(a) A phonograph record carries a uniform density of ``static electricity'' σ. If it rotates at angular velocity ω, what is the surface current density K at a distance r from the center?
(b) A uniformly charged solid sphere, of radius R and total charge Q, is centered at the origin and spinning at a constant angular velocity ω about the z axis. Find the current density J at any point (r,θ,ϕ) within the sphere.