Question
a metallic sphere having inner radius a and outer radius b has thermal conductivity k = ko(k not)/r² (a ≤ r ≤ b). the thermal resistance between inner and outer surface for radiant heat flow is
Joshi sir comment
consider a sphere of radius r and thickness dr
formula used is H = KAdT/L so dT/H = L/KA or R = L/KA, dT/H is thermal resistance
thermal resistance of layer of thickness dr is dr/K4πr2, on putting the value of K we get
dR = r2dr/4πr2k0
or dR = dr/4πk0
now integrate within the proper limits