In a composite rod, when two rods of different lengths $l_1$ and $l_2$ and of the same cross-sectional area are joined from end to end then if $K$ is the effective coefficient of thermal conductivity, the value of $(l_1 + l_2)/K$ is

- A
$\frac{{{l_1}}}{{{K_1}}} + \frac{{{l_2}}}{{{K_2}}}$

- B
$\frac{{{l_1}}}{{{K_2}}} + \frac{{{l_2}}}{{{K_1}}}$

- C
$\frac{{{l_1}}}{{{K_1}}} - \frac{{{l_2}}}{{{K_2}}}$

- D
$\frac{{{l_1}}}{{{K_2}}} - \frac{{{l_2}}}{{{K_1}}}$

Which of the following cylindrical rods will coduct most heat when their ends are maintained at the same steady temperatures?

Radiated energy at $TK$ temperature is $E$ for a body of diameter $'d'$. If temperature becomes $(2T)$ and diameter becomes $\frac{d}{4}$ then radiated energy will be :-

Two rods having same area are used to connect two reservoirs at temperature $100\,^oC$ and $0\,^oC$ as shown. The temperature of junction is $70\,^oC$. If the rods are now interchanged, the temperature of junction will be ......... $^oC$

A cylindrical metallic rod, in thermal contact with two reservoirs of heat at its two ends, conducts an amount of heat $Q$ in time $t.$ The metallic rod is melted and the material is formed into a rod of half the radius of the original rod. Amount of heat conducted by the new rod, when placed in thermal contact with same two reservoirs in time $t$ , is

The rate of emission of radiation of a black body at $273^o C$ is $E$, then the rate of emission of radiation of this body at $0^o C$ will be