In Electromagnets and Motors you saw that a current in a coil makes a magnet. That is the whole story of magnetism in one line: moving charge makes magnetic fields. A bar magnet works the same way. Its electrons spin and orbit, and in iron those tiny currents line up.
A magnetic field $\vec{B}$ is a vector at every point in space. A compass needle is a tiny magnet, and its north end points along $\vec{B}$. Field lines follow the compass: they leave a magnet's north pole and enter its south pole. Where the lines crowd together, the field is strong.
Key idea
Magnetic field lines are closed loops. Outside a magnet they run from N to S; inside they run from S back to N. The field is measured in teslas: $1\ \text{T} = 1\ \text{N/(A·m)}$.
Why it works
Electric field lines start on positive charges and end on negative ones, because charges exist on their own. No one has ever found a magnetic "charge", a lone north or south pole. Every magnetic field comes from moving charge, and the field of a small current loop curls around and closes on itself. Add up loops and you still get closed lines. That is why cutting a magnet never isolates a pole: each piece is a complete magnet.
Common mistake
Thinking that field lines begin on the north pole and end on the south pole, like electric field lines on charges. Inside the magnet the lines keep going, from S to N, so every line is a loop. Check: a line that "ends" would need a magnetic charge there, and none has ever been found.
How strong is a tesla?
| Source | Field |
|---|---|
| Earth, at the surface | about $25$ to $65\ \mu\text{T}$ |
| A refrigerator magnet, close up | about $5\ \text{mT}$ |
| A hospital MRI scanner | $1.5$ to $3\ \text{T}$ |
Worked example: Comparing fields
An MRI magnet makes $3.0\ \text{T}$. How many times stronger is that than Earth's field of $50\ \mu\text{T}$?
- Put both in teslas: $50\ \mu\text{T} = 50 \times 10^{-6}\ \text{T} = 5.0 \times 10^{-5}\ \text{T}$.
- Divide: $$\frac{3.0\ \text{T}}{5.0 \times 10^{-5}\ \text{T}} = 6.0 \times 10^{4}.$$
- The units cancel, so the answer is a pure number.
Answer: About $60{,}000$ times stronger
Worked example: Earth is a magnet
A compass's north end points toward Earth's geographic North Pole. What kind of magnetic pole lies under the Arctic?
- Opposite poles attract, so the north end of the needle is pulled toward a south magnetic pole.
- So a magnetic south pole lies near the geographic North Pole. Outside Earth, field lines run from the southern hemisphere toward the northern one.
- The name "north pole" of a magnet is short for "north-seeking pole".
Answer: A magnetic south pole