Flux through a hoop: 3D

Grab the large dot and drag it to rotate the hoop. The blue hoop sits in a uniform field of vectors. The flux enclosed by the hoop depends on its rotation angle, and is proportional to the number of vectors (green) enclosed within the hoop. The blue vector indicates the normal vector to the flat surface bounded by the hoop. Use shift-drag to pan the view, shift-mousewheel to zoom, and shift-drag with right-click to rotate the view.