Magnets: domains, distance, Earth, and electromagnets

Inside a magnet: domains. A piece of iron is not one single atomic magnet. It contains many tiny regions called magnetic domains. In each domain, the magnetic fields of many atoms point in the same direction. In an unmagnetised iron nail, the domains point in random directions, so their fields mostly cancel. When a strong magnet is stroked along the nail, the domains can be pulled into alignment. Then the whole nail behaves like a magnet. Materials such as iron, cobalt, and nickel are called ferromagnetic because their domains can align. Materials such as wood, plastic, glass, and copper do not form aligned domains in the same way, so they are not attracted strongly. The source Curiosity: Textbook of Science for Grade 6, Chapter 4 (Exploring Magnets) introduces magnetic and non-magnetic materials; the domain picture is a deeper model used to explain the pattern.

Why the pull fades with distance. A magnet does not need to touch a paper clip to attract it; the magnetic field reaches through space. But the field is strongest near the poles and becomes weaker farther away. The weakening is steep, not gradual in a simple half-half way. If you move a paper clip from 1 cm to 2 cm from a small bar magnet, the pull does not simply become half as strong; it becomes much weaker than half. A useful rule is: doubling the distance reduces the field from a small magnet very sharply, so a chain of paper clips that holds at 1 cm may hold far fewer at 2 cm and almost none at 5 cm. This is why small magnets can feel strong when touched but seem useless a few centimetres away.

Earth as a magnet. Earth behaves as if it has a giant bar magnet inside it, tilted about 11 degrees from the spin axis. The magnetic pole near the geographic North Pole is actually a magnetic south pole, because it attracts the north-seeking end of a compass needle. The magnetic pole near the geographic South Pole is a magnetic north pole. A compass needle lines up with Earth's magnetic field, so its north end points roughly toward geographic north. The difference between magnetic north and true geographic north is called magnetic declination.

Electromagnets: current, turns, and core. When electric current flows through a coil of wire, it creates a magnetic field. Current is moving electric charge, as described in Electricity explained: How electricity is generated. Wrapping the coil around an iron core makes the field much stronger because the iron's domains align and add to the field. Three changes usually increase the strength: increasing the current, adding more turns of wire, and using an iron core instead of air or plastic. Motors use electromagnets because the magnetic field can be switched on and off and reversed by changing the current. A permanent magnet provides a steady field, while the electromagnet's changing field pushes and pulls the rotor.