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The equation for heat current through a rod is H=k*A*DeltaT/L where L is the length of the rod.

For this sperical shell the area which is perpendicular to the flow of the heat is changing with the radius. So I have to integrate the area. Am I right here?

I have tried to integrate the area by doing:

A=integrate(4*pi*r^2) from a to b

But I end up with something far from the right answer, don't want to get into that.

Could anyone please give me a hint to this problem?

Thanks

The right answer is H=4*pi*k*a*b*DeltaT/(b-a)