Two thin metallic spherical shells of radii r_1 and r_2( r_1< r_2) are placed with their centres coinciding. A material of thermal conductivity K is…

Two thin metallic spherical shells of radii $\displaystyle \mathrm{r}_1$ and $\displaystyle \mathrm{r}_2\left(\mathrm{r}_1<\mathrm{r}_2\right)$ 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 $\displaystyle \theta_1$ and the outer shell at temperature $\displaystyle \theta_2\left(\theta_1<\theta_2\right)$. The rate at which heat flows radially through the material is :