A smartphone can contain a magnetometer: a sensor that measures magnetic fields. It detects components of the magnetic field around the phone. Earth behaves approximately like a giant magnet, so its field provides a direction that a compass can use. Software combines magnetometer readings with other sensors to estimate which way the phone is pointing. Nearby magnets, steel structures and electrical equipment can disturb the reading, which is why digital compasses sometimes need recalibration. The phone is not detecting geographic north directly; it is measuring a magnetic field and using that information to infer direction.

The useful part of this physics is that it helps people predict and design real things. Engineers often begin with the same simplified model used in school, test where it works, then add more detail when measurements show that the real situation is more complicated.

Magnetism comes from magnetic fields, which exert forces without objects needing to touch. In magnetic materials, many tiny magnetic regions can become aligned, producing a larger overall field. Electric current also produces a magnetic field, which is why coils of wire can act as electromagnets and why magnetic effects can be controlled in motors, speakers, relays and sensors.