If two metallic plates of equal thicknesses and thermal conductivities ${K_1}$ and ${K_2}$ are put together face to face and a common plate is constructed, then the equivalent thermal conductivity of this plate will be

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 $\left( {\frac{{A({T_2} - {T_1})K}}{x}} \right)f$, with $f $ which equal to

One end of a thermally insulated rod is kept at a temperature $T_1$ and the other at $T_2$. The rod is composed of two sections of lengths $l_1$ and $l_2$ and thermal conductivities $K_1$ and $K_2$ respectively. The temperature at the interface of the two sections is

Temperature of water at the surface of lake is $ - {20^o}C$ Then temperature of water just below the lower surface of ice layer is ...... $^oC$

Two different rods $A$ and $B$ are kept as shown in figure. The ratio of thermal conductivities of $A$ and $B$ is

$Assertion :$ Two thin blankets put together are warmer than a single blanket of double the thickness.
$Reason :$ Thickness increases because of air layer enclosed between the two blankets.