A magnetic field line shows the direction a free north pole would move: out of a north pole, into a south pole, and never crossing. Switch the source between a bar magnet, a straight current-carrying wire, and a solenoid, and watch the compasses line up with the field.
MissionSwitch to the wire, then predict the shape of the magnetic field around a long straight current-carrying conductor.Streak 0Best 0
The magnetic field around a long straight current-carrying wire is shaped like:
sourcebar magnet
field-line shapeloops, north to south
direction ruleout of N, into S
Field lines run from north to south outside a magnet, never cross, and are closer where the field is stronger.