Magnets & Field Lines
A short lesson with diagrams, a worked example, a problem to try, and a quick check. Then quiz yourself on 5 practice questions.
Every magnet has a north and a south pole; like poles repel and unlike poles attract. Magnetic field lines leave the north pole, loop around, and enter the south pole. Unlike electric field lines, they always form closed loops: there’s no magnetic “charge” for them to start or end on.
1. Mapping the field
2. Poles always come in pairs
Break a bar magnet in half and you get two smaller magnets, each with its own N and S. No one has ever found a single, isolated pole.
A magnet is made of tiny aligned regions called domains. Iron and nickel are attracted to a magnet because their domains line up with its field.
Earth is a giant magnet. A compass’s north end points toward Earth’s geographic North Pole, so the pole there is actually a magnetic south pole (opposites attract).
3. Example, then try it
A compass is carried around a bar magnet. Where does its north end point when it sits right above the middle of the magnet?
Just outside the magnet, the field lines run from the N end back toward the S end, parallel to the magnet.
So the needle lies parallel to the magnet, with its north end pointing toward the magnet’s S end.
A bar magnet lies with its N pole on the right. Which way does a compass needle’s north end point at A (on the axis, just right of the N pole), B (on the axis, just left of the S pole), and C (inside the magnet, if a tiny compass could fit there)?
A: field lines leave the N pole, so the needle points to the right, away from the magnet.
B: field lines enter the S pole, so the needle points to the right, toward the magnet.
C: inside, the lines run S → N, so it points to the right too.
Notice: along the whole axis the field points the same way: the lines form one continuous loop.
Answer: to the right at all three points.
4. Common mistakes
The north poles of two bar magnets are brought close together. What happens?
Like poles repel. The field lines from the two north poles push against each other between the magnets.
Ready to practice?
5 questions on Magnets & Field Lines, each with a hint and an explanation for every wrong answer.