1. The problem statement, all variables and given/known data A charged plate (record) is rotating with frequency "f". What is the magnetic field generated in the middle of the record? 2. Relevant equations R...radius f...frequency Q...electric charge B=? 3. The attempt at a solution My idea was that the field in the middle is the sum of the elemental fields generated by the thin belts in distance r and thickness Δr with electric current I(r). But i dont know how to continue. Thanks for help.