A body cools from $62\,^oC$ to $50\,^oC$ in $10\, minutes$ and to $42\,^oC$ in next $10\, minutes$. The temperature of the surrounding is ........... $^oC$

Gravitation force is required for

A liquid in a beaker has temperature $\theta \left( t \right)$ at time $t$ and ${\theta _0}$ is temperature of surroundings, then according to Newton's law of cooling the correct graph between loge ${\log _e}(\theta  - {\theta _0})$ and $t$ is

A hot liquid kept in a beaker cools from $80\,^oC$ to $70\,^oC$ in $2\, min$. If the  surrounding temperature is $30\,^oC$, then the time of cooling of the same liquid from  $60\,^oC$ to $50\,^oC$ ........ $\sec$

The intensity of radiation emitted by the Sun has its maximum value at a wavelength of $510\,\, nm$ and that emitted by the North Star has the maximum value at $350\,\, nm$. If these stars behave like black bodies then the ratio of the surface temperature of the Sun and the North Star is

A cylinder of radius $R$ made of a material of thermal conductivity$ k_1$ is surrounded by a cylindrical shell of inner radius $R$ and outer radius $2R$ made of a material of thermal conductivity $k_2$. The two ends of the combined system are maintained at different temperatures. There is no loss of heat from the cylindrical surface and the system is in steady state. The effective thermal conductivity of the system is