Fig. shows a system of two concentric spheres of radii r1 and r2 kept at temperatures T1 and T2 respectively. The radial rate of flow of heat in a substance between the two concentric spheres is proportional to

We know that  Q=−KAdTdror     Q=−K4πr2dTdr ∵A=4πr2        ∴ drr2=−4πKQdTIntegrating this expression within proper limits, we get           ∫r1I2 drr2=−4πKQ∫T1T2 dTor     1r1−1r2=4πKQT1−T2r2−r1r1r2=4πKQT1−T2or   Q=4πKT1−T2r1r2r2−r1∝r1r2r2−r1.