Electric and magnetic forces: electromagnets, motors and generators

The Danish scientist Hans Oersted noticed that a compass needle near a wire turns when a current flows in the wire. So a current has a magnetic effect: while it flows, a magnetic field surrounds the wire. An electromagnet is made by coiling insulated copper wire round a soft iron core. The current magnetises the core, and when the current stops, the magnetism goes: it is a temporary magnet that can be switched on and off. This is why it is used in scrap-yard cranes, in sorting metal for recycling and in electric bells. The strength grows with more turns of wire, a bigger current and the soft iron core, and the number of paper clips it lifts is a way to measure it. In an electric bell, the electromagnet pulls a springy strip away from a contact, which breaks the circuit; the magnet then switches off, the strip springs back, the circuit closes again, and the cycle repeats and the hammer rings the bell. Careful: an electromagnet is not a permanent magnet, a soft iron core is not a steel one, and a compass is not “attracted” by the wire: it turns because of the magnetic field.