A slab consists of two parallel layers of copper and brass of the same thickness and having thermal conductivities in the ratio 1 : 4. If the free face of brass is at 100°C and that of copper at 0°C, the temperature of interface is
1. 80°C               

2. 20°C

3. 60°C               

4. 40°C

Two thin blankets keep more hotness than one blanket of thickness equal to these two. The reason is

(1) Their surface area increases

(2) A layer of air is formed between these two blankets, which is bad conductor

(3) These have more wool

(4) They absorb more heat from outside

Ice formed over lakes has

Wires A and B have identical lengths and have circular cross-sections. The radius of A is twice the radius of B i.e. ${\mathrm{r}}_{\mathrm{A}}=2{\mathrm{r}}_{\mathrm{B}}$ . For a given temperature difference between the two ends, both wires conduct heat at the same rate. The relation between the thermal conductivities is given by

(1) ${\mathrm{K}}_{\mathrm{A}}=4{\mathrm{K}}_{\mathrm{B}}$                     
(2) ${\mathrm{K}}_{\mathrm{A}}=2{\mathrm{K}}_{\mathrm{B}}$
(3) ${\mathrm{K}}_{\mathrm{A}}={\mathrm{K}}_{\mathrm{B}}/2$                   
(4) ${\mathrm{K}}_{\mathrm{A}}={\mathrm{K}}_{\mathrm{B}}/4$

Two identical plates of different metals are joined to form a single plate whose thickness is double the thickness of each plate. If the coefficients of conductivity of each plate are 2 and 3 respectively, then the conductivity of the composite plate will be:
1. 5
2. 2.4
3. 1.5                 
4. 1.2

The coefficients of thermal conductivity of copper, mercury and glass are respectively ${\mathrm{K}}_{\mathrm{c}}$, ${\mathrm{K}}_{\mathrm{m}}$ and ${\mathrm{K}}_{\mathrm{g}}$ such that ${\mathrm{K}}_{\mathrm{c}}>{\mathrm{K}}_{\mathrm{m}}>{\mathrm{K}}_{\mathrm{g}}$. If the same quantity of heat is to flow per second per unit area of each and corresponding temperature gradients are ${\mathrm{X}}_{\mathrm{c}}$, ${\mathrm{X}}_{\mathrm{m}}$ and ${\mathrm{X}}_{\mathrm{g}}$, then 
(1) ${\mathrm{X}}_{\mathrm{c}}={\mathrm{X}}_{\mathrm{m}}={\mathrm{X}}_{\mathrm{g}}$                 

(2) ${\mathrm{X}}_{\mathrm{c}}>{\mathrm{X}}_{\mathrm{m}}>{\mathrm{X}}_{\mathrm{g}}$

(3) ${\mathrm{X}}_{\mathrm{c}}<{\mathrm{X}}_{\mathrm{m}}<{\mathrm{X}}_{\mathrm{g}}$                 

(4) ${\mathrm{X}}_{\mathrm{m}}<{\mathrm{X}}_{\mathrm{c}}<{\mathrm{X}}_{\mathrm{g}}$

If two metallic plates of equal thicknesses and thermal conductivities ${\mathrm{K}}_1$ and ${\mathrm{K}}_2$ are put together face to face and a common plate is constructed, then the equivalent thermal conductivity of this plate will be

(1) $\frac{{\mathrm{K}}_1{\mathrm{K}}_2}{{\mathrm{K}}_1+{\mathrm{K}}_2}$     
               
(2) $\frac{2{\mathrm{K}}_1{\mathrm{K}}_2}{{\mathrm{K}}_1+{\mathrm{K}}_2}$

(3) $\frac{{\left({\mathrm{K}}_1^2+{\mathrm{K}}_2^2\right)}^{3/2}}{{\mathrm{K}}_1{\mathrm{K}}_2}$ 
           
(4) $\frac{{\left({\mathrm{K}}_1^2+{\mathrm{K}}_2^2\right)}^{3/2}}{2{\mathrm{K}}_1{\mathrm{K}}_2}$

The quantity of heat which crosses unit area of a metal plate during conduction depends upon

(1) The density of the metal

(2) The temperature gradient perpendicular to the area

(3) The temperature to which the metal is heated

(4) The area of the metal plate

Two spheres of different materials one with double the radius and one-fourth wall thickness of the other, are filled with ice. If the time taken for complete melting of ice in the larger sphere is 25 minutes and that for smaller sphere 16 minutes, the ratio of thermal conductivities of the materials of larger sphere to the smaller sphere is 
(1) 4 : 5                   

(2) 5 : 4

(3) 25 : 1                 

(4) 8 : 25

The mud houses are cooler in the summer and warmer in the winter because: