b a)Free electrons of metal disc are rotating, so there has to be some force that is responsible for centripetal acceleration of electrons. That force exists because free electrons go to the rim of the disc and create electric field, so we have: Fcp=e*E, E=m*ω^2*r/e. So potential diference is Δφ=∫Edr (from 0 to R), Δφ=mω^2*r^2/2e.
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