This is an alternating current generator: a conducting coil (shown in yellow) rotates inside a magnetic field (the field lines are green here). The vector "A" is perpendicular to the area of the coil (its magnitude is the area of the coil). During rotation of the coil, the magnetic flux changes and an alternating current is induced in the coil. The first graph show the magnetic flux, the other graph shows current intensity.
Other animations by Yves Pelletier
The title says it all: this blog features physics videos found everywhere on the web: animations, demonstrations, lectures, documentaries.