6. A sphere of radius R2 has a concentric central cavity of radius R1. The material between R1 and R2 has thermal conductivity k. Radius R2 is maintained at temperature T2, and radius R1 is maintained at temperature T1. Find the rate of heat ow across any spherical surface within the spherical shell. Hint: the rate of heat ow is the same for all such surfaces (why?), and remember that the area of a spherical surface depends on the radius