Question
The figure shows a system of two concentric spheres of radii and and kept at temperatures and respectively. The radial rate of flow of heat in a substance between the two concentric spheres, is proportional to

 In


The correct answer is:
To measure the radial rate of heat flow, we have to go for integration technique as here the area of the surface through which heat will flow is not constant.
Let us consider an element (spherical shell) of thickness d and radius as shown in figure. Let us first find the equivalent thermal resistance between inner and outer sphere.
Resistance of shell=
=
Rate of heat flow = H
=
=
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