(a) A phonograph record carries a uniform density of static electricity . If it rotates at angular...

Question:

(a) A phonograph record carries a uniform density of "static electricity" σ. If it rotates at angular velocity w, 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 w about the z axis. Find the current density J at any point (r, θ, Ф) within the sphere.