I need help with this problem.

Use Darcy's Law to derive an equation for the volumetric flow rate $(Q_v)$ through a porous

sphere. The sphere has a radius $R_e$ and a hollow core with radius $R_i.$ A cross$-$section of

the sphere is provided below. Assume that the pressure inside the sphere $(P_i)$ is greater

than the pressure outside the sphere $(P_e),$ thus driving the flow outward radially.