Magnetic field

Symbol $B$, unit the tesla, and one tesla is one newton per ampere metre. Because it is a vector field, a full description is an arrow at every point: direction along the local field, length for relative strength. A compass needle is the cheapest field probe there is, since a dipole placed in a field turns to lie along it.

What it acts on is a closed list of three: moving electric charges, electric currents, and magnetic materials. A charge sitting still is not on that list. Put $v = 0$ into $F_B = qvB\sin\theta$ and the force is gone, which is the first structural break from the electric field.

Field lines close on themselves. They begin nowhere and end nowhere, so there is no place for a lone source to sit: the smallest possible source is a dipole. Snap a bar magnet in half and you get two dipoles, not a free north pole. Outside a magnet the field runs away from the north pole and back into the south, which is how those poles are defined in the first place.

For sources, the AP Physics 2 sheet prints one field equation only, $B = \dfrac{\mu_0 I}{2\pi r}$ for a long straight wire. The field of a bar magnet, of a current loop, or of a single moving charge is described in this course, never computed.

Topic 12.1 carries the CED treatment, including ferromagnetic, paramagnetic and diamagnetic materials.