Surface current density, K is defined as: K = σv where σ is surface charge density and v is velocity. Given a uniformly charged spherical shell with radius R, spinning at constant angular velocity ω, find the current. So, I start with this formula: dI = K dl dI = σ Rω dl and I placed the spherical shell at cartesian coordinate with its center at origin and try to solve the question in spherical coordinate. What path should I take to express dl? Professor explained, dl = R dθ. What I don't understand is, why do we have to take R? Why not R sin(θ)?