Introduction to Magnetism

Magnetism is a phenomenon of nature that shows up at every scale, from the orbital motion of a single electron to the magnetic field of the entire Earth. The word "magnetism" itself comes from magnetite (Fe$_3$O$_4$), the natural iron ore that was used as the first magnetic compass. In everyday modern life, magnets sit inside motors, cycle dynamos, loudspeakers, audio/video tapes, mobile phones, headphones, CDs, pen-drives, laptop hard discs and generators.

Until 1820, electricity and magnetism were treated as two completely separate branches of physics. In that year Hans Christian Oersted, while preparing a lecture, noticed that a compass needle placed near a current-carrying wire was deflected -- and that reversing the current reversed the deflection. This single observation unified electricity and magnetism into one subject, electromagnetism, and is the reason this unit first develops the properties of bar magnets and then shows that a steady (non-time-varying) current-carrying conductor behaves exactly like a magnet.