how are magnet useful in junkyard?

Rather than "permanent" magnets, the large electromagnets used in junkyards only create a magnetic field when electric current flows through them. They are used to lift iron and steel scrap, and to separate it from non-magnetic scrap. The cranes have an electric generator connected to a wire coil in the large round lifting disc. When the power is on, its magnetic field attracts heavy iron and steel to it. When the scrap is positioned where they want it, they cut the power to the electromagnet and the scrap drops.