A spherical tungsten pices of radius `1.0cm` is suspended in an evacuated chamber maintained at `300K. The pices is maintained at 1000K by heating it electrically. Find the rate at which the electrical energy must be supplied. The emissivity of tungsten is `0.30` and the Stefan constant sigma is `6.0xx10^(-s)Wm^(-2)K^(-4)` .

Answer

B

Explanation

`r=1cm =1xx10^(-2)m,`
`A =4 pi (10^(-2))^(2)`
`=4 pi xx10^(-4)m^(2)`
`e=0.3, sigma =6xx10^(-8)`
`(E/t)e sigma A (T_(2)^(4)-T_(1)^(4))`
`=0.3xx6xx10^(-8)xx4 pixx10^(-4)`
`xx[(1000)^4-(300)^4]`
`=0.3xx6xx4 pi xx10^(-12)`
`xx[1-0.0081]xx1012`
`=0.3xx64xx3.14xx9919xx10^(-5)`
`=22.4=22W`.