10. Imagine two concentric metal sphericar snells. The inner one (radius a) carries a charge Q(t), and the outer one (radius b) an opposite charge -Q(t). The space between them is filled with a material of conductivity o. a) Show using symmetries involved in the problem that magnetic field can not be in 6, o or f direction, and only possible value of magnetic field is zero. b) Calculate current density and its direction. c) Why there exist a current but not magnetic field in this case.