In spherical coordinates (r, θ, φ), where dΩ = dθ × sin θdφ is the infinitesimal solid angle (in steradians) and ρ(r, \θ, \φ) depends only on the radius but not on the angular coordinates θ or φ. E(r, θ, φ) = E(r only)ˆr . To find the radial dependence of E(r) we use the Gauss Law. Let S be a sphere of some radius r centered at the origin.