(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.