The radiant energy from sun incident normally at the surface of earth is $20\, k\,cal/m^2- min$. What would have been the radiant energy incident normally on the earth if the sun had a temp. twice of the present one ? ........... $k\,cal/m^2- min$

A wall is made up of two layers $A$  and $B.$ The thickness of the two layers is the same, but materials are different. The thermal conductivity of $A$  is double than that of $B.$ In thermal equilibrium the temperature difference between the two ends is $36\,^oC.$ Then the difference of temperature at the two surfaces of  $A$  will be......... $^oC$

Which of the following cylindrical rods will coduct most heat when their ends are maintained at the same steady temperatures?

The temperature of body is increased from $27\,^oC$ to $127\,^oC$ the radiation emitted by it increases by a factor of

Six wires each of cross-sectional area $A$ and length $l$ are combined as shown in the figure. The thermal conductivities of copper and iron are $K_1$ and $K_2$ respectively. The equivalent thermal resistance between points $A$ and $C$ is

A cylinder of radius $R$ made of a material of thermal conductivity$ k_1$ is surrounded by a cylindrical shell of inner radius $R$ and outer radius $2R$ made of a material of thermal conductivity $k_2$. The two ends of the combined system are maintained at different temperatures. There is no loss of heat from the cylindrical surface and the system is in steady state. The effective thermal conductivity of the system is