A wall is made of two layers of thickness $1\ cm$ and $4\ cm$ and thermal conductivities $K$ and $3K$ respectively. If temperature difference for the wall is $50\ ^oC$ then temperature difference for the thin layer will be

- A
$\frac{120}{7}\ ^oC$

- B
$\frac{150}{7}\ ^oC$

- C
$\frac{180}{7}\ ^oC$

- D
None

The radiant energy from sun incident normally at the surface of earth is $20\, k\,cal/m^2- min$. What would have been the radiant energy incident normally on the earth if the sun had a temp. twice of the present one ? ........... $k\,cal/m^2- min$

A black body at a high temperature $T\, K$ radiates energy at the rate $E\, watt/m^2$ ; when the temperature falls to $(T/2)\, K$ the radiated energy will be

Assuming the sun to be a spherical body of radius $R$ at a temperature of $T$ $K$, evaluate the total radiant power, incident on earth, at a distance $r$ from the sun- (when radius of earth is $r_0$)

Three rods $A, B,$ and $C$ of thermal conductivities $K, 2K$ and $4K$ and equal, cross-sectional areas and lengths $2l, l$ and $l$ respectively are connected as shown in the figure. If the ends of the rods are maintained at temperatures $100\ ^oC, 50\ ^oC,$ and $0\ ^oC$ respectively, then the temperature $\theta$ of the junction is

Five wires each of cross-sectional area $A$ and length $l$ are combined as shown. The thermal conductivity of copper and steel are $k_1$ and $k_2$ respectively. The equivalent thermal resistance between $A$ and $C$ is