The dimensions of $\sigma b^4$, where $\sigma$ is Stefan's constant and $b$ is Wien's constant, are:

If $120 \ J$ of thermal energy is incident on an area of $3 \ m^2$,the amount of heat transmitted is $12 \ J$,and the coefficient of absorption is $0.6$,then the amount of heat reflected is: (in $J$)

$A$ heated body emits radiation which has maximum intensity at frequency $f_m$. If the temperature of the body is doubled,then:

Heat loss takes place from a body maintained at a temperature of $400^{\circ} C$ to the surrounding air at $30^{\circ} C$ by convection and to the surrounding surfaces at $30^{\circ} C$ by radiation. The Newton's cooling coefficient is $20 \ W / m^2 \ K$ and the Stefan-Boltzmann constant is $5.67 \times 10^{-8} \ W / m^2 \ K^4$. If the rate of heat loss by convection is equal to the rate of heat loss by radiation, the emissivity of the body surface is

Three identical rods $AB$,$CD$ and $PQ$ are joined as shown. $P$ and $Q$ are midpoints of $AB$ and $CD$ respectively. Ends $A, B, C$ and $D$ are maintained at $0^{\circ} C, 100^{\circ} C, 30^{\circ} C$ and $60^{\circ} C$ respectively. The direction of heat flow in $PQ$ is

Three rods of Copper,Brass,and Steel are welded together to form a $Y$ shaped structure. The area of cross-section of each rod is $4 \ cm^2$. The end of the copper rod is maintained at $100^\circ C$,whereas the ends of the brass and steel rods are kept at $0^\circ C$. The lengths of the copper,brass,and steel rods are $46 \ cm$,$13 \ cm$,and $12 \ cm$ respectively. The rods are thermally insulated from the surroundings except at the ends. The thermal conductivities of copper,brass,and steel are $0.92$,$0.26$,and $0.12 \ CGS$ units respectively. The rate of heat flow through the copper rod is ....... $cal \ s^{-1}$.