Magnetic fields and electromagnets

A magnetic field is the region around a magnet where a force acts on another magnet or magnetic material. Field lines go from north to south; they are closest where the field is strongest, at the poles. Like poles repel and unlike poles attract. A permanent magnet has its own field; an induced magnet becomes magnetic only when placed in a field, and always attracts. Iron, steel, nickel and cobalt are magnetic. A compass contains a small bar magnet that points along the Earth's magnetic field.

A current in a wire produces a magnetic field around it, in circles. Making the wire into a coil (a solenoid) gives a field like a bar magnet's. Adding an iron core makes an electromagnet much stronger; so does increasing the current or the number of turns. Electromagnets can be switched off, which makes them useful in scrapyard cranes, relays and electric bells.