Two thin metallic spherical shells of radii \( r_{1} \) and \( r_{2}\left(r_{1}<r_{2}\right) \) are placed with their…

Two thin metallic spherical shells of radii \( r_{1} \) and \( r_{2}\left(r_{1}<r_{2}\right) \) are placed with their centres coinciding. A material of thermal conductivity \( K \) is filled in the space between the shells. The inner shell is maintained at temperature \( \theta_{1} \) and the outer shell at temperature \( \theta_{2}\left(\theta_{1}<\theta_{2}\right) \). The rate at which heat flows radially through the material is