Heat And Thermodynamics

Two thin metallic spherical shells of radii r₁ and r₂ (r₁ < r₂) 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(\(\theta\)₁ < \(\theta\)₂). The rate at which heat flows radially through the material is :-