Electrical power systems rely heavily on two closely related machines that appear, on the surface, to do opposite jobs. A generator converts mechanical rotation, from a turbine spun by steam, water or wind, into electrical current. An electric motor does the reverse, converting electrical current into mechanical rotation that can drive a wheel, a fan or a compressor. Both machines actually rely on essentially the same underlying electromagnetic principle, just applied in opposite directions.
A generator uses motion through a magnetic field to induce current
A generator works by rotating a coil of wire within a magnetic field, or equivalently rotating magnets around a stationary coil, so that the magnetic field passing through the coil is constantly changing as the rotation proceeds. That changing magnetic field induces an electrical current in the coil, a direct consequence of electromagnetic induction: any time the magnetic field through a conductor changes, a current is induced in that conductor. A generator's entire function reduces to mechanically forcing exactly this kind of continuous change, converting the mechanical energy driving the rotation into electrical energy carried by the induced current.
A motor runs the same physics forward: current produces rotational force
A motor exploits a closely related electromagnetic effect running in the opposite direction: when an electrical current flows through a conductor sitting within a magnetic field, that conductor experiences a physical force. By arranging current-carrying coils within a magnetic field so that this force consistently pushes the coil around a central axis, a motor converts the electrical energy of the input current directly into mechanical rotational energy. Because generators and motors share so much of their underlying physical mechanism, many electrical machines are actually built to be usable as either one, depending on whether mechanical energy or electrical energy is being supplied as the input.
What we're still unsure about
The basic electromagnetic principles behind generators and motors are precisely defined, extremely well established nineteenth and twentieth-century physics and electrical engineering, confirmed across an enormous range of practical machines built on exactly this basis. What's more genuinely an active area of applied engineering research is designing motors and generators that squeeze more usable power out of a given size and weight of machine while wasting less energy as heat, a persistent challenge given real materials' unavoidable electrical resistance and mechanical friction — engineers continue to develop improved materials, magnet designs and control electronics to push these efficiency limits further, without any real machine ever reaching perfect, lossless conversion.
This sits inside Power Systems & Electrical Machines, one of eight topics in Electrical Engineering, one of four domains in Engineering, one of seventeen subjects the app can quiz you on.