Critical thickness of insulation for
spheres is given by
k/h
k/4h
h/2k
2k/h
2k/h
Critical thickness of insulation for sphere is given by,
Hence, the correct option is (D).
&Key Points:-
Critical thickness of insulation for cylinder
is given by 
Critical thickness of insulations
means maximum heat loss with minimum thermal resistance.
Critical radius of insulation means
the insulation at which resistance to heat flow is minimum.
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