#### Question

A spherical ball of surface area 20 cm^{2} absorbs any radiation that falls on it. It is suspended in a closed box maintained at 57°C. (a) Find the amount of radiation falling on the ball per second. (b) Find the net rate of heat flow to or from the ball at an instant when its temperature is 200°C. Stefan constant = 6.0 × 10^{−8} W m^{−2} K^{−4}.

#### Solution

(a)

Area of the ball,* A* = 20 × 10^{−4} m^{2}

Temperature of the ball, *T* = 57°C = 57 + 273 = 330 K

Amount of heat radiated per second = *AσT*^{4}

= 20 × 10^{−4} × 6 × 10^{−8} × (330)^{4}

= 1.42 J

(b)

Net rate of heat flow from the ball when its

emperature is 200 °C is given by

eAσ (T_{1}^{4} - T_{2}^{4})

= 20 × 10^{-4 }× 6 × 10^{-8} × 1 ((473)^{4} - (330)^{4} [∴ e = 1]

= 4.58 W

Is there an error in this question or solution?

Solution Calculate the Amount of Heat Radiated per Second by a Body of Surface Area 12 Cm2 Kept in Thermal Equilibrium in a Room at Temperature 20°C. the Emissivity of the Surface = 0.80 and Concept: Thermal Expansion of Solids.