Correct Answer - A
Consider a concentric spherical shell of radius r and thickness dr as shown in the The radial rate of flow of heat through this shell in steady state will be
`H = (dQ)/(dt) = - KA (dT)/(dr) = -K (4pir^(2))(dT)/(dr)`
`rArrunderset(r_(1))overset(r_(2))int(dr)/(r^(2))=-(4piK)/(H)underset(T_(1))overset(T_(2))intdT`
Which on integration and simplifiction gives
`H=(dQ)/(dt) = (4piKr_(1)r_(2)(T_(1)-T_(2)))/(r_(2)-r_(1))`
`rArr(dQ)/(dt)prop(r_(1)r_(2))/((r_(2)-r_(1)))`
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