AP® Physics 2: Algebra-Based · Unit 12 · Topic 12.1
Magnetic Fields and Field Lines
A magnetic field line map shows the direction a compass needle would point at every location around a magnet, with lines leaving north, curling around, and entering south.
Iron filings scattered near a bar magnet snap into curved chains because each tiny filing becomes a miniature compass, twisting to align with the magnetic field at its location. Line up millions of these tiny compasses and you see the field's shape directly: lines bunched tightly near the poles where the field is strong, spreading out and curving through space farther away. By convention, field lines point out of the north pole, arc through the surrounding space, and dive back into the south pole. Inside the magnet itself, the lines continue from south back to north, so every line forms a closed loop with no beginning or end.
Two magnets facing like poles together, north to north, show field lines pushing away from each other, bowing outward and refusing to connect, because both regions are fields pointing away from their own north pole. Flip one magnet so north faces south, and the lines leaving one pole bend directly into the other, connecting the two magnets with a dense, continuous path, the signature of attraction. This pattern is why cutting a magnet in half never isolates a single pole: magnetism comes from enormous numbers of aligned atomic-scale dipoles running through the entire material, so slicing it just creates two smaller magnets, each still a complete north-south pair.
This field-line picture sets up everything else in the unit. Once you can read field direction and density from a diagram, you're ready to predict how a moving charge or current-carrying wire feels a force when it crosses those lines, which is where the magnetic force law takes over.
Key terms
- Magnetic field line
- An imagined curve showing the direction a compass needle would point at each location, running from north pole to south pole outside the magnet.
- Magnetic dipole
- A pair of inseparable north and south poles, the basic unit from which all magnetic fields, down to atomic scale, are built.
- Field density
- How closely spaced field lines are drawn; tightly packed lines mean a stronger magnetic field at that location.
- Like poles
- Two north poles or two south poles facing each other, producing field lines that bow apart and a repulsive force.
Watch it explained
- How it worksHow do magnetic field lines leave a north pole, curl around, and close on themselves?
- How it worksDraw the field pattern between two bar magnets facing north to north and then north to south.
- Common mistakeWhy doesn't cutting a bar magnet in half ever give you a separate north and south piece?
- Going furtherWhat happens to a compass needle as you walk it in a loop around a bar magnet?
Check yourself
End of topic.