Consider a compound slab consisting of two different materials having an equal thickness and thermal conductivities K and 2K respectively. The equivalent thermal conductivity of the slab if the materials are joined by ends is:
(a) $\sqrt{2\mathrm{K}}$                  (b) 3K
(c) $\frac{4}{3}\mathrm{K}$                    (d) $\frac{2}{3}\mathrm{K}$

Two rods having thermal conductivity in the ratio of 5 : 3 having equal lengths and equal cross-sectional area are joined by face to face. If the temperature of the free end of the first rod is 100$°\mathrm{C}$ and free end of the second rod is 20$°\mathrm{C}$. Then temperature of the junction is

(1) 70 $°\mathrm{C}$                   

(2) 50 $°\mathrm{C}$

(3) 50 $°\mathrm{C}$                  

(4) 90 $°\mathrm{C}$

Woollen clothes are used in winter season because woollen clothes

(1) Are good sources for producing heat

(2) Absorb heat from surroundings

(3) Are bad conductors of heat

(4) Provide heat to body continuously

Two metal cubes and of same size are arranged as shown in the figure. The extreme ends of the combination are maintained at the indicated temperatures. The arrangement is thermally insulated. The coefficients of thermal conductivity of A and B are 300 W/m$°\mathrm{C}$ and 200 W/m$°\mathrm{C}$, respectively. After steady state is reached, the temperature of the interface will be 

(1) 45$°\mathrm{C}$
(2) 90$°\mathrm{C}$
(3) 30$°\mathrm{C}$
(4) 60$°\mathrm{C}$

A cylindrical rod having temperature ${\mathrm{T}}_1$ and ${\mathrm{T}}_2$ at its ends. The rate of flow of heat is ${\mathrm{Q}}_1$ cal/sec. If all the linear dimensions are doubled keeping temperature constant then rate of flow of heat ${\mathrm{Q}}_2$ will be
(a) $4{\mathrm{Q}}_1$           (b) $2{\mathrm{Q}}_1$
(c)$\frac{{\mathrm{Q}}_1}{4}$            (d) $\frac{{\mathrm{Q}}_1}{2}$

A body of length 1m having cross sectional area 0.75 ${\mathrm{m}}^2$ has heat flow through it at the rate of 6000 Joule/sec. Then find the temperature difference if K = 200 ${\mathrm{Jm}}^{-1}{\mathrm{K}}^{-1}$ 

(a) 20°C                     (b) 40°C
(c) 80°C                     (d) 100°C

A wall has two layers A and B made of different materials. The thickness of both the layers is the same. The thermal conductivity of A and B are ${\mathrm{K}}_{\mathrm{A}}$ and ${\mathrm{K}}_{\mathrm{B}}$ such that  ${\mathrm{K}}_{\mathrm{A}}$ = 3${\mathrm{K}}_{\mathrm{B}}$. The temperature across the wall is 20°C. In thermal equilibrium 

(1) The temperature difference across A = 15°C

(2) The temperature difference across A = 5°C

(3) The temperature difference across A is 10°C

(4) The rate of transfer of heat through A is more than that through B.

             
1. \(80^{\circ}\text{C}\)
2. \(85^{\circ}\text{C}\)
3. \(90^{\circ}\text{C}\)
4. \(95^{\circ}\text{C}\)

Which of the following circular rods (given radius r and length l) each made of the same material as whose ends are maintained at the same temperature will conduct most heat ?

(1) $\mathrm{r}=2{\mathrm{r}}_0 ; \mathrm{l}=2{\mathrm{l}}_0$           

(2) $\mathrm{r}=2{\mathrm{r}}_0 ; \mathrm{l}={\mathrm{l}}_0$

(3) $\mathrm{r}={\mathrm{r}}_0 ; \mathrm{l}={\mathrm{l}}_0$               

(4) $\mathrm{r}={\mathrm{r}}_0 ; \mathrm{l}=2{\mathrm{l}}_0$

It is hotter for the same distance over the top of fire than it is inside of it, mainly because

(1) Air conducts heat upwards

(2) Heat is radiated upwards

(3) Convection takes more heat upwards

(4) Convection, conduction and radiation all contribute significantly transferring heat upwards