The temperature of the two outer surfaces of a composite slab, consisting of two materials having coefficients of thermal conductivity $K$ and $2K$ and thickness $x$ and $4x$, respectively are $T_2$ and $T_1(T_2 > T_1)$. The rate of heat transfer through the slab, in a steady state is $\left( {\frac{{A\left( {{T_2} - {T_1}} \right)K}}{x}} \right)f$, with $f$ equals to

A body cools from $62\,^oC$ to $50\,^oC$ in $10\, minutes$ and to $42\,^oC$ in next $10\, minutes$. The temperature of the surrounding is ........... $^oC$

A wall has two layers $A$ and $B,$ each made of different material. Both the layers have the same thickness. The thermal conductivity for $A$ is twice that of $B$ and under steady condition, the temperature difference across the wall is $36\,^oC.$ The temperature difference across the layer $A$ is....... $^oC$

If the temperature of the sun were to increase from $T$ to $2T$ and its radius from $R$ to $2R$, then the ratio of the radiant energy received on earth to what it was previously will be

Two rods of crossectional area $A$ and $2A$ and equal length having thermal conductivities $2K$ and $3K$ are joined in parallel, then equivalent thermal conductivity of their combination will be-

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