Break a bar magnet in half and you do not get a north pole and a south pole. You get two smaller magnets, each with both. Break those and you get four. Keep going down to individual atoms and the pattern holds — magnetism never comes in isolated poles, no matter how hard you try.
The Short Answer
What is magnetism? It is a force produced by moving electric charge. In a permanent magnet, the motion is the spin and orbital motion of electrons inside atoms. In an electromagnet, it is current flowing through a wire. Both produce a magnetic field, and that field exerts forces on other moving charges and on magnetic materials.
Magnetism and electricity are not separate phenomena. They are two aspects of one thing, electromagnetism, described by Maxwell’s equations in the 1860s.
Domains: Why Only Some Metals Are Magnetic
Every electron has a tiny magnetic moment, like a compass needle that can only point one of two ways. In most materials these moments point randomly and cancel out. In iron, cobalt, and nickel, the moments interact in a way that makes neighbors align — but only within small regions called domains, typically microns across.
In unmagnetized iron, domains point in random directions and the net field is zero. Put the iron in an external field and domains aligned with the field grow at the expense of the others. Remove the field and most of that alignment persists, which is why the iron stays magnetized. That persistence is called hysteresis, and it is what makes a permanent magnet permanent.
Poles and Field Lines
Magnetic field lines leave the north pole and enter the south pole outside the magnet, forming closed loops. They never cross. Where lines are dense, the field is strong.
Like poles repel, opposite poles attract. But the names are historical, not physical. The “north” pole of a compass needle points toward Earth’s geographic north because Earth’s magnetic south pole is up there. The labels are backwards by 400 years of convention. That is also why the simple answer to what is magnetism — opposites attract — describes the behavior without explaining the cause.
Cutting a Magnet
Since magnetism comes from aligned domains, cutting a magnet does not separate poles. It just creates two smaller sets of aligned domains, each with a full north-south pair. There is no magnetic monopole, despite decades of searching. Every experiment so far has found none.
Earth’s Field
Earth’s magnetic field is generated by liquid iron moving in the outer core — a dynamo driven by convection and rotation. It is not a permanent magnet; it is an active, churning system, and it flips polarity every few hundred thousand years.
The field is weak by human standards: about 25 to 65 microteslas at the surface, roughly 1/200th of a fridge magnet. But it is enough to deflect the solar wind, and without it the atmosphere would erode away over geological time. Mars lost its field and lost most of its air.
Electromagnets
Run current through a coil and you get a magnetic field proportional to current times turns. Add an iron core and the field multiplies by hundreds, because the core’s domains align with the coil’s field.
This is the basis of motors, generators, transformers, relays, hard drives, speakers, and every electric vehicle. It is also why the relationship between electricity and magnetism was not understood until Ørsted noticed a compass twitch next to a wire in 1820.
How Strong Do Fields Get?
| Source | Field strength |
|---|---|
| Earth’s surface | 25 to 65 microtesla |
| Refrigerator magnet | ~5 millitesla |
| Strong neodymium magnet | ~1 tesla |
| MRI scanner | 1.5 to 3 tesla |
| Strongest continuous lab magnet | ~45 tesla |
| Pulsed lab magnet (record) | ~100 tesla |
| Neutron star surface | 10^8 to 10^11 tesla |
That last row is not a typo. A neutron star’s field is strong enough to distort atoms into thin cylinders and would erase a credit card from 100,000 km away.
What Magnetism Really Is
Special relativity gives the deepest explanation. A moving charge creates a magnetic field because electric fields look different in different reference frames. Two parallel wires carrying current in the same direction attract each other — not because of a mysterious magnetic force, but because length contraction makes the positive and negative charge densities appear different to observers moving with the electrons.
So magnetism is what electricity looks like when you change your point of view. That is not a metaphor. It is the actual derivation, and it is one of the most elegant results in physics.
Keep Exploring
- What is static electricity? — the electric half of the same coin
- How do batteries work? — chemistry that drives electric current
- How does GPS work? — and why it needs relativity to function
