$A$ composite slab is made of two different metals of the same thickness,having thermal conductivities $K$ and $2K$. The equivalent thermal conductivity of the composite slab is:

$A$ wall consists of alternating blocks of length $d$ and coefficients of thermal conductivity $K_{1}$ and $K_{2}$ respectively,as shown in the figure. The cross-sectional areas of the blocks are the same. The equivalent coefficient of thermal conductivity of the wall between the left and right sides is

Three identical rods $A, B$ and $C$ are placed end to end. $A$ temperature difference is maintained between the free ends of $A$ and $C$. The thermal conductivity of $B$ is thrice that of $C$ and half of that of $A$. The effective thermal conductivity of the system will be ($K_{A}$ is the thermal conductivity of rod $A$).

In a composite rod,when two rods of different lengths and of the same area are joined end to end,then if $K$ is the effective coefficient of thermal conductivity,$\frac{{\ell _1} + {\ell _2}}{K}$ is equal to

Two diagonally opposite corners of a square made of four thin rods of the same material and same dimensions are at temperatures $40^{\circ} C$ and $10^{\circ} C$. If only heat conduction takes place,then the temperature difference between the other two corners will be .......... $^{\circ} C$.

Consider two insulating sheets with thermal resistances $R_1$ and $R_2$ as shown in the figure. The temperature $\theta$ at the junction is: