Magnetism is a class of physical phenomena that are mediated by magnetic fields. Electric currents and the moments of elementary particles give rise to a magnetic field, which acts on other currents and moments. The most familiar effects occur in materials, which have permanent magnetic moments, called magnets. However, all materials experience the force of the Earth’s magnetic field; this is called the lodestone effect. Magnetism is also important in electromagnetism and electrical engineering, for example in motors, generators and transformers.
In classical physics, magnetism is explained using magnetostatics, as an interaction between moving electric charges with electric and magnetic fields. In quantum mechanics it is an interaction between moving charged particles with spin (intrinsic angular momentum) with both electric and magnetic fields.
Classical electrodynamics explains how moving charges create electromagnetic fields via special relativity: in addition to a charge’s own electric field there will be a contribution from its motion through space-time creating a ‘magnetic’ component to the electromagnetic field tensor Fμν . This additional field due to motion appears even when there are no Permanent magnets or current loops anywhere near – it is just an effect of Special Relativity. It turns out that this extra ‘magnetic’ piece couples very nicely to intrinsic spin angular momentum S – i.e., any moving electrically charged particle also creates a tiny little magnet along its direction of travel!
This explains why electrons orbiting around atomic nuclei can produce stable atoms: they create their own little ‘atom-sized’ magnets which oppose any external applied magnetic fields trying to line up the electron spins all parallel (which would make the atom unstable). In addition these same principles explain various more complicated molecular level phenomena such as Nuclear Magnetic Resonance spectroscopy used in modern medical MRI machines.
The combined theory of Special Relativity and Quantum Mechanics leads us quite naturally then to consider what happens when we put together lots of tiny little magnets – like those due to spinning electrons – into one big macroscopic object made from many atoms…