|MLA Citation:||Bloomfield, Louis A. "Question 940"|
How Everything Works 21 Jan 2018. 21 Jan 2018 <http://howeverythingworks.org/print1.php?QNum=940>.
In an electromagnet, an iron or steel core is surrounded by a coil of wire. When you run current through that coil of wire, the magnetic field of the current causes the core's magnetic domains to change sizes—the domains that are aligned with the field grow at the expense of the domains misaligned with the field and the whole piece of iron or steel becomes highly magnetic. When you stop current from flowing through the coil of wire, the domains may return to their original sizes and shapes and the iron or steel may become nonmagnetic again.
The abilities for magnetic domains to change sizes depends on the chemical and physical properties of the metal, particularly its crystalline structure. In some magnetic materials, the domains change size extremely easily. These materials are considered to be "soft"—they magnetize easily in the presence of a magnetic field and demagnetize easily when that field is removed. Most electromagnets are made from such soft magnetic materials because it takes only a small current in a wire coil to magnetize the electromagnet's soft core and that core quickly becomes nonmagnetic when you stop the current from flowing.
But in other magnetic materials, the domains don't change size easily. These materials are considered to be "hard"—they are both difficult to magnetize and difficult to demagnetize. You must put lots of current through the coil of wire around a hard magnetic material in order to magnetize that material. But once you turn off the current, the material will retain its magnetization and it will be a permanent magnet.