Magnetic domain

Look for this term in the current CEDs and you will find it once, in a clause. Essential knowledge 12.1.B.3.i, which AP Physics 2 and AP Physics C: Electricity and Magnetism print in identical words, says ferromagnetic materials such as iron, nickel and cobalt can be permanently magnetized by an external field that causes "the alignment of magnetic domains or atomic magnetic dipoles." That "or" is doing real work: the CED offers domains as one of two ways to say the same thing, and then never defines a domain, never draws one, and never asks you to calculate anything about one. The other two CEDs do not mention magnetism of materials at all.

So treat the domain as vocabulary, not as content. The physics the exam does want is 12.1.B.1: magnetic dipoles come from the circular or rotational motion of electric charges, and in magnetic materials that can be the motion of electrons. Statement 12.1.B.1.i then says permanent magnetism and induced magnetism both result from the alignment of magnetic dipoles within a system.

The domain picture is the everyday version of that alignment. A bar of unmagnetised iron is not free of dipoles; its dipoles are already aligned in small regions that point every which way, so the outside field averages to nothing. Bring up an external field and the alignment spreads, and the bar becomes a magnet. Ferromagnetic materials keep that arrangement after the field is removed; 12.1.B.3.ii says the dipoles in a paramagnetic material do not.

Topic 12.1 carries the three material classes in full.