So imagine a world where you never have to charge your cell phone or MP3 player. While the technology may sound near future, the principal it depends on is straight out the 19th century.
English chemist and physicist Michael Faraday put forth his law of induction in 1831, which outlined how moving a conductor through a magnetic field induces a current in the conductor proportional to the speed of movement. Faraday's law of induction underlies the functionality of the world's power plants. From coal-burning plants to nuclear facilities, the idea is to create enough kinetic energy (the extra energy produced by motion) to move the conductor through the magnetic field.
Various companies have applied this technology to common gadgetry. An emergency radio boasts a crank on the side, which you can turn to produce the minimal electricity necessary to power it. More recently, developers have tweaked this design to produce a stronger electric current without the need for active cranking. These include shake flashlights and power packs, which boast special coil assemblies inside a short tube. At each end of the tube is a repellent magnet and, in the middle of the tube, you have the all-important magnetic assembly.
Repelled by the magnets on either end, the magnetic assembly is forced to hang out in the middle -- until it's shaken. When that happens, it's forced to bounce back between the two repellent forces, generating an electric current as it passes back and forth through the coil assembly.
This leads to two kinds of motion-powered electronics: those that require the active application of kinetic energy, such as cranking, and those that require passive kinetic energy, such as the up-and-down movement produced by walking or jogging.
The downside, of course, is that passive kinetic energy varies greatly depending on what you're doing. Going for a vigorous run? Great, enjoy a fully charged cell phone. But unless your nights are full of sleep walking or night-terror-induced thrashing, then don't expect to charge your batteries while sleeping out under the stars. Developers M2E Power (Motion to Energy) claim that, in tests, their batteries can turn six hours of average human movement into 30 to 60 minutes of cell phone power [source: LaMonica]