A magnetic field is a region around a magnet or current-carrying wire where magnetic forces act. Every magnet has a north pole and a south pole. Opposite poles attract, like poles repel. If you cut a magnet in half, you get two smaller magnets, each with both poles. There is no such thing as a magnetic monopole, at least as far as we know.
Earth itself is a giant magnet. Its magnetic field is generated by convection currents in the liquid iron outer core, a process called the geodynamo. The field extends far into space, deflecting charged particles from the solar wind. This magnetosphere protects us from harmful radiation. Without it, Earth’s atmosphere would be stripped away, as happened to Mars.
Electromagnets are coils of wire that produce magnetic fields when current flows through them. The strength depends on the number of turns, the current, and the core material. Wrapping a coil around an iron core amplifies the field by a factor equal to the relative permeability of iron, about 5,000. This is why scrap yard electromagnets can lift entire cars with relatively modest electrical power.
MRI machines use superconducting magnets cooled by liquid helium. These produce fields of 1.5 to 7 tesla, tens of thousands of times stronger than Earth’s magnetic field. The strong field aligns hydrogen nuclei in the body. Radio waves then knock them out of alignment, and as they relax back, they emit signals that are reconstructed into detailed images of soft tissue.
Magnetic storage has been fundamental to computing. Hard disk drives use magnetic domains on spinning platters to store data. Each domain is a tiny magnet that can point one way or the other, representing a 0 or 1. Modern drives can store terabytes by making these domains incredibly small. The limit is superparamagnetism, where thermal fluctuations randomly flip tiny magnetic domains, destroying data.
Magnetic levitation uses repulsive magnetic forces to suspend objects without contact. Maglev trains achieve speeds over 600 km/h by floating on a magnetic cushion. Friction is eliminated, leaving only air resistance. The same principle operates at a smaller scale in magnetic bearings used in high-speed turbines and flywheels.