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.

A metal rod of length 2m has cross sectional areas 2A and A as shown in figure. The ends are maintained at temperatures 100°C and 70°C. The temperature at middle point C is

1. 80°C

2. 85°C

3. 90°C

4. 95°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

One likes to sit in the sunshine in the winter season, because:

Air is a bad conductor of heat or partly conducts heat. Still, a vacuum is to be placed between the walls of the thermos flask because: