United StatesPhysics 2Syllabus dot point

What is a magnetic field, and why do magnetic field lines always form closed loops?

Topic 12.1 Magnetic Fields: describe magnetic fields, their sources, the dipole nature of magnets, and the representation of fields with field lines.

A focused answer to AP Physics 2 Topic 12.1, covering magnetic fields and their units, the dipole nature of all magnets, why field lines form closed loops with no magnetic monopoles, the field of a bar magnet and the Earth, and ferromagnetism, with full worked examples.

Generated by Claude Opus 4.810 min answerUpdated 2026-06-04

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Quick answer

A magnetic field $\vec{B}$ is the region of magnetic influence around a magnet or a moving charge, measured in teslas (T). Every magnet is a dipole with a north and a south pole: there are no magnetic monopoles, so cutting a magnet in half just makes two smaller dipoles. Magnetic field lines point from north to south outside a magnet and continue south to north inside it, forming closed loops with no beginning or end (unlike electric field lines, which start and stop on charges). The field is stronger where the lines are closer together, and lines never cross. Magnetism comes from moving charges, including the electron motions inside ferromagnetic materials like iron, which can align to make a permanent magnet. The Earth itself is a giant magnet, which is why a compass needle aligns with its field. Magnetic fields set the stage for the forces on moving charges and the induction that follow in this unit.

Jump to a section

What this topic is asking
What a magnetic field is
The dipole nature: no monopoles
Field lines as closed loops
Try this

What this topic is asking

The College Board (Topic 12.1) wants you to describe magnetic fields, identify their sources, explain the dipole nature of all magnets (no magnetic monopoles), and represent magnetic fields with field lines that form closed loops.

What a magnetic field is

A magnetic field fills the space around any magnet or any moving charge, and it makes its presence known by exerting forces, on other magnets, on moving charges (Topic 12.2) and on current-carrying wires (Topic 12.3). The tesla is a large unit: the Earth's field is only about $5 \times 10^{-5}$ T, while a strong lab magnet might reach a few teslas. The field's direction is defined as the way a compass north pole points.

The dipole nature: no monopoles

This is the deepest structural fact about magnetism and the one most often tested. Unlike electric charge, which comes in isolated positive and negative pieces, magnetic poles always come in pairs. No matter how finely you divide a magnet, you never isolate a single pole, you only get more dipoles. This is why magnetic field lines behave so differently from electric ones.

Field lines as closed loops

Because there are no monopoles for field lines to start or stop on, magnetic field lines form closed loops with no beginning or end. Outside a magnet they run from the north pole to the south pole; inside the magnet they continue from south back to north, completing the loop. As with all field-line diagrams, the field is stronger where the lines are denser, and lines never cross. The contrast with electric fields is a favorite exam point: electric field lines begin on positive and end on negative charges, but magnetic field lines close on themselves because there are no isolated poles. The source of all magnetism is moving charge: a magnet's field comes from the aligned motion of its electrons, and in ferromagnetic materials (iron, nickel, cobalt) these can lock into alignment to give a permanent magnet. The strategic role of this topic is to define the field that the rest of the unit acts with: the forces on moving charges (Topic 12.2) and currents (Topic 12.3) depend on $\vec{B}$ , and changing $\vec{B}$ drives the electromagnetic induction of Topic 12.4.

Cutting a bar magnet

A bar magnet with a north pole on the left and a south pole on the right is cut exactly in half. Describe the poles of each piece and the field lines.

step 1 Apply the no-monopole rule

There are no magnetic monopoles, so each piece must still be a complete dipole with both a north and a south pole.

step 2 Identify the new poles

The left piece keeps the original north pole on its left and gains a new south pole at the cut. The right piece gains a new north pole at the cut and keeps the original south pole on its right.

step 3 Describe the field lines

Around each smaller magnet, field lines run from its north pole to its south pole outside, and from south to north inside, forming closed loops, just like the original.

step 4 Interpret

No matter how many times the magnet is cut, you only ever produce more dipoles, never an isolated pole. This is the defining feature of magnetism.

Try this

Q1. State the direction of magnetic field lines outside a bar magnet. [1 point]

Cue. From the north pole to the south pole.

Q2. State what you get if you cut a bar magnet in half. [1 point]

Cue. Two smaller magnets, each with its own north and south pole (no isolated pole).

Exam-style practice questions

Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AP 2024 (style)5 marksSection II (short FRQ). A student examines a bar magnet. (a) Describe the direction of the magnetic field lines outside the magnet and inside it. (b) Explain why cutting the magnet in half does not produce an isolated north pole. (c) State how the strength of a magnetic field is indicated on a field-line diagram.

Show worked answer →

A 5-point FRQ on magnetic fields.

(a) Direction (2 points): outside the magnet, field lines run from the north pole to the south pole; inside the magnet they continue from south back to north, so each line forms a closed loop.
(b) No monopole (2 points): there are no magnetic monopoles. Every magnet is a dipole, so cutting it in half produces two smaller magnets, each with its own north and south pole, not an isolated pole.
(c) Strength (1 point): the field is stronger where the field lines are closer together (denser).

Markers reward the closed-loop direction, the dipole/no-monopole reasoning, and line density for field strength.

AP 2023 (style)1 marksSection I (multiple choice). Which statement about magnetic field lines is correct? (A) they begin on north poles and end nowhere (B) they form closed loops with no beginning or end (C) they begin on positive charges (D) they can cross each other. Justify your reasoning.

Show worked answer →

A 1-point MCQ on magnetic field lines. The answer is (B).

Because there are no magnetic monopoles, magnetic field lines never start or stop; they form continuous closed loops, passing out of the north pole, around, and back in through the south, continuing through the magnet. The trap is (A): unlike electric field lines, magnetic lines have no isolated source to begin on.

Related dot points

Sources & how we know this

AP Physics 2: Algebra-Based Course and Exam Description — College Board (2024)