Bar magnets give the clearest picture of magnetic behavior. For AP Physics 2, focus on two ideas: the direction of a bar magnet’s external field and the attraction or repulsion between poles.

Bar Magnets and Their Poles

A bar magnet is a simple permanent magnet with two distinct ends that behave differently from the middle of the magnet and are treated as opposite poles.

Each end acts as a magnetic pole, so the magnet is described by the behavior of its two ends rather than by separate pieces.

By convention, one end is called north and the other south. These names are labels for the ends of one magnet, not separate substances stored inside it. For AP Physics 2, the key idea is that the poles determine both the direction of the external magnetic field and the way two bar magnets interact.

Direction of the External Field

The external magnetic field is the magnetic field in the space around the magnet, not inside the magnetic material itself.

Outside a bar magnet, the field points away from the north pole and toward the south pole.

Magnetic field mapping around a bar magnet using many small compasses: the compass needles align tangent to the field, pointing away from the magnet’s north pole and toward its south pole. The connected arrows form continuous field lines that curve through space while preserving the overall outside-the-magnet direction from $N$ to $S$. Source

If arrows are shown on a diagram, they leave the north end and enter the south end. This is the main directional rule you must know for this subsubtopic.

This rule applies at every point outside the magnet, even though the field direction changes from place to place. Near the north pole, the field points outward from the magnet. Near the south pole, it points inward toward the magnet. In regions off to the side, the direction curves, but it still follows the same overall north-to-south pattern outside the magnet.

This makes unlabeled diagrams easier to interpret:

• If field arrows come out of an end, that end is north.

• If field arrows go into an end, that end is south.

• If arrows in the gap between two facing poles point from one magnet toward the other, they point from north to south.

A common error is to memorize only the letters $N$ and $S$ without thinking about arrow direction. In exam questions, field direction is often the quickest way to identify which pole is which.

Pole Interactions

The second core idea is how poles behave when two bar magnets are brought near each other. Their interaction depends on which poles face.

Like poles repel

When north faces north or south faces south, the magnets push apart. This is called repulsion.

Repulsion may appear as:

• the magnets sliding away from each other

• one magnet turning so the like poles no longer face directly

• resistance when you try to force the poles together

Repulsion does not require contact. The magnetic force acts across the space between the magnets.

Unlike poles attract

When north faces south, the magnets pull together. This is called attraction.

Attraction may appear as:

• the magnets moving closer together

• a free magnet rotating until opposite poles face

• the magnets sticking together when they are close enough

For quick recall, the rule is simple:

• like poles repel

• unlike poles attract

If a magnet is free to move, the force is not always just a straight push or pull. The magnet may rotate first, because the interaction tends to bring opposite poles into a facing position.

Connecting Field Direction to Pole Interaction

Field direction and pole interaction support the same picture of a bar magnet. Because the external field leaves the north pole and enters the south pole, a region between unlike poles has a clear direction across the gap, from north to south. That arrangement is associated with attraction.

When like poles face, the magnets do not naturally settle into a facing arrangement. Instead, the interaction favors motion that separates the magnets or turns one of them so different poles face. This is why like-pole configurations are unstable when the magnets are free to move.

You do not need a mathematical treatment here. The important qualitative links are:

• a bar magnet produces a magnetic field outside itself

• the field direction identifies north and south

• the pole types predict attraction or repulsion

Reading Common Bar-Magnet Diagrams

In many AP-style diagrams, a bar magnet is drawn as a rectangle with ends labeled $N$ and $S$, or with one label missing. The problem may then ask you to identify the missing pole or predict the motion of a second magnet.

Useful habits:

• First identify any labeled pole.

• Next use the rule that the external field goes away from $N$ and toward $S$.

• Then compare the poles that face each other.

• Predict attraction for opposite poles and repulsion for matching poles.

When two unlike poles face, arrows in the gap are commonly drawn from the north pole toward the south pole. When like poles face, the field in the region between them is not drawn as a simple direct connection, and the magnets tend to push apart instead.

Be careful with language. A north pole is not "positive," and a south pole is not "negative." Magnetic poles are not the same as electric charges. For this subsubtopic, the most important statements are the two syllabus rules: the external field of a bar magnet points away from north and toward south, and like poles repel while unlike poles attract.