It is proposed that the so-called radial spokes observed in rotation across Saturn's B ring are composed of fine charged dust levitated off the surfaces of larger bodies by electrostatic forces when the latter are sporadically charged to high electrostatic potentials. From the observed wedge-shaped morphology of these spokes it is deduced that the dust grains are negatively charged. The dynamical structure of a spoke feature consisting of grains of different sizes is considered. It is shown that the surface density of dust within these wedge-shaped spokes is not uniform. Each spoke has a fine structure consisting of a number straight sharp 'ribs' radiating out from points on the synchronous orbit. The time evolution of a spoke can be likened to the continuous unfolding of a fan whose vertex is at the synchronous orbit and one of whose edges (the corotating one) is fixed in the radial direction. The model also predicts that the measured spoke velocities correspond to phase velocities rather than particle velocities, as is generally believed. According to this model, the angular velocity of the leading edge of the fan depends on the age of the spoke. Inside the synchronous orbit it is super-Kepler up to about 2.5 hours after formation, but sub-Kepler between 2.5 hours and 6 hours. Those spokes surviving the ring plane crossing will once again have a super-Kepler leading edge after 6 hours. Outside the synchronous orbit the reverse would be the case. Consequently the model anticipates the observed scatter of the measured velocities about the Kepler value as real rather than due to errors of measurements, although they lie within the expected error bars. Finally, it is shown that the resettling of the grains on the larger bodies in the ring plane following their initial levitation results in a differential transport of grains across the ring plane. A consequence of this is the establishment of different multimodal size distribution of dust within the spokes at different planetocentric distances.