Magnetic Fields

Magnetic field lines form closed loops: outside a magnet they run from the north end to the south end and inside the material they continue from south back to north. No magnetic monopole exists, so no line begins or ends anywhere, and cutting a magnet produces two complete dipoles rather than separated poles, because the magnetism comes from circulating charge inside the material. A material's response depends on what its dipoles do, which makes iron strongly attracted, aluminium almost unaffected, and everything faintly diamagnetic. A compass is a dipole that a torque aligns with the local field, so it lies tangent to the field line through it.

Three errors dominate. Treating a pole as a magnetic charge that lines start on, which predicts that sawing a bar magnet in half yields one north piece and one south piece. Sorting objects by whether they look like metal instead of by how their dipoles respond, so an aluminium can is expected to behave like an iron nail. And pointing a compass needle straight at the nearest pole rather than along the field line where the compass actually sits, which gets every off-axis reading wrong.