A spherical ball of surface area `20cm^(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 `=6.0xx10^(-s)Wm^(-2)K^(-4)` .

Answer

A, B, C, D

Explanation

`(a) A=20cm^2=20xx10^(-4)m^2`
`T=57^(2)C=57+273=330K`
`E=A sigma T^4`
`=20xx10^(-4)xx6xx10^(-8)xx(330)^(4)`
`=1.42J.`
`(b)(E/t)=(A sigma e)(T_(1)^(4)-(T_(2)^(4)`
`[A=20 cm^2=20xx10^(-4)m^2, sigma= 6xx10^(-8)`
`t_1=473K, t_2=330 K`
`=20xx10^(-4)xx6xx10^(-8)`
`xx1[(473)^4-(330)^(4))]`
`=20xx6xx10^(-12)`
`[5.005xx10^(10)-1.185xx10^(10)] =20xx6xx3.82xx10^(-2)`
`=4.58W from the ball.`