Magnetism and electromagnets
Physics · Key Stage 3 · The School
Fields
A magnet is surrounded by a MAGNETIC FIELD — the region where it exerts a force. Field lines run from north to south outside the magnet, and where they are closer together the field is stronger. Iron filings or a plotting compass reveal the shape, which is how a field that cannot be seen becomes something you can measure.
Electricity makes magnetism
A current flowing in a wire creates a magnetic field around it. Wind the wire into a coil and the fields add up; put an iron core inside and it becomes far stronger still. That is an ELECTROMAGNET. Its strength depends on the current, the number of turns, and the core — all three of which can be changed.
Why that matters
Unlike a permanent magnet, an electromagnet can be SWITCHED OFF, which is what makes it useful: scrapyard cranes pick up and drop, doorbells and relays use the pull to move a part, and electric motors turn because a coil in a field experiences a force. Almost anything that turns under electrical power depends on this.
Wind 20 turns of insulated wire round an iron nail and connect it to a cell: it will pick up a few paperclips. Now change ONE thing at a time. Add 20 more turns: it picks up more. Add a second cell: more again. Take the nail out and use just the coil: far fewer, because the iron core concentrated the field. Disconnect the cell and every clip falls — which no permanent magnet can do.