60. The figure shows a system of two concentric spheres of radii  and  are kept at temperatures  and , respectively. The radial rate of flow of heat in a substance between the two concentric 

spheres is proportional to                                                                                                                      

A slab of stone of area  and thickness  is exposed on the lower surface to steam at . A block of ice at  rests on the upper surface of the slab. In one hour  of ice is melted. The thermal conductivity of slab is :

(Given latent heat of fusion of ice  ):

The coefficient of thermal conductivity of copper, mercury and glass are respectively  and  such that . If the same quantity of heat is to flow per sec per unit area of each and corresponding temperature gradients are  and  then. 

Consider a pair of insulating blocks with thermal resistances  and  as shown in the figure. The temperature  at the boundary between the two blocks is

12 identical rods made of same material are arranged in the form of a cube. The temperature of  and  are maintained at  and  respectively. Then the temperature of point , when steady state is reached is

65. The temperature  at the junction of two insulating sheets, having thermal resistances  and  as well as top and bottom temperatures  and  (as shown in figure) is given by :                                        

66. Two rods of same length and transfer a given amount of heat 12 second, when they are joined as shown in figure (i). But when they are joined as shown in figure (ii), then they will transfer same heat in same conditions in

A partition wall has two layers of different materials  and  in contact with each other. They have the same thickness but the thermal conductivity of layer  is twice that of layer B. At steady state the temperature difference across the layer  is , then the corresponding difference across the layer  is     

Two identical metal wires of thermal conductivities  and  respectively are connected in series. The effective thermal conductivity of the combination is:

Heat is transmitted from higher to lower temperature through actual mass motion of the molecules in