align stably, there are no domains that orient themselves, so copper does not become appreciably magnetised and is not attracted by the magnet.
Links
Topics: Magnetism Concepts: Magnetic field